Desert Dueler

Desert Dueler

Posted by alexandriabrangwin on 2017-11-22 08:56:20

Tagged: , Alexandria Brangwin , Second Life , 3D , CGI , Computer , Graphics , Virtual , world , photography , Mercedes Benz , AMG , G63 , 6×6 , off , road , truck , silver , desert , highway , stopped , headlights , FBI , field , agent , outfit , Sicario , Emily , Blunt , massive , tires , tread , sand , dust , dirt , beam , differential , leather , boots , cargo , pants , tactilneck , Archer

Mad Alex – Second Life Road

Mad Alex - Second Life Road

www.youtube.com/watch?v=hEJnMQG9ev8

Posted by alexandriabrangwin on 2015-05-14 09:12:07

Tagged: , Alexandria Brangwin , Second Life , 3D , CGI , Computer , Graphics , Virtual , world , Mad , Max , Fury , Road , post , apocalyptic , wasteland , movie , tribute , fan , art , Alex , Ford , XB , Falcon , GT , Supercharger , Weiand , shotgun , fight , battle , aim , window , angry , yelling , driving , dust , wind , black , leather , jacket , haze , sun , sunset , rust , warrior , interceptor , V8 , MFP

Eat my dust

Eat my dust

‘Eat my dust’ On Black

I had a major hard disk crash today. I managed to save most of my data, including a massive amount of raw files (phew!). This means working all weekend to get my computer running again, because on this laptop here, it’s no fun to edit the pictures at all…

What do I learn from that?
– Do backups more often
– Keep your important data on a different hard drive than your windows installation
– Think about buying an analog camera 🙂

Posted by Andreas Reinhold on 2006-07-14 23:19:09

Tagged: , 123bw , bw , sw , black , white , blackandwhite , truck , mine , mining , stones , rocks , rock , dust , dusty , road , gravel , huge , bergischesland , wuppertal , dornap , mettmann , smoke

Around The Mountain

Around The Mountain

Alex breaks off as Mondy enters Innuendo Tunnel

Posted by alexandriabrangwin on 2014-12-24 22:05:32

Tagged: , Alexandria Brangwin , Second Life , 3D , CGI , Computer , Graphics , Virtual , world , The Great Race , Mercedes Benz , SL65 , AMG , V12 , BiTurbo , twin , turbo , convertible , race , dust , cloud , fast , speeding , racing , dangerous , perilous , forest , mountains , driving , dirt , road , pine , spruce , woman

quot;Arts test,” good and bad talent on the road, “single-log bridge”? – Arts test, useful, single-plank bridge – Education

quot;Arts test,” good and bad talent on the road, “single-log bridge”? – Arts test, useful, single-plank bridge – Education

Related Topics Recommended: Art exam Art exam _ _2008-year education sector project

Not much interested in art and accumulation, to participate in arts Test Only because of mediocre grades, to more easily obtain a university. In recent years, with the art institutions of higher enrollment, “Arts test,” seems to have become more and more how much candidates into the University’s “shortcut.”

Statistics, according to the education sector in Shanxi Province, the province registered for the 2008 College Art Major Candidates 4.3 million, an increase of 10,000 over 2007 people. Enrollment in the Shanxi University, a fine arts course in front of his family from the Shanxi Xinzhou of Liu Yan, who came to this application. Liu Yan increase this year, three have not previously had any art foundation, where the acceptance period of 7 months of art training, recruitment in the examination room will be artsy.

At this course, no art such as Liu Yan basis, through the arts for the purpose of examination candidates, accounting for 70% of course. In addition to several thousand dollars to pay tuition fees, ranging from first million, the field to the candidates and pay accommodation costs in varying amounts. However, their family members are all very supportive, although many come from low income rural families. Xiao Zhang

university entrance students choose to repeat next year in the arts to participate in college entrance examination. She said that if she adopted the art of professional courses, cultural courses next year, her performance as long as a slight increase, it is expected to be one colleges and universities.

Appears in the real art lovers, the training of the students are “dressed experts coat of amateurs.” He studied double reed, he won the ranking in the country’s strict Kai, said: “Under the weight of the test, which rely on short relentless training, the practice of temporary battlefield, easily disgusted students in art, art itself is a blasphemy. “

“Test arts” course good and bad

Want quick, you have to participate in the examination for the purpose of the course, many good and bad of course emerged. Hsu, Shanxi University, in the vicinity of West Street, not far apart you will see an entrance arts training. According to one informed source disclosed that the majority of these courses is not eligible for school.

Number of training courses based in the poor condition of the hotel, the students in the beauty salon and restaurant upstairs study. In the “good business” of course “classroom” where each student per capita area of less than 1 square meter, all standing, support a drawing board and, apart from no more room. Although crude

condition, charges are not vague. 800 yuan per month to 1,600 per month. Some “deep intelligence,” the long-term toll of 1.5 million per class and asked Yicijiaoqing. A training course students range from dozens of people, and more to 200.

Months of intensive training, really good foundation with the launch competition? Faced with these questions, almost every course the students can show their achievements over the years, adding that access to an art school candidates do not have any basis, they should be able to learn by.

However, these students and their parents come here especially do not know, a lot of the organizers of training courses and teaching are not as they imagined as a “hard air.” To attract talented students, many teacher training courses play a certain speaker of the signs, but the actual teaching, the “teacher” is only linked to a title or a face exposed to disappear, most of course was in undergraduate and graduate school for art .

Taiyuan Normal University art professor Hu Jianfeng some of the “black studio” of the scam are summarized as follows: to “ensure that students pass the professional examination” or “enrollment rate reached 100 percent” and false advertising; play “in the candidates list,” where teachers Roll Recommended name; the guise of trick with a low-tuition students; “small classes, one on one counseling”; “entrance marking teachers” teach themselves; and mature relationship between the school admissions, college entrance examination and admission when the school for Latin America; list of so-called ” candidates in the list, “such brilliant results.

We are high quality suppliers, our products such as 3D Cinema System , 4D Theater System for oversee buyer. To know more, please visits 3D Cinema System.

Fury Road

Fury Road

www.youtube.com/watch?v=akX3Is3qBpw

Posted by alexandriabrangwin on 2014-10-09 18:45:58

Tagged: , Alexandria Brangwin , Second Life , 3D , CGI , Computer , Graphics , Virtual , world , Mad , Max , Fury , Road , new , movie , post , apocalypse , wasteland , Furiosa , Charlise , Theron , warrior , chase , interceptor , truck , ford , falcon , pursuit , special , dust , smoke , cyber , arm , metal , pose , action , sequence , Desert , woman

One Night in October

One Night in October

Quelle:
de.wikipedia.org/wiki/Milchstra%C3%9Fe

en.wikipedia.org/wiki/Milky_Way

Die Milchstraße, auch Galaxis, ist die Galaxie, in der sich unser Sonnensystem mit der Erde befindet. Entsprechend ihrer Form als flache Scheibe, die aus Milliarden von Sternen besteht, ist die Milchstraße von der Erde aus als bandförmige Aufhellung am Nachthimmel sichtbar, die sich ĂŒber 360° erstreckt. Ihrer Struktur nach zĂ€hlt die Milchstraße zu den Balkenspiralgalaxien.
Den Namen Milchstraßensystem trĂ€gt unser Sternsystem nach der Milchstraße, die als freiĂ€ugige Innenansicht des Systems von der Erde aus wie ein quer ĂŒber das Firmament gesetzter milchiger Pinselstrich erscheint. Dass dieses weißliche Band sich in Wirklichkeit aus unzĂ€hligen einzelnen Sternen zusammensetzt, wurde erst 1609 von Galileo Galilei erkannt, der die Erscheinung als Erster durch ein Fernrohr betrachtete. Es sind nach heutiger SchĂ€tzung ca. 100 bis 300 Milliarden Sterne.

Schon im Altertum war die Milchstraße als heller, schmaler Streifen am Nachthimmel bekannt. Ihr altgriechischer Name galaxias (ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚) – von dem auch der heutige Fachausdruck „Galaxis“ stammt – ist von dem Wort gala (γΏλα, Milch) abgeleitet.[1] Wie dem deutschen Wort „Milchstraße“ liegt also auch dem altgriechischen Begriff das „milchige“ Aussehen zugrunde.

Eine antike griechische Sage versucht, diesen Begriff mythologisch zu erklĂ€ren: Danach habe Zeus seinen Sohn Herakles, den ihm die sterbliche Frau Alkmene geschenkt hatte, an der Brust seiner göttlichen Frau Hera trinken lassen, als diese schlief. Herakles sollte auf diese Weise göttliche KrĂ€fte erhalten. Aber er saugte so ungestĂŒm, dass Hera erwachte und den ihr fremden SĂ€ugling zurĂŒckstieß; dabei wurde ein Strahl ihrer Milch ĂŒber den ganzen Himmel verspritzt.

Einer germanischen Sage zufolge erhielt die Milchstraße nach dem Gott des Lichtes, Heimdall, auch Iring genannt, den Namen Iringsstraße (laut Felix Dahn, Walhall – germanische Götter- und Heldensagen). Die afrikanischen San gaben der Milchstraße den Namen „RĂŒckgrat der Nacht“.

Zur ersten Vorstellung der Scheibenform des Milchstraßensystems gelangte bereits Wilhelm Herschel im Jahr 1785 aufgrund systematischer SternzĂ€hlungen (Stellarstatistik). Diese Methode konnte aber nicht zu einem realistischen Bild fĂŒhren, da das Licht weiter entfernter Sterne stark durch interstellare Staubwolken abgeschwĂ€cht wird, ein Effekt, dessen wahre Bedeutung erst in der ersten HĂ€lfte des 20. Jahrhunderts vollstĂ€ndig erfasst wurde. Durch Untersuchungen zur Verteilung der Kugelsternhaufen im Raum gelangte Harlow Shapley 1919 zu realistischen AbschĂ€tzungen der GrĂ¶ĂŸe des Milchstraßensystems und zu der Erkenntnis, dass die Sonne nicht – wie bis dahin, z. B. von Jacobus Kapteyn, angenommen – im Zentrum der Galaxis sitzt, sondern eher an deren Rand. Edwin Hubbles Messungen der Entfernungen von Spiralnebeln zeigten, dass diese außerhalb des Milchstraßensystems liegen und tatsĂ€chlich wie dieses eigenstĂ€ndige Galaxien sind.
Das Band der Milchstraße erstreckt sich als unregelmĂ€ĂŸig breiter, schwach milchig-heller Streifen ĂŒber dem Firmament.[2] Seine Erscheinung rĂŒhrt daher, dass in ihm mit bloßem Auge keine Einzelsterne wahrgenommen werden, sondern eine Vielzahl lichtschwacher Sterne der galaktischen Scheibe und des Bulges (in Richtung des galaktischen Zentrums). Von der SĂŒdhalbkugel aus steht das helle Zentrum der Milchstraße hoch am Himmel, wĂ€hrend man von der Nordhalbkugel zum Rand hin blickt. Daher kann man das Band der Milchstraße am besten von der SĂŒdhalbkugel aus beobachten. Im Dezember und Januar kann der hellste Bereich der Milchstraße nicht oder nur sehr schlecht beobachtet werden, weil sich die Sonne zwischen dem Zentrum der Galaxis und der Erde befindet. Gute Beobachtungsbedingungen sind bei klarer Luft und bei nur geringer Lichtverschmutzung durch kĂŒnstliche Lichtquellen gegeben. Alle der maximal 6000 mit bloßem Auge sichtbaren Sterne des Nachthimmels gehören zum Milchstraßensystem.

Das Milchstraßenband verlĂ€uft unter anderem durch die Sternbilder SchĂŒtze (in dieser Richtung liegt auch das galaktische Zentrum), Adler, Schwan, Kassiopeia, Perseus, Fuhrmann, Zwillinge, Orion, Kiel des Schiffs, Zentaur, Kreuz des SĂŒdens und Skorpion. Die mittlere Ebene des Milchstraßensystems ist gegenĂŒber dem HimmelsĂ€quator um einen Winkel von etwa 63° gekippt.

Astronomen verwenden gelegentlich ein spezielles, an die Geometrie des Milchstraßensystems angepasstes galaktisches Koordinatensystem, bestehend aus LĂ€nge l und Breite b. Die galaktische Breite betrĂ€gt 0° in der Ebene des Milchstraßensystems, +90° am galaktischen Nordpol und −90° am galaktischen SĂŒdpol. Die galaktische LĂ€nge, die ebenfalls in Grad angegeben wird, hat ihren Ursprung (l = 0°) in Richtung des galaktischen Zentrums und nimmt nach Osten hin zu.

Die Erforschung der Struktur des Milchstraßensystems ist schwieriger als die der Strukturen anderer Galaxien, da Beobachtungen nur von einem Punkt innerhalb der Scheibe gemacht werden können. Wegen der erwĂ€hnten Absorption sichtbaren Lichts durch interstellaren Staub ist es nicht möglich, durch visuelle Beobachtungen ein vollstĂ€ndiges Bild des Milchstraßensystems zu erhalten. Große Fortschritte wurden erst gemacht, als Beobachtungen in anderen WellenlĂ€ngenbereichen, insbesondere im Radiofrequenzbereich und im Infraroten möglich wurden. Dennoch sind viele Details des Aufbaus der Galaxis noch nicht bekannt.

Das Milchstraßensystem wurde frĂŒher als vier- oder fĂŒnfarmig betrachtet, nun gilt es als zweiarmige Balkenspiralgalaxie.[3] Es besteht aus etwa 100 bis 300 Milliarden Sternen und großen Mengen interstellarer Materie, die nochmals 600 Millionen bis einige Milliarden Sonnenmassen ausmacht (die Anzahl der Sterne und damit auch die Gesamtmasse unserer Galaxis kann auf Basis von Berechnungen und Beobachtungen nur geschĂ€tzt werden, woraus sich der große Toleranzbereich der Zahlen ergibt). Die Masse dieses inneren Bereichs der Galaxis wird mit ungefĂ€hr 180 Milliarden Sonnenmassen veranschlagt. Ihre Ausdehnung in der galaktischen Ebene betrĂ€gt etwa 100.000 Lichtjahre (30 kpc), die Dicke der Scheibe etwa 3000 Lichtjahre (920 pc) und die der zentralen Ausbauchung (engl. Bulge) etwa 16.000 Lichtjahre (5 kpc). Zum Vergleich: Der Andromedanebel hat eine Ausdehnung von etwa 150.000 Lj. und das drittgrĂ¶ĂŸte Mitglied der lokalen Gruppe, der Dreiecksnebel M 33, ca. 50.000 Lj. Die Angaben der Dicke mĂŒssen aber eventuell noch bis zum Doppelten nach oben korrigiert werden, wie der australische Wissenschaftler Bryan Gaensler und sein Team im Januar 2008 Ă€ußerten.[4][5] Aus der Bewegung interstellaren Gases und der Sternverteilung im Bulge ergibt sich fĂŒr diesen eine lĂ€ngliche Form. Dieser Balken bildet mit der Verbindungslinie des Sonnensystems zum Zentrum des Milchstraßensystems einen Winkel von 45°. Die Galaxis ist also vermutlich eine Balkenspiralgalaxie vom Hubble-Typ SBc. GemĂ€ĂŸ einer Bestimmung mithilfe des Infrarot-Weltraumteleskops Spitzer ist die Balkenstruktur mit einer Ausdehnung von 27.000 Lichtjahren ĂŒberraschend lang.

Basierend auf der bekannten Umlaufzeit der Sonne und ihrem Abstand vom galaktischen Zentrum kann nach dem dritten keplerschen Gesetz zumindest die Gesamtmasse berechnet werden, die sich innerhalb der Sonnenbahn befindet.[6] Die Gesamtmasse des Milchstraßensystems wird auf etwa 400 Milliarden Sonnenmassen geschĂ€tzt,[7][8] damit ist sie neben dem Andromedanebel (800 Milliarden Sonnenmassen) die massereichste Galaxie der Lokalen Gruppe.

Galaktischer Halo
Umgeben ist die Galaxis vom kugelförmigen galaktischen Halo mit einem Durchmesser von etwa 165.000 Lichtjahren (50 kpc), einer Art von galaktischer „AtmosphĂ€re“. In ihm befinden sich neben den etwa 150 bekannten Kugelsternhaufen nur weitere alte Sterne, darunter RR Lyrae-VerĂ€nderliche, und Gas sehr geringer Dichte. Ausnahme sind die heißen Blue-Straggler-Sterne. Dazu kommen große Mengen Dunkle Materie mit etwa 1 Billion Sonnenmassen, darunter auch so genannte MACHOs. Anders als die galaktische Scheibe ist der Halo weitgehend staubfrei und enthĂ€lt fast ausschließlich Sterne der Ă€lteren, metallarmen Population II, deren Orbit sehr stark gegen die galaktische Ebene geneigt ist. Das Alter des inneren Teils des Halo wurde in einer im Mai 2012 vorgestellten neuen Methode zur Altersbestimmung vom Space Telescope Science Institute in Baltimore mit 11,4 Milliarden Jahren (mit einer Unsicherheit von 0,7 Milliarden Jahren) angegeben. Dem Astronomen Jason Kalirai vom Space Telescope Science Institute gelang diese Altersbestimmung durch den Vergleich der Halo-Zwerge der Milchstraße mit den gut untersuchten Zwergen im Kugelsternhaufen Messier 4, die im Sternbild Skorpion liegen.[9]

Galaktische Scheibe
Der Großteil der Sterne innerhalb der Galaxis ist annĂ€hernd gleichmĂ€ĂŸig auf die galaktische Scheibe verteilt. Sie enthĂ€lt im Gegensatz zum Halo vor allem Sterne der Population I, welche sich durch einen hohen Anteil schwerer Elemente auszeichnen.

Spiralarme
Teil der Scheibe sind auch die fĂŒr das Milchstraßensystem charakteristischen Spiralarme. In den Spiralarmen befinden sich enorme Ansammlungen von Wasserstoff und auch die grĂ¶ĂŸten HII-Regionen, die Sternentstehungsgebiete der Galaxis. Daher befinden sich dort auch viele Protosterne, junge Sterne des T-Tauri-Typs und Herbig-Haro-Objekte. WĂ€hrend ihrer Lebenszeit bewegen sich Sterne von ihren GeburtsstĂ€tten weg und verteilen sich auf die Scheibe. Besonders massereiche und leuchtkrĂ€ftige Sterne entfernen sich allerdings aufgrund ihrer kĂŒrzeren Lebensdauer nicht so weit von den Spiralarmen, weswegen diese hervortreten. Daher gehören zu den dort befindlichen stellaren Objekten vor allem Sterne der Spektralklassen O und B, Überriesen und Cepheiden, alle jĂŒnger als 100 Millionen Jahre. Sie stellen jedoch nur etwa ein Prozent der Sterne im Milchstraßensystem. Der grĂ¶ĂŸte Teil der Masse der Galaxis besteht aus alten, massearmen Sternen. Der „Zwischenraum“ zwischen den Spiralarmen ist also nicht leer, sondern ist einfach nur weniger leuchtstark.
Die Spiralstruktur der Galaxis konnte durch die Beobachtung der Verteilung von neutralem Wasserstoff bestÀtigt werden. Die entdeckten Spiralarme wurden nach den in ihrer Richtung liegenden Sternbildern benannt.

Die Zeichnung rechts stellt den Aufbau des Milchstraßensystems schematisch dar. Das Zentrum ist im sichtbaren Licht nicht direkt beobachtbar, ebenso wie der hinter ihm liegende Bereich. Die Sonne (gelber Kreis) liegt zwischen den Spiralarmen Sagittarius (nach Sternbild SchĂŒtze) und Perseus im Orionarm. Vermutlich ist dieser Arm nicht vollstĂ€ndig, siehe braune Linie in der Abbildung. Im VerhĂ€ltnis zu dieser unmittelbaren Umgebung bewegt sich die Sonne mit etwa 30 km/s in Richtung des Sternbildes Herkules. Der innerste Arm ist der Norma-Arm (nach Sternbild Winkelmaß, auch 3-kpc-Arm), der Ă€ußerste (nicht in der Abbildung) ist der Cygnus-Arm (nach Sternbild Schwan), welcher vermutlich die Fortsetzung des Scutum-Crux-Arms (nach Sternbildern Schild und Kreuz des SĂŒdens) ist .

Wissenschaftler der UniversitĂ€t von Wisconsin veröffentlichten im Juni 2008 Auswertungen von Infrarotaufnahmen des Spitzer-Teleskopes, die das Milchstraßensystem nun als zweiarmige Galaxie darstellen. Sagittarius und Norma sind in dieser Darstellung nur noch als dĂŒnne Nebenarme erkenntlich, da diese nur durch eine ĂŒberschĂŒssige Verteilung von Gas gekennzeichnet sind wĂ€hrend die restlichen beiden Arme durch eine hohe Dichte alter rötlicher Sterne gekennzeichnet sind.[10] Eine jĂŒngere Untersuchung der Verteilung von Sternentstehungsgebieten und junger Sterne scheint hingegen die bekannte vierarmige Struktur der Milchstraße zu bestĂ€tigen.[11] Die Milchstraße besteht daher scheinbar aus vier Spiralarmen die sich primĂ€r durch Gaswolken und junge Sterne abzeichnen, wobei zwei Arme zusĂ€tzlich durch eine hohe Konzentration Ă€lterer Sterne charakterisiert sind. Neben diesen unterschiedlichen Auffassungen bezĂŒglich der Struktur der Galaxis sollte beachtet werden, dass ein klar definiertes logarithmisches Spiralmuster nur in seltenen FĂ€llen bei anderen Spiralgalaxien ĂŒber die Gesamtheit der Scheibe beobachtet werden kann und die vorhandenen Arme oft extreme Abzweigungen, VerĂ€stelungen und VerschrĂ€nkungen aufweisen.[12][13] Die wahrscheinliche Natur des lokalen Arms als solche UnregelmĂ€ĂŸigkeit ist ein Hinweis darauf, dass solche Strukturen in der Milchstraße hĂ€ufig auftreten könnten.[14]

Welche Prozesse fĂŒr die Entstehung der Spiralstruktur verantwortlich sind, ist bislang noch nicht eindeutig geklĂ€rt. Jedoch ist klar, dass die zu den Spiralarmen gehörigen Sterne keine starre Struktur sind, die sich in Formation um das galaktische Zentrum dreht. WĂ€re dies der Fall, wĂŒrde sich die Spiralstruktur des Milchstraßensystems und anderer Spiralgalaxien aufgrund der unterschiedlichen Bahngeschwindigkeiten innerhalb relativ kurzer Zeit aufwickeln und unkenntlich werden. Eine ErklĂ€rung bietet die Dichtewellentheorie, nach der die Spiralarme Zonen erhöhter Materiedichte und Sternentstehung sind, die sich unabhĂ€ngig von den Sternen durch die Scheibe bewegen. Die durch die Spiralarme verursachten Störungen in den Bahnen der Sterne können zu Lindblad-Resonanzen fĂŒhren.

Sterne der galaktischen Scheibe
Die zur Population I zĂ€hlenden Sterne der galaktischen Scheibe lassen sich mit zunehmender Streuung um die Hauptebene und Alter in drei Unterpopulationen einteilen. Die so genannte „Thin Disk“ in einem Bereich von 700 bis 800 Lichtjahren ĂŒber und unterhalb der galaktischen Ebene enthĂ€lt neben den oben genannten leuchtkrĂ€ftigen Sternen der Spiralarme, die sich nur maximal 500 Lichtjahre von der Ebene entfernen, Sterne der Spektralklassen A und F, einige Riesen der Klassen A, F, G und K, sowie Zwergsterne der Klassen G, K und M und auch einige Weiße Zwerge. Die MetallizitĂ€t dieser Sterne ist vergleichbar mit der der Sonne, meist aber auch doppelt so hoch, ihr Alter liegt bei etwa einer Milliarde Jahren.

Eine weitere Gruppe ist die der mittelalten Sterne (Alter bis zu fĂŒnf Milliarden Jahre). Dazu zĂ€hlen die Sonne und weitere Zwergsterne der Spektraltypen G, K und M, sowie einige Unter- und Rote Riesen. Der Metallgehalt ist hier deutlich geringer mit nur etwa 50 bis 100 Prozent dessen der Sonne. Auch ist die BahnexzentrizitĂ€t der galaktischen Orbits dieser Sterne höher und sie befinden sich nicht weiter als 1500 Lichtjahre ĂŒber oder unterhalb der galaktischen Ebene.

Zwischen maximal 2500 Lichtjahren ober- und unterhalb der Hauptebene erstreckt sich die „Thick Disk“. Dort befinden sich rote K- und M-Zwerge, Weiße Zwerge, sowie einige Unterriesen und Rote Riesen, aber auch langperiodische VerĂ€nderliche. Ihr Alter erreicht bis zu zehn Milliarden Jahre und sie sind vergleichsweise metallarm (etwa ein Viertel der SonnenmetallizitĂ€t). Diese Population Ă€hnelt auch vielen Sternen im Bulge.

Die galaktische Scheibe ist nicht vollkommen gerade, durch gravitative Wechselwirkung mit den Magellanschen Wolken ist sie leicht in deren Richtung gebogen.

Das Zentrum des Milchstraßensystems liegt im Sternbild SchĂŒtze und ist hinter dunklen Staub- und Gaswolken verborgen, so dass es im sichtbaren Licht nicht direkt beobachtet werden kann. Beginnend in den 1950er Jahren ist es gelungen, im Radiowellenbereich sowie mit Infrarotstrahlung und Röntgenstrahlung zunehmend detailreichere Bilder aus der nahen Umgebung des galaktischen Zentrums zu gewinnen. Man hat dort eine starke Radioquelle entdeckt, bezeichnet als Sagittarius A* (Sgr A*), die aus einem sehr kleinen Gebiet strahlt. Diese Massenkonzentration wird von einer Gruppe von Sternen in einem Radius von weniger als einem halben Lichtjahr mit einer Umlaufzeit von etwa 100 Jahren sowie einem Schwarzen Loch mit 1300 Sonnenmassen in drei Lichtjahren Entfernung umkreist. Der dem zentralen Schwarzen Loch am nĂ€chsten liegende Stern S2 umlĂ€uft das galaktische Zentrum in einer Entfernung von etwa 17 Lichtstunden in einem Zeitraum von nur 15,2 Jahren. Seine Bahn konnte inzwischen ĂŒber einen vollen Umlauf hinweg beobachtet werden. Aus den Beobachtungen der Bewegungen der Sterne des zentralen Sternhaufens ergibt sich, dass sich innerhalb dieser Region von 15,4 Millionen km Durchmesser eine Masse von geschĂ€tzten 4,31 Millionen Sonnenmassen befinden muss.[15] Die im Rahmen der RelativitĂ€tstheorie plausibelste und einzige mit allen Beobachtungen konsistente ErklĂ€rung fĂŒr diese große Massenkonzentration ist die Anwesenheit eines Schwarzen Lochs.

Am 9. November 2010 machte Doug Finkbeiner vom Harvard-Smithsonian Center for Astrophysics bekannt, dass er zwei riesenhafte kugelförmige Blasen entdeckt habe, die aus der Mitte der Milchstraße nach Norden und SĂŒden hinausgreifen. Die Entdeckung ist mit der Hilfe von Daten des Fermi Gamma-ray Space Telescope gelungen. Der Durchmesser der Blasen betrĂ€gt jeweils etwa 25.000 Lichtjahre; sie erstrecken sich am sĂŒdlichen Nachthimmel von der Jungfrau bis zum Kranich. Ihr Ursprung ist bisher noch nicht geklĂ€rt.[16][17]

GrĂ¶ĂŸenvergleich
Man bekommt eine anschauliche Vorstellung von der GrĂ¶ĂŸe unserer Galaxis mit ihren 100 bis 300 Milliarden Sternen, wenn man sie sich im Maßstab 1:1017 verkleinert als Schneetreiben auf einem Gebiet von 10 km Durchmesser und einer Höhe von etwa 1 km im Mittel vorstellt. Jede Schneeflocke entspricht dabei einem Stern und es gibt etwa drei StĂŒck pro Kubikmeter. Unsere Sonne hĂ€tte in diesem Maßstab einen Durchmesser von etwa 10 nm, wĂ€re also kleiner als ein Virus. Selbst die Plutobahn, die sich im Mittel etwa 40-mal so weit von der Sonne befindet wie die Bahn der Erde, lĂ€ge mit einem Durchmesser von 0,1 mm an der Grenze der visuellen Sichtbarkeit. Pluto selbst hĂ€tte ebenso wie die Erde lediglich atomare Dimension. Damit demonstriert dieses Modell auch die geringe durchschnittliche Massendichte unserer Galaxis.
Die Sonne umkreist das Zentrum des Milchstraßensystems in einem Abstand von 25.000 bis 28.000 Lichtjahren (≈ 250 Em oder 7,94 ± 0,42 kpc)[18] und befindet sich nördlich der Mittelebene der galaktischen Scheibe innerhalb des Orion-Arms, in einem weitgehend staubfreien Raumgebiet, das als „Lokale Blase“ bekannt ist. FĂŒr einen Umlauf um das Zentrum der Galaxis, ein so genanntes galaktisches Jahr, benötigt sie 220 bis 240 Millionen Jahre, was einer Rotationsgeschwindigkeit von etwa 220 km/s entspricht. Die Erforschung dieser Rotation ist mittels der Eigenbewegung und der Radialgeschwindigkeit vieler Sterne möglich; aus ihnen wurden um 1930 die Oortschen Rotationsformeln abgeleitet. Heutzutage kann auch die durch die Umlaufbewegung des Sonnensystems bedingte scheinbare Bewegung des Milchstraßenzentrums gegenĂŒber Hintergrundquellen direkt beobachtet werden, so dass die Umlaufgeschwindigkeit des Sonnensystems unmittelbar messbar ist.[19] Neuere Messungen haben eine Umlaufgeschwindigkeit von ca. 267 km/s (961.200 km/h) ergeben.[20]

Das Sonnensystem umlĂ€uft das galaktische Zentrum nicht auf einer ungestörten ebenen Keplerbahn. Die in der Scheibe des Milchstraßensystems verteilte Masse ĂŒbt eine starke Störung aus, so dass die Sonne zusĂ€tzlich zu ihrer Umlaufbahn um das Zentrum auch regelmĂ€ĂŸig durch die Scheibe auf und ab oszilliert. Die Scheibe durchquert sie dabei etwa alle 30 bis 45 Millionen Jahre einmal.[21] Vor ca. 1,5 Millionen Jahren hat sie die Scheibe in nördlicher Richtung passiert und befindet sich jetzt etwa 65 Lichtjahre (ca. 20 pc)[22] ĂŒber ihr. Die grĂ¶ĂŸte Entfernung wird etwa 250 Lichtjahre (80 pc) betragen, dann kehrt sich die Bewegung wieder um.[21]

GrĂ¶ĂŸere datierbare Krater auf der Erde sowie erdgeschichtliche Massenaussterben scheinen eine PeriodizitĂ€t von 34 bis 37 Millionen Jahren aufzuweisen, was auffĂ€llig mit der PeriodizitĂ€t der Scheibenpassagen ĂŒbereinstimmt. Möglicherweise stören wĂ€hrend einer Scheibendurchquerung die in ScheibennĂ€he stĂ€rker werdenden Gravitationsfelder die Oortsche Wolke des Sonnensystems, so dass eine grĂ¶ĂŸere Anzahl von Kometen ins innere Sonnensystem gelangt und die Anzahl schwerer Impakte auf der Erde zunimmt. Die betreffenden Perioden sind jedoch bisher nicht genau genug bekannt, um definitiv einen Zusammenhang festzustellen;[21] neuere Ergebnisse (Scheibendurchgang alle 42 ± 2 Millionen Jahre) sprechen eher dagegen.[23] Eine neue Studie des Max-Planck Instituts fĂŒr Astronomie hat gezeigt, dass es sich bei der scheinbaren PeriodizitĂ€t der EinschlĂ€ge um statistische Artefakte handelt und es keinen solchen Zusammenhang gibt.

Um das Milchstraßensystem herum sind einige Zwerggalaxien versammelt. Die bekanntesten davon sind die Große und die Kleine Magellansche Wolke, mit denen das Milchstraßensystem ĂŒber eine etwa 300.000 Lichtjahre lange WasserstoffgasbrĂŒcke, dem Magellanschen Strom, verbunden ist.

Die dem Milchstraßensystem am nĂ€chsten gelegene Galaxie ist der Canis-Major-Zwerg, mit einer Entfernung von 42.000 Lichtjahren vom Zentrum des Milchstraßensystems und 25.000 Lichtjahren von unserem Sonnensystem. Die Zwerggalaxie wird zurzeit von den GezeitenkrĂ€ften des Milchstraßensystems auseinandergerissen und hinterlĂ€sst dabei ein Filament aus Sternen, das sich um die Galaxis windet, den so genannten Monoceros-Ring. Ob es sich dabei allerdings tatsĂ€chlich um die Überreste einer Zwerggalaxie oder um eine zufĂ€llige, projektionsbedingte HĂ€ufung handelt, ist derzeit noch nicht sicher. Andernfalls wĂ€re die 50.000 Lichtjahre vom galaktischen Zentrum entfernte Sagittarius-Zwerggalaxie die nĂ€chste Galaxie, die ebenfalls gerade durch das Milchstraßensystem einverleibt wird.

Das Milchstraßensystem verleibt sich bestĂ€ndig Zwerggalaxien ein und nimmt dadurch an Masse zu. WĂ€hrend der Verschmelzung hinterlassen die Zwergsysteme Ströme aus Sternen und interstellarer Materie, die durch die GezeitenkrĂ€fte des Milchstraßensystems aus den kleinen Galaxien herausgerissen werden (siehe auch: Wechselwirkende Galaxien). Dadurch entstehen Strukturen wie der Magellansche Strom, der Monoceros-Ring und der Virgo-Strom, sowie die anderen Hochgeschwindigkeitswolken in der Umgebung unserer Galaxis.

Lokale Gruppe
Mit der Andromeda-Galaxie, dem Dreiecksnebel (M 33) und einigen anderen kleineren Galaxien bildet das Milchstraßensystem die Lokale Gruppe, wobei das Milchstraßensystem die massereichste Galaxie darunter ist, obwohl es nicht die grĂ¶ĂŸte Ausdehnung besitzt. Die Lokale Gruppe ist Bestandteil des Virgo-Superhaufens, der nach dem Virgohaufen in seinem Zentrum benannt ist. Auf diesen bewegt sich die Lokale Gruppe zu. Der lokale Superhaufen strebt mit anderen Großstrukturen dem Shapley-Superhaufen entgegen (die frĂŒhere Annahme, Ziel dieses Strebens sei der Große Attraktor, ist ĂŒberholt).[25]

Die Andromeda-Galaxie ist eine der wenigen Galaxien im Universum, deren Spektrum eine Blauverschiebung aufweist: Die Andromeda-Galaxie und das Milchstraßensystem bewegen sich mit einer Geschwindigkeit von 120 km/s aufeinander zu. Allerdings gibt die Blauverschiebung nur Aufschluss ĂŒber die Geschwindigkeitskomponente parallel zur Verbindungslinie beider Systeme, wĂ€hrend die Komponente senkrecht zu dieser Linie unbekannt ist. Vermutlich werden die beiden Galaxien in etwa drei Milliarden Jahren zusammenstoßen und zu einer grĂ¶ĂŸeren Galaxie verschmelzen. FĂŒr den Ablauf der Kollision können mangels Kenntnis der Raumgeschwindigkeiten und wegen der KomplexitĂ€t der beim Zusammenstoß ablaufenden Prozesse nur Wahrscheinlichkeitsaussagen gemacht werden.[26] Nach der Verschmelzung der beiden Galaxien wird das Endprodukt voraussichtlich eine massereiche elliptische Galaxie sein. Als Name fĂŒr diese Galaxie wird von Cox-Loeb 2008 in ihrem Artikel der Arbeitsname „Milkomeda“ benutzt, eine Verschmelzung des englischen Milky Way und Andromeda.[26]

Alter
Messungen aus dem Jahr 2004 zufolge ist das Milchstraßensystem etwa 13,6 Milliarden Jahre alt. Die Genauigkeit dieser AbschĂ€tzung, die das Alter anhand des Berylliumanteils einiger Kugelsternhaufen bestimmt, wird mit etwa 800 Millionen Jahren angegeben. Da das Alter des Universums von 13,8 Milliarden Jahren als recht verlĂ€sslich bestimmt gilt, hieße das, dass die Entstehung der Milchstraße auf die FrĂŒhzeit des Universums datiert.

2007 wurde zunĂ€chst fĂŒr den Stern HE 1523-0901 im galaktischen Halo von der ESO-Sternwarte in Hamburg ein Alter von 13,2 Milliarden Jahren festgestellt[27]. 2014 wurde dann fĂŒr den Stern SM0313, 6000 Lj von der Erde entfernt, von der Australian National University ein Alter von 13,6 Milliarden Jahren dokumentiert. Als Ă€lteste bekannte Objekte der Milchstraße setzen diese Datierungen eine unterste Grenze, die im Bereich der Messgenauigkeit der AbschĂ€tzung von 2004 liegt.

Nach derselben Methode kann das Alter der dĂŒnnen galaktischen Scheibe durch die Ă€ltesten dort gemessenen Objekte abgeschĂ€tzt werden, wodurch sich ein Alter von etwa 8,8 Milliarden Jahren mit einer SchĂ€tzbreite von etwa 1,7 Milliarden Jahren ergibt. Auf dieser Basis ergĂ€be sich eine zeitliche LĂŒcke von etwa drei bis sieben Milliarden Jahren zwischen der Bildung des galaktischen Zentrums und der Ă€ußeren Scheibe.

The Milky Way is the galaxy that contains our Solar System.[15][16][17][nb 1] Its name “milky” is derived from its appearance as a dim glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. The term “Milky Way” is a translation of the Latin via lactea, from the Greek ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚ ÎșύÎșÎ»ÎżÏ‚ (galaxĂ­as kĂœklos, "milky circle").[18][19][20] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Up until the early 1920s, most astronomers thought that all of the stars in the universe were contained inside of the Milky Way. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[21] observations by Edwin Hubble definitively showed that the Milky Way is just one of many billions of galaxies.[22]

The Milky Way is a barred spiral galaxy some 100,000–120,000 light-years in diameter, which contains 100–400 billion stars. It may contain at least as many planets as well.[23][24] The Solar System is located within the disk, about 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole.

Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name “dark matter”.[25] The rotational period is about 240 million years at the position of the Sun.[11] The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest known star in the Milky Way is at least 13.82 [26] billion years old and thus must have formed shortly after the Big Bang.[7]

Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster, which again forms a subcomponent of the Laniakea supercluster.
When observing the night sky, the term “Milky Way” is limited to the hazy band of white light some 30 degrees wide arcing across the sky.[29] Although all of the individual stars that can be seen in the entire sky with the naked eye are part of the Milky Way Galaxy,[30] the light in this band originates from the accumulation of un-resolved stars and other material when viewed in the direction of the Galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by interstellar dust.

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. It is readily visible when the limiting magnitude is +5.1 or better and shows a great deal of detail at +6.1.[31] This makes the Milky Way difficult to see from any brightly lit urban or suburban location, but very prominent when viewed from a rural area when the Moon is below the horizon.[nb 2]

As viewed from Earth, the visible region of the Milky Way’s Galactic plane occupies an area of the sky that includes 30 constellations. The center of the Milky Way lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass westward to the Galactic anticenter in Auriga. The band then continues westward the rest of the way around the sky, back to Sagittarius. The band divides the night sky into two roughly equal hemispheres.

The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of Earth’s orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth’s equatorial plane and the plane of the ecliptic, relative to the Galactic plane. The north Galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near ÎČ Comae Berenices, and the south Galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the arc of Milky Way may appear relatively low or relatively high in the sky. For observers from approximately 65 degrees north to 65 degrees south on Earth’s surface, the Milky Way passes directly overhead twice a day.
The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (30 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick.[2][3] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 mm in width, about the size of a grain of sand. The nearest star, Proxima Centauri, would be 4.2 mm distant.[nb 3] Alternatively visualized, if the Solar System out to Neptune were the size of a US quarter (25mm), the Milky Way would have a diameter of 4,000 kilometers, or approximately the breadth of the United States.

Estimates for the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8×1011 solar masses (M☉), somewhat smaller than the Andromeda Galaxy.[33][34][35] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s for stars at the outer edge of the Milky Way.[36] As the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M☉ within 160,000 ly (49 kpc) of its center.[37] A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M☉.[38] According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5×1011 M☉,[39] which is about half the mass of the Andromeda Galaxy.[39]

Most of the mass of the Milky Way appears to be matter of unknown form that interacts with other matter through gravitational but not electromagnetic forces, which is dubbed dark matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggest that the total mass of the entire Galaxy lies in the range 1–1.5×1012 M☉.[8] More recent studies indicate a mass as large as 4.5×1012 M☉ [40] and as small as 0.8×1012 M☉.[41] The Milky Way contains at least 100 billion planets[42] and between 200 and 400 billion stars.[43][44] The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[45] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[46] whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[47][48] Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way[23][49] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[50][51] The Milky Way Galaxy contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory.[24] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way Galaxy.[52] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way Galaxy.[53][54][55] 11 billion of these estimated planets may be orbiting sun-like stars.[56] The nearest such planet may be 12 light-years away, according to the scientists.[53][54] Such Earth-sized planets may be more numerous than gas giants.[23] Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way Galaxy.[52]

The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.[57] Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic Center is about 180,000 ly (55 kpc).[58] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be −20.9.

The Milky Way consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Milky Way closely resembles the type SBc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms.[1] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[61] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[62] that showed the Milky Way’s central bar to be larger than previously suspected.

Galactic quadrants
Main article: Galactic quadrant
A galactic quadrant, or quadrant of the galaxy, refers to one of four circular sectors in the division of the Milky Way. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the pole of the mapping system.

Quadrants are described using ordinals—for example, "1st galactic quadrant",[63] "second galactic quadrant",[64] or "third quadrant of the Galaxy".[65] Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are as follow:

1st galactic quadrant – 0° ≀ longitude (ℓ) ≀ 90°[66] 2nd galactic quadrant – 90° ≀ ℓ ≀ 180°[64] 3rd galactic quadrant – 180° ≀ ℓ ≀ 270°[65] 4th galactic quadrant – 270° ≀ ℓ ≀ 360° (0°)[63] The Sun is 26,000–28,000 ly (8.0–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.[10][67][68][69][70] In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[71] It has been proposed that the Milky Way lacks a bulge formed due to a collision and merger between previous galaxies and that instead has a pseudobulge formed by its central bar.[72]

The Galactic Center is marked by an intense radio source named Sagittarius A*. The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.[73] This concentration of mass is best explained as a supermassive black hole[nb 4][10][67] with an estimated mass of 4.1–4.5 million times the mass of the Sun.[67] Observations indicate that there are supermassive black holes located near the center of most normal galaxies.[74][75]

The nature of the Milky Way’s bar is actively debated, with estimates for its half-length and orientation spanning from 1–5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[69][70][76] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[77] In most galaxies, Wang et al. report, the rate of accretion of the supermassive black hole is slow, but the Milky Way seems to be an important exception. X-ray emission is aligned with the massive stars surrounding the central bar.[78] However, RR Lyr variables do not trace a prominent Galactic bar.[70][79][80] The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way’s star-formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[81]

In 2010, two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere.[82][83] Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Milky Way.[84]

Spiral arms
Outside the gravitational influence of the Galactic bars, astronomers generally organize the structure of the interstellar medium and stars in the disk of the Milky Way into four spiral arms.[85] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions[86][87] and molecular clouds.[88]

Maps of the Milky Way’s spiral structure are notoriously uncertain and exhibit striking differences.[60][85][87][89][90][91][92][93] Some 150 years after Alexander (1852)[94] first suggested that the Milky Way was a spiral, there is currently no consensus on the nature of the Milky Way’s spiral arms. Perfect logarithmic spiral patterns only crudely describe features near the Sun,[87][92] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[70][92][93] The possible scenario of the Sun within a spur / Local arm[87] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.[92]

As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from about 7° to 25°.[95][96] There are thought to be four spiral arms that all start near the Milky Way’s center. These are named as follows, with the positions of the arms shown in the image at right:
Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun’s orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm.[95][98] In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars.[60] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.[99][100][101] Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.[101]

The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen).[102][103] It was found to be expanding away from the center of the Milky Way at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the Galactic Center. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard-Smithsonian CfA). It’s located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center.[103][104]

A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.[105]

It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring[106] and the two patterns would be connected by the Cygnus arm.[107]

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.[108]

Halo
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[109] However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of the Milky Way’s clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[110] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.[111]

Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars.[11] Open clusters are also located primarily in the disk.[112]

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way’s structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[113] the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm[97][114] and of a similar extension of the Scutum-Centaurus Arm.[115] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[116]

Gaseous halo
In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself.[117][118][119] The temperature of this halo gas is between 1 million and 2.5 million kelvin, a few hundred times hotter than the surface of the sun.[120]

Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[121] If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.

The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center.[10][67][122] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.[123] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly).[124] The Sun, and thus the Solar System, is found in the Galactic habitable zone.

There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsec, or one star per 2,360 cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsec, or one per 284 cubic light-year (from List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14.[125]

The apex of the Sun’s way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun’s Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun’s orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth.[126] However, a reanalysis of the effects of the Sun’s transit through the spiral structure based on CO data has failed to find a correlation.[127]

It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a Galactic year),[11] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).

The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s.[131] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.

If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[25] The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the strongest proof of the existence of dark matter in galaxies. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.
The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[133][134]

Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Milky Way’s growth) and accretion of gas directly from the Galactic halo.[134] The Milky Way is currently accreting material from two of its nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high-velocity clouds like the Smith Cloud.[135][136] However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[137][138]

According to recent studies, the Milky Way as well as Andromeda lie in what in the galaxy color–magnitude diagram is known as the green valley, a region populated by galaxies in transition from the blue cloud (galaxies actively forming new stars) to the red sequence (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy.[139] In fact, measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.[140]

Age[edit] The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 12.5 ± 3 billion years for CS 31082-001[141] and 13.8 ± 4 billion years for BD +17° 3248.[142] Once a white dwarf is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Globular clusters are among the oldest objects in the Milky Way Galaxy, which thus set a lower limit on the age of the Milky Way. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[143]

In 2007, a star in the galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, ≈0.5 billion years less than the age of the universe. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.[144] This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[144]

The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk.
The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster. The Virgo Supercluster forms part of a greater structure, called Laniakea.[146]

Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 14,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a stream of neutral hydrogen gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.[147] Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri.

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.[148]

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. In 3 to 4 billion years, there may be an Andromeda–Milky Way collision, depending on the importance of unknown lateral components to the galaxies’ relative motion. If they collide, the chance of individual stars colliding with each other is extremely low, but instead the two galaxies will merge to form a single elliptical galaxy or perhaps a large disk galaxy[149] over the course of about a billion years.[150]

Velocity
Although special relativity states that there is no "preferred" inertial frame of reference in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological frames of reference.

One such frame of reference is the Hubble flow, the apparent motions of galaxy clusters due to the expansion of space. Individual galaxies, including the Milky Way, have peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km per second with respect to this local co-moving frame of reference.[151] The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.[152] The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[153]

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km/s[13] with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[13]

Etymology and mythology
Main articles: List of names for the Milky Way and Milky Way (mythology)
In western culture the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚, short for ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚ ÎșύÎșÎ»ÎżÏ‚ (pr. galaktikos kyklos, "milky circle"). The Ancient Greek ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚ (galaxias), from root γαλαÎșτ-, γΏλα (milk) + -ÎŻÎ±Ï‚ (forming adjectives), is also the root of "galaxy", the name for our, and later all such, collections of stars.[18][154][155][156] The Milky Way "milk circle" was just one of 11 circles the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.

In Meteorologica (DK 59 A80), Aristotle (384–322 BC) wrote that the Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."[158] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.[159]

The Persian astronomer AbĆ« Rayhān al-BÄ«rĆ«nÄ« (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars".[160] The Andalusian astronomer Avempace (d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth’s atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.[158] Ibn Qayyim Al-Jawziyya (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.[161]

According to Jamil Ragep, the Persian astronomer NaáčŁÄ«r al-DÄ«n al-áčŹĆ«sÄ« (1201–1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."[162]

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars.[163][164] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[165] speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.[166] The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[167][168][169]

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center.[170]

In 1845, Lord Rosse construct

Posted by !!! Painting with Light !!! #schauer on 2014-10-19 14:40:33

Tagged: , Schauer , Christian , Oberdiendorf , Passau , Hauzenberg , Painting , with , Licht , Bayern , Bavaria , Germany , Europe , Canon , Tamron , Nikon , Lightroom , Photoshop , Milkyway , Milchstraße , Street , Road , Highway , Nature , Landscape , Building , Nacht , Night , Nuit , Noir , Dark , Sky , Heaven , Cloud , Star , ISO , Long , Exposure , Life , Live , Crane , Kran , Vehicle , Milf , Car , Blue , Midnight , Bau , Cold , Airplane , Plane , Flugzeug , Trail , Baum , Tree , Moon , No , Non , High , Highlight , Low , Upper , Welt , World

Milkyway Oberdiendorf

Milkyway Oberdiendorf

Quelle:
de.wikipedia.org/wiki/Milchstra%C3%9Fe

en.wikipedia.org/wiki/Milky_Way

Die Milchstraße, auch Galaxis, ist die Galaxie, in der sich unser Sonnensystem mit der Erde befindet. Entsprechend ihrer Form als flache Scheibe, die aus Milliarden von Sternen besteht, ist die Milchstraße von der Erde aus als bandförmige Aufhellung am Nachthimmel sichtbar, die sich ĂŒber 360° erstreckt. Ihrer Struktur nach zĂ€hlt die Milchstraße zu den Balkenspiralgalaxien.
Den Namen Milchstraßensystem trĂ€gt unser Sternsystem nach der Milchstraße, die als freiĂ€ugige Innenansicht des Systems von der Erde aus wie ein quer ĂŒber das Firmament gesetzter milchiger Pinselstrich erscheint. Dass dieses weißliche Band sich in Wirklichkeit aus unzĂ€hligen einzelnen Sternen zusammensetzt, wurde erst 1609 von Galileo Galilei erkannt, der die Erscheinung als Erster durch ein Fernrohr betrachtete. Es sind nach heutiger SchĂ€tzung ca. 100 bis 300 Milliarden Sterne.

Schon im Altertum war die Milchstraße als heller, schmaler Streifen am Nachthimmel bekannt. Ihr altgriechischer Name galaxias (ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚) – von dem auch der heutige Fachausdruck „Galaxis“ stammt – ist von dem Wort gala (γΏλα, Milch) abgeleitet.[1] Wie dem deutschen Wort „Milchstraße“ liegt also auch dem altgriechischen Begriff das „milchige“ Aussehen zugrunde.

Eine antike griechische Sage versucht, diesen Begriff mythologisch zu erklĂ€ren: Danach habe Zeus seinen Sohn Herakles, den ihm die sterbliche Frau Alkmene geschenkt hatte, an der Brust seiner göttlichen Frau Hera trinken lassen, als diese schlief. Herakles sollte auf diese Weise göttliche KrĂ€fte erhalten. Aber er saugte so ungestĂŒm, dass Hera erwachte und den ihr fremden SĂ€ugling zurĂŒckstieß; dabei wurde ein Strahl ihrer Milch ĂŒber den ganzen Himmel verspritzt.

Einer germanischen Sage zufolge erhielt die Milchstraße nach dem Gott des Lichtes, Heimdall, auch Iring genannt, den Namen Iringsstraße (laut Felix Dahn, Walhall – germanische Götter- und Heldensagen). Die afrikanischen San gaben der Milchstraße den Namen „RĂŒckgrat der Nacht“.

Zur ersten Vorstellung der Scheibenform des Milchstraßensystems gelangte bereits Wilhelm Herschel im Jahr 1785 aufgrund systematischer SternzĂ€hlungen (Stellarstatistik). Diese Methode konnte aber nicht zu einem realistischen Bild fĂŒhren, da das Licht weiter entfernter Sterne stark durch interstellare Staubwolken abgeschwĂ€cht wird, ein Effekt, dessen wahre Bedeutung erst in der ersten HĂ€lfte des 20. Jahrhunderts vollstĂ€ndig erfasst wurde. Durch Untersuchungen zur Verteilung der Kugelsternhaufen im Raum gelangte Harlow Shapley 1919 zu realistischen AbschĂ€tzungen der GrĂ¶ĂŸe des Milchstraßensystems und zu der Erkenntnis, dass die Sonne nicht – wie bis dahin, z. B. von Jacobus Kapteyn, angenommen – im Zentrum der Galaxis sitzt, sondern eher an deren Rand. Edwin Hubbles Messungen der Entfernungen von Spiralnebeln zeigten, dass diese außerhalb des Milchstraßensystems liegen und tatsĂ€chlich wie dieses eigenstĂ€ndige Galaxien sind.
Das Band der Milchstraße erstreckt sich als unregelmĂ€ĂŸig breiter, schwach milchig-heller Streifen ĂŒber dem Firmament.[2] Seine Erscheinung rĂŒhrt daher, dass in ihm mit bloßem Auge keine Einzelsterne wahrgenommen werden, sondern eine Vielzahl lichtschwacher Sterne der galaktischen Scheibe und des Bulges (in Richtung des galaktischen Zentrums). Von der SĂŒdhalbkugel aus steht das helle Zentrum der Milchstraße hoch am Himmel, wĂ€hrend man von der Nordhalbkugel zum Rand hin blickt. Daher kann man das Band der Milchstraße am besten von der SĂŒdhalbkugel aus beobachten. Im Dezember und Januar kann der hellste Bereich der Milchstraße nicht oder nur sehr schlecht beobachtet werden, weil sich die Sonne zwischen dem Zentrum der Galaxis und der Erde befindet. Gute Beobachtungsbedingungen sind bei klarer Luft und bei nur geringer Lichtverschmutzung durch kĂŒnstliche Lichtquellen gegeben. Alle der maximal 6000 mit bloßem Auge sichtbaren Sterne des Nachthimmels gehören zum Milchstraßensystem.

Das Milchstraßenband verlĂ€uft unter anderem durch die Sternbilder SchĂŒtze (in dieser Richtung liegt auch das galaktische Zentrum), Adler, Schwan, Kassiopeia, Perseus, Fuhrmann, Zwillinge, Orion, Kiel des Schiffs, Zentaur, Kreuz des SĂŒdens und Skorpion. Die mittlere Ebene des Milchstraßensystems ist gegenĂŒber dem HimmelsĂ€quator um einen Winkel von etwa 63° gekippt.

Astronomen verwenden gelegentlich ein spezielles, an die Geometrie des Milchstraßensystems angepasstes galaktisches Koordinatensystem, bestehend aus LĂ€nge l und Breite b. Die galaktische Breite betrĂ€gt 0° in der Ebene des Milchstraßensystems, +90° am galaktischen Nordpol und −90° am galaktischen SĂŒdpol. Die galaktische LĂ€nge, die ebenfalls in Grad angegeben wird, hat ihren Ursprung (l = 0°) in Richtung des galaktischen Zentrums und nimmt nach Osten hin zu.

Die Erforschung der Struktur des Milchstraßensystems ist schwieriger als die der Strukturen anderer Galaxien, da Beobachtungen nur von einem Punkt innerhalb der Scheibe gemacht werden können. Wegen der erwĂ€hnten Absorption sichtbaren Lichts durch interstellaren Staub ist es nicht möglich, durch visuelle Beobachtungen ein vollstĂ€ndiges Bild des Milchstraßensystems zu erhalten. Große Fortschritte wurden erst gemacht, als Beobachtungen in anderen WellenlĂ€ngenbereichen, insbesondere im Radiofrequenzbereich und im Infraroten möglich wurden. Dennoch sind viele Details des Aufbaus der Galaxis noch nicht bekannt.

Das Milchstraßensystem wurde frĂŒher als vier- oder fĂŒnfarmig betrachtet, nun gilt es als zweiarmige Balkenspiralgalaxie.[3] Es besteht aus etwa 100 bis 300 Milliarden Sternen und großen Mengen interstellarer Materie, die nochmals 600 Millionen bis einige Milliarden Sonnenmassen ausmacht (die Anzahl der Sterne und damit auch die Gesamtmasse unserer Galaxis kann auf Basis von Berechnungen und Beobachtungen nur geschĂ€tzt werden, woraus sich der große Toleranzbereich der Zahlen ergibt). Die Masse dieses inneren Bereichs der Galaxis wird mit ungefĂ€hr 180 Milliarden Sonnenmassen veranschlagt. Ihre Ausdehnung in der galaktischen Ebene betrĂ€gt etwa 100.000 Lichtjahre (30 kpc), die Dicke der Scheibe etwa 3000 Lichtjahre (920 pc) und die der zentralen Ausbauchung (engl. Bulge) etwa 16.000 Lichtjahre (5 kpc). Zum Vergleich: Der Andromedanebel hat eine Ausdehnung von etwa 150.000 Lj. und das drittgrĂ¶ĂŸte Mitglied der lokalen Gruppe, der Dreiecksnebel M 33, ca. 50.000 Lj. Die Angaben der Dicke mĂŒssen aber eventuell noch bis zum Doppelten nach oben korrigiert werden, wie der australische Wissenschaftler Bryan Gaensler und sein Team im Januar 2008 Ă€ußerten.[4][5] Aus der Bewegung interstellaren Gases und der Sternverteilung im Bulge ergibt sich fĂŒr diesen eine lĂ€ngliche Form. Dieser Balken bildet mit der Verbindungslinie des Sonnensystems zum Zentrum des Milchstraßensystems einen Winkel von 45°. Die Galaxis ist also vermutlich eine Balkenspiralgalaxie vom Hubble-Typ SBc. GemĂ€ĂŸ einer Bestimmung mithilfe des Infrarot-Weltraumteleskops Spitzer ist die Balkenstruktur mit einer Ausdehnung von 27.000 Lichtjahren ĂŒberraschend lang.

Basierend auf der bekannten Umlaufzeit der Sonne und ihrem Abstand vom galaktischen Zentrum kann nach dem dritten keplerschen Gesetz zumindest die Gesamtmasse berechnet werden, die sich innerhalb der Sonnenbahn befindet.[6] Die Gesamtmasse des Milchstraßensystems wird auf etwa 400 Milliarden Sonnenmassen geschĂ€tzt,[7][8] damit ist sie neben dem Andromedanebel (800 Milliarden Sonnenmassen) die massereichste Galaxie der Lokalen Gruppe.

Galaktischer Halo
Umgeben ist die Galaxis vom kugelförmigen galaktischen Halo mit einem Durchmesser von etwa 165.000 Lichtjahren (50 kpc), einer Art von galaktischer „AtmosphĂ€re“. In ihm befinden sich neben den etwa 150 bekannten Kugelsternhaufen nur weitere alte Sterne, darunter RR Lyrae-VerĂ€nderliche, und Gas sehr geringer Dichte. Ausnahme sind die heißen Blue-Straggler-Sterne. Dazu kommen große Mengen Dunkle Materie mit etwa 1 Billion Sonnenmassen, darunter auch so genannte MACHOs. Anders als die galaktische Scheibe ist der Halo weitgehend staubfrei und enthĂ€lt fast ausschließlich Sterne der Ă€lteren, metallarmen Population II, deren Orbit sehr stark gegen die galaktische Ebene geneigt ist. Das Alter des inneren Teils des Halo wurde in einer im Mai 2012 vorgestellten neuen Methode zur Altersbestimmung vom Space Telescope Science Institute in Baltimore mit 11,4 Milliarden Jahren (mit einer Unsicherheit von 0,7 Milliarden Jahren) angegeben. Dem Astronomen Jason Kalirai vom Space Telescope Science Institute gelang diese Altersbestimmung durch den Vergleich der Halo-Zwerge der Milchstraße mit den gut untersuchten Zwergen im Kugelsternhaufen Messier 4, die im Sternbild Skorpion liegen.[9]

Galaktische Scheibe
Der Großteil der Sterne innerhalb der Galaxis ist annĂ€hernd gleichmĂ€ĂŸig auf die galaktische Scheibe verteilt. Sie enthĂ€lt im Gegensatz zum Halo vor allem Sterne der Population I, welche sich durch einen hohen Anteil schwerer Elemente auszeichnen.

Spiralarme
Teil der Scheibe sind auch die fĂŒr das Milchstraßensystem charakteristischen Spiralarme. In den Spiralarmen befinden sich enorme Ansammlungen von Wasserstoff und auch die grĂ¶ĂŸten HII-Regionen, die Sternentstehungsgebiete der Galaxis. Daher befinden sich dort auch viele Protosterne, junge Sterne des T-Tauri-Typs und Herbig-Haro-Objekte. WĂ€hrend ihrer Lebenszeit bewegen sich Sterne von ihren GeburtsstĂ€tten weg und verteilen sich auf die Scheibe. Besonders massereiche und leuchtkrĂ€ftige Sterne entfernen sich allerdings aufgrund ihrer kĂŒrzeren Lebensdauer nicht so weit von den Spiralarmen, weswegen diese hervortreten. Daher gehören zu den dort befindlichen stellaren Objekten vor allem Sterne der Spektralklassen O und B, Überriesen und Cepheiden, alle jĂŒnger als 100 Millionen Jahre. Sie stellen jedoch nur etwa ein Prozent der Sterne im Milchstraßensystem. Der grĂ¶ĂŸte Teil der Masse der Galaxis besteht aus alten, massearmen Sternen. Der „Zwischenraum“ zwischen den Spiralarmen ist also nicht leer, sondern ist einfach nur weniger leuchtstark.
Die Spiralstruktur der Galaxis konnte durch die Beobachtung der Verteilung von neutralem Wasserstoff bestÀtigt werden. Die entdeckten Spiralarme wurden nach den in ihrer Richtung liegenden Sternbildern benannt.

Die Zeichnung rechts stellt den Aufbau des Milchstraßensystems schematisch dar. Das Zentrum ist im sichtbaren Licht nicht direkt beobachtbar, ebenso wie der hinter ihm liegende Bereich. Die Sonne (gelber Kreis) liegt zwischen den Spiralarmen Sagittarius (nach Sternbild SchĂŒtze) und Perseus im Orionarm. Vermutlich ist dieser Arm nicht vollstĂ€ndig, siehe braune Linie in der Abbildung. Im VerhĂ€ltnis zu dieser unmittelbaren Umgebung bewegt sich die Sonne mit etwa 30 km/s in Richtung des Sternbildes Herkules. Der innerste Arm ist der Norma-Arm (nach Sternbild Winkelmaß, auch 3-kpc-Arm), der Ă€ußerste (nicht in der Abbildung) ist der Cygnus-Arm (nach Sternbild Schwan), welcher vermutlich die Fortsetzung des Scutum-Crux-Arms (nach Sternbildern Schild und Kreuz des SĂŒdens) ist .

Wissenschaftler der UniversitĂ€t von Wisconsin veröffentlichten im Juni 2008 Auswertungen von Infrarotaufnahmen des Spitzer-Teleskopes, die das Milchstraßensystem nun als zweiarmige Galaxie darstellen. Sagittarius und Norma sind in dieser Darstellung nur noch als dĂŒnne Nebenarme erkenntlich, da diese nur durch eine ĂŒberschĂŒssige Verteilung von Gas gekennzeichnet sind wĂ€hrend die restlichen beiden Arme durch eine hohe Dichte alter rötlicher Sterne gekennzeichnet sind.[10] Eine jĂŒngere Untersuchung der Verteilung von Sternentstehungsgebieten und junger Sterne scheint hingegen die bekannte vierarmige Struktur der Milchstraße zu bestĂ€tigen.[11] Die Milchstraße besteht daher scheinbar aus vier Spiralarmen die sich primĂ€r durch Gaswolken und junge Sterne abzeichnen, wobei zwei Arme zusĂ€tzlich durch eine hohe Konzentration Ă€lterer Sterne charakterisiert sind. Neben diesen unterschiedlichen Auffassungen bezĂŒglich der Struktur der Galaxis sollte beachtet werden, dass ein klar definiertes logarithmisches Spiralmuster nur in seltenen FĂ€llen bei anderen Spiralgalaxien ĂŒber die Gesamtheit der Scheibe beobachtet werden kann und die vorhandenen Arme oft extreme Abzweigungen, VerĂ€stelungen und VerschrĂ€nkungen aufweisen.[12][13] Die wahrscheinliche Natur des lokalen Arms als solche UnregelmĂ€ĂŸigkeit ist ein Hinweis darauf, dass solche Strukturen in der Milchstraße hĂ€ufig auftreten könnten.[14]

Welche Prozesse fĂŒr die Entstehung der Spiralstruktur verantwortlich sind, ist bislang noch nicht eindeutig geklĂ€rt. Jedoch ist klar, dass die zu den Spiralarmen gehörigen Sterne keine starre Struktur sind, die sich in Formation um das galaktische Zentrum dreht. WĂ€re dies der Fall, wĂŒrde sich die Spiralstruktur des Milchstraßensystems und anderer Spiralgalaxien aufgrund der unterschiedlichen Bahngeschwindigkeiten innerhalb relativ kurzer Zeit aufwickeln und unkenntlich werden. Eine ErklĂ€rung bietet die Dichtewellentheorie, nach der die Spiralarme Zonen erhöhter Materiedichte und Sternentstehung sind, die sich unabhĂ€ngig von den Sternen durch die Scheibe bewegen. Die durch die Spiralarme verursachten Störungen in den Bahnen der Sterne können zu Lindblad-Resonanzen fĂŒhren.

Sterne der galaktischen Scheibe
Die zur Population I zĂ€hlenden Sterne der galaktischen Scheibe lassen sich mit zunehmender Streuung um die Hauptebene und Alter in drei Unterpopulationen einteilen. Die so genannte „Thin Disk“ in einem Bereich von 700 bis 800 Lichtjahren ĂŒber und unterhalb der galaktischen Ebene enthĂ€lt neben den oben genannten leuchtkrĂ€ftigen Sternen der Spiralarme, die sich nur maximal 500 Lichtjahre von der Ebene entfernen, Sterne der Spektralklassen A und F, einige Riesen der Klassen A, F, G und K, sowie Zwergsterne der Klassen G, K und M und auch einige Weiße Zwerge. Die MetallizitĂ€t dieser Sterne ist vergleichbar mit der der Sonne, meist aber auch doppelt so hoch, ihr Alter liegt bei etwa einer Milliarde Jahren.

Eine weitere Gruppe ist die der mittelalten Sterne (Alter bis zu fĂŒnf Milliarden Jahre). Dazu zĂ€hlen die Sonne und weitere Zwergsterne der Spektraltypen G, K und M, sowie einige Unter- und Rote Riesen. Der Metallgehalt ist hier deutlich geringer mit nur etwa 50 bis 100 Prozent dessen der Sonne. Auch ist die BahnexzentrizitĂ€t der galaktischen Orbits dieser Sterne höher und sie befinden sich nicht weiter als 1500 Lichtjahre ĂŒber oder unterhalb der galaktischen Ebene.

Zwischen maximal 2500 Lichtjahren ober- und unterhalb der Hauptebene erstreckt sich die „Thick Disk“. Dort befinden sich rote K- und M-Zwerge, Weiße Zwerge, sowie einige Unterriesen und Rote Riesen, aber auch langperiodische VerĂ€nderliche. Ihr Alter erreicht bis zu zehn Milliarden Jahre und sie sind vergleichsweise metallarm (etwa ein Viertel der SonnenmetallizitĂ€t). Diese Population Ă€hnelt auch vielen Sternen im Bulge.

Die galaktische Scheibe ist nicht vollkommen gerade, durch gravitative Wechselwirkung mit den Magellanschen Wolken ist sie leicht in deren Richtung gebogen.

Das Zentrum des Milchstraßensystems liegt im Sternbild SchĂŒtze und ist hinter dunklen Staub- und Gaswolken verborgen, so dass es im sichtbaren Licht nicht direkt beobachtet werden kann. Beginnend in den 1950er Jahren ist es gelungen, im Radiowellenbereich sowie mit Infrarotstrahlung und Röntgenstrahlung zunehmend detailreichere Bilder aus der nahen Umgebung des galaktischen Zentrums zu gewinnen. Man hat dort eine starke Radioquelle entdeckt, bezeichnet als Sagittarius A* (Sgr A*), die aus einem sehr kleinen Gebiet strahlt. Diese Massenkonzentration wird von einer Gruppe von Sternen in einem Radius von weniger als einem halben Lichtjahr mit einer Umlaufzeit von etwa 100 Jahren sowie einem Schwarzen Loch mit 1300 Sonnenmassen in drei Lichtjahren Entfernung umkreist. Der dem zentralen Schwarzen Loch am nĂ€chsten liegende Stern S2 umlĂ€uft das galaktische Zentrum in einer Entfernung von etwa 17 Lichtstunden in einem Zeitraum von nur 15,2 Jahren. Seine Bahn konnte inzwischen ĂŒber einen vollen Umlauf hinweg beobachtet werden. Aus den Beobachtungen der Bewegungen der Sterne des zentralen Sternhaufens ergibt sich, dass sich innerhalb dieser Region von 15,4 Millionen km Durchmesser eine Masse von geschĂ€tzten 4,31 Millionen Sonnenmassen befinden muss.[15] Die im Rahmen der RelativitĂ€tstheorie plausibelste und einzige mit allen Beobachtungen konsistente ErklĂ€rung fĂŒr diese große Massenkonzentration ist die Anwesenheit eines Schwarzen Lochs.

Am 9. November 2010 machte Doug Finkbeiner vom Harvard-Smithsonian Center for Astrophysics bekannt, dass er zwei riesenhafte kugelförmige Blasen entdeckt habe, die aus der Mitte der Milchstraße nach Norden und SĂŒden hinausgreifen. Die Entdeckung ist mit der Hilfe von Daten des Fermi Gamma-ray Space Telescope gelungen. Der Durchmesser der Blasen betrĂ€gt jeweils etwa 25.000 Lichtjahre; sie erstrecken sich am sĂŒdlichen Nachthimmel von der Jungfrau bis zum Kranich. Ihr Ursprung ist bisher noch nicht geklĂ€rt.[16][17]

GrĂ¶ĂŸenvergleich
Man bekommt eine anschauliche Vorstellung von der GrĂ¶ĂŸe unserer Galaxis mit ihren 100 bis 300 Milliarden Sternen, wenn man sie sich im Maßstab 1:1017 verkleinert als Schneetreiben auf einem Gebiet von 10 km Durchmesser und einer Höhe von etwa 1 km im Mittel vorstellt. Jede Schneeflocke entspricht dabei einem Stern und es gibt etwa drei StĂŒck pro Kubikmeter. Unsere Sonne hĂ€tte in diesem Maßstab einen Durchmesser von etwa 10 nm, wĂ€re also kleiner als ein Virus. Selbst die Plutobahn, die sich im Mittel etwa 40-mal so weit von der Sonne befindet wie die Bahn der Erde, lĂ€ge mit einem Durchmesser von 0,1 mm an der Grenze der visuellen Sichtbarkeit. Pluto selbst hĂ€tte ebenso wie die Erde lediglich atomare Dimension. Damit demonstriert dieses Modell auch die geringe durchschnittliche Massendichte unserer Galaxis.
Die Sonne umkreist das Zentrum des Milchstraßensystems in einem Abstand von 25.000 bis 28.000 Lichtjahren (≈ 250 Em oder 7,94 ± 0,42 kpc)[18] und befindet sich nördlich der Mittelebene der galaktischen Scheibe innerhalb des Orion-Arms, in einem weitgehend staubfreien Raumgebiet, das als „Lokale Blase“ bekannt ist. FĂŒr einen Umlauf um das Zentrum der Galaxis, ein so genanntes galaktisches Jahr, benötigt sie 220 bis 240 Millionen Jahre, was einer Rotationsgeschwindigkeit von etwa 220 km/s entspricht. Die Erforschung dieser Rotation ist mittels der Eigenbewegung und der Radialgeschwindigkeit vieler Sterne möglich; aus ihnen wurden um 1930 die Oortschen Rotationsformeln abgeleitet. Heutzutage kann auch die durch die Umlaufbewegung des Sonnensystems bedingte scheinbare Bewegung des Milchstraßenzentrums gegenĂŒber Hintergrundquellen direkt beobachtet werden, so dass die Umlaufgeschwindigkeit des Sonnensystems unmittelbar messbar ist.[19] Neuere Messungen haben eine Umlaufgeschwindigkeit von ca. 267 km/s (961.200 km/h) ergeben.[20]

Das Sonnensystem umlĂ€uft das galaktische Zentrum nicht auf einer ungestörten ebenen Keplerbahn. Die in der Scheibe des Milchstraßensystems verteilte Masse ĂŒbt eine starke Störung aus, so dass die Sonne zusĂ€tzlich zu ihrer Umlaufbahn um das Zentrum auch regelmĂ€ĂŸig durch die Scheibe auf und ab oszilliert. Die Scheibe durchquert sie dabei etwa alle 30 bis 45 Millionen Jahre einmal.[21] Vor ca. 1,5 Millionen Jahren hat sie die Scheibe in nördlicher Richtung passiert und befindet sich jetzt etwa 65 Lichtjahre (ca. 20 pc)[22] ĂŒber ihr. Die grĂ¶ĂŸte Entfernung wird etwa 250 Lichtjahre (80 pc) betragen, dann kehrt sich die Bewegung wieder um.[21]

GrĂ¶ĂŸere datierbare Krater auf der Erde sowie erdgeschichtliche Massenaussterben scheinen eine PeriodizitĂ€t von 34 bis 37 Millionen Jahren aufzuweisen, was auffĂ€llig mit der PeriodizitĂ€t der Scheibenpassagen ĂŒbereinstimmt. Möglicherweise stören wĂ€hrend einer Scheibendurchquerung die in ScheibennĂ€he stĂ€rker werdenden Gravitationsfelder die Oortsche Wolke des Sonnensystems, so dass eine grĂ¶ĂŸere Anzahl von Kometen ins innere Sonnensystem gelangt und die Anzahl schwerer Impakte auf der Erde zunimmt. Die betreffenden Perioden sind jedoch bisher nicht genau genug bekannt, um definitiv einen Zusammenhang festzustellen;[21] neuere Ergebnisse (Scheibendurchgang alle 42 ± 2 Millionen Jahre) sprechen eher dagegen.[23] Eine neue Studie des Max-Planck Instituts fĂŒr Astronomie hat gezeigt, dass es sich bei der scheinbaren PeriodizitĂ€t der EinschlĂ€ge um statistische Artefakte handelt und es keinen solchen Zusammenhang gibt.

Um das Milchstraßensystem herum sind einige Zwerggalaxien versammelt. Die bekanntesten davon sind die Große und die Kleine Magellansche Wolke, mit denen das Milchstraßensystem ĂŒber eine etwa 300.000 Lichtjahre lange WasserstoffgasbrĂŒcke, dem Magellanschen Strom, verbunden ist.

Die dem Milchstraßensystem am nĂ€chsten gelegene Galaxie ist der Canis-Major-Zwerg, mit einer Entfernung von 42.000 Lichtjahren vom Zentrum des Milchstraßensystems und 25.000 Lichtjahren von unserem Sonnensystem. Die Zwerggalaxie wird zurzeit von den GezeitenkrĂ€ften des Milchstraßensystems auseinandergerissen und hinterlĂ€sst dabei ein Filament aus Sternen, das sich um die Galaxis windet, den so genannten Monoceros-Ring. Ob es sich dabei allerdings tatsĂ€chlich um die Überreste einer Zwerggalaxie oder um eine zufĂ€llige, projektionsbedingte HĂ€ufung handelt, ist derzeit noch nicht sicher. Andernfalls wĂ€re die 50.000 Lichtjahre vom galaktischen Zentrum entfernte Sagittarius-Zwerggalaxie die nĂ€chste Galaxie, die ebenfalls gerade durch das Milchstraßensystem einverleibt wird.

Das Milchstraßensystem verleibt sich bestĂ€ndig Zwerggalaxien ein und nimmt dadurch an Masse zu. WĂ€hrend der Verschmelzung hinterlassen die Zwergsysteme Ströme aus Sternen und interstellarer Materie, die durch die GezeitenkrĂ€fte des Milchstraßensystems aus den kleinen Galaxien herausgerissen werden (siehe auch: Wechselwirkende Galaxien). Dadurch entstehen Strukturen wie der Magellansche Strom, der Monoceros-Ring und der Virgo-Strom, sowie die anderen Hochgeschwindigkeitswolken in der Umgebung unserer Galaxis.

Lokale Gruppe
Mit der Andromeda-Galaxie, dem Dreiecksnebel (M 33) und einigen anderen kleineren Galaxien bildet das Milchstraßensystem die Lokale Gruppe, wobei das Milchstraßensystem die massereichste Galaxie darunter ist, obwohl es nicht die grĂ¶ĂŸte Ausdehnung besitzt. Die Lokale Gruppe ist Bestandteil des Virgo-Superhaufens, der nach dem Virgohaufen in seinem Zentrum benannt ist. Auf diesen bewegt sich die Lokale Gruppe zu. Der lokale Superhaufen strebt mit anderen Großstrukturen dem Shapley-Superhaufen entgegen (die frĂŒhere Annahme, Ziel dieses Strebens sei der Große Attraktor, ist ĂŒberholt).[25]

Die Andromeda-Galaxie ist eine der wenigen Galaxien im Universum, deren Spektrum eine Blauverschiebung aufweist: Die Andromeda-Galaxie und das Milchstraßensystem bewegen sich mit einer Geschwindigkeit von 120 km/s aufeinander zu. Allerdings gibt die Blauverschiebung nur Aufschluss ĂŒber die Geschwindigkeitskomponente parallel zur Verbindungslinie beider Systeme, wĂ€hrend die Komponente senkrecht zu dieser Linie unbekannt ist. Vermutlich werden die beiden Galaxien in etwa drei Milliarden Jahren zusammenstoßen und zu einer grĂ¶ĂŸeren Galaxie verschmelzen. FĂŒr den Ablauf der Kollision können mangels Kenntnis der Raumgeschwindigkeiten und wegen der KomplexitĂ€t der beim Zusammenstoß ablaufenden Prozesse nur Wahrscheinlichkeitsaussagen gemacht werden.[26] Nach der Verschmelzung der beiden Galaxien wird das Endprodukt voraussichtlich eine massereiche elliptische Galaxie sein. Als Name fĂŒr diese Galaxie wird von Cox-Loeb 2008 in ihrem Artikel der Arbeitsname „Milkomeda“ benutzt, eine Verschmelzung des englischen Milky Way und Andromeda.[26]

Alter
Messungen aus dem Jahr 2004 zufolge ist das Milchstraßensystem etwa 13,6 Milliarden Jahre alt. Die Genauigkeit dieser AbschĂ€tzung, die das Alter anhand des Berylliumanteils einiger Kugelsternhaufen bestimmt, wird mit etwa 800 Millionen Jahren angegeben. Da das Alter des Universums von 13,8 Milliarden Jahren als recht verlĂ€sslich bestimmt gilt, hieße das, dass die Entstehung der Milchstraße auf die FrĂŒhzeit des Universums datiert.

2007 wurde zunĂ€chst fĂŒr den Stern HE 1523-0901 im galaktischen Halo von der ESO-Sternwarte in Hamburg ein Alter von 13,2 Milliarden Jahren festgestellt[27]. 2014 wurde dann fĂŒr den Stern SM0313, 6000 Lj von der Erde entfernt, von der Australian National University ein Alter von 13,6 Milliarden Jahren dokumentiert. Als Ă€lteste bekannte Objekte der Milchstraße setzen diese Datierungen eine unterste Grenze, die im Bereich der Messgenauigkeit der AbschĂ€tzung von 2004 liegt.

Nach derselben Methode kann das Alter der dĂŒnnen galaktischen Scheibe durch die Ă€ltesten dort gemessenen Objekte abgeschĂ€tzt werden, wodurch sich ein Alter von etwa 8,8 Milliarden Jahren mit einer SchĂ€tzbreite von etwa 1,7 Milliarden Jahren ergibt. Auf dieser Basis ergĂ€be sich eine zeitliche LĂŒcke von etwa drei bis sieben Milliarden Jahren zwischen der Bildung des galaktischen Zentrums und der Ă€ußeren Scheibe.

The Milky Way is the galaxy that contains our Solar System.[15][16][17][nb 1] Its name “milky” is derived from its appearance as a dim glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. The term “Milky Way” is a translation of the Latin via lactea, from the Greek ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚ ÎșύÎșÎ»ÎżÏ‚ (galaxĂ­as kĂœklos, "milky circle").[18][19][20] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Up until the early 1920s, most astronomers thought that all of the stars in the universe were contained inside of the Milky Way. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[21] observations by Edwin Hubble definitively showed that the Milky Way is just one of many billions of galaxies.[22]

The Milky Way is a barred spiral galaxy some 100,000–120,000 light-years in diameter, which contains 100–400 billion stars. It may contain at least as many planets as well.[23][24] The Solar System is located within the disk, about 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole.

Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name “dark matter”.[25] The rotational period is about 240 million years at the position of the Sun.[11] The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest known star in the Milky Way is at least 13.82 [26] billion years old and thus must have formed shortly after the Big Bang.[7]

Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster, which again forms a subcomponent of the Laniakea supercluster.
When observing the night sky, the term “Milky Way” is limited to the hazy band of white light some 30 degrees wide arcing across the sky.[29] Although all of the individual stars that can be seen in the entire sky with the naked eye are part of the Milky Way Galaxy,[30] the light in this band originates from the accumulation of un-resolved stars and other material when viewed in the direction of the Galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by interstellar dust.

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. It is readily visible when the limiting magnitude is +5.1 or better and shows a great deal of detail at +6.1.[31] This makes the Milky Way difficult to see from any brightly lit urban or suburban location, but very prominent when viewed from a rural area when the Moon is below the horizon.[nb 2]

As viewed from Earth, the visible region of the Milky Way’s Galactic plane occupies an area of the sky that includes 30 constellations. The center of the Milky Way lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass westward to the Galactic anticenter in Auriga. The band then continues westward the rest of the way around the sky, back to Sagittarius. The band divides the night sky into two roughly equal hemispheres.

The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of Earth’s orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth’s equatorial plane and the plane of the ecliptic, relative to the Galactic plane. The north Galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near ÎČ Comae Berenices, and the south Galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the arc of Milky Way may appear relatively low or relatively high in the sky. For observers from approximately 65 degrees north to 65 degrees south on Earth’s surface, the Milky Way passes directly overhead twice a day.
The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (30 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick.[2][3] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 mm in width, about the size of a grain of sand. The nearest star, Proxima Centauri, would be 4.2 mm distant.[nb 3] Alternatively visualized, if the Solar System out to Neptune were the size of a US quarter (25mm), the Milky Way would have a diameter of 4,000 kilometers, or approximately the breadth of the United States.

Estimates for the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8×1011 solar masses (M☉), somewhat smaller than the Andromeda Galaxy.[33][34][35] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s for stars at the outer edge of the Milky Way.[36] As the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M☉ within 160,000 ly (49 kpc) of its center.[37] A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M☉.[38] According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5×1011 M☉,[39] which is about half the mass of the Andromeda Galaxy.[39]

Most of the mass of the Milky Way appears to be matter of unknown form that interacts with other matter through gravitational but not electromagnetic forces, which is dubbed dark matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggest that the total mass of the entire Galaxy lies in the range 1–1.5×1012 M☉.[8] More recent studies indicate a mass as large as 4.5×1012 M☉ [40] and as small as 0.8×1012 M☉.[41] The Milky Way contains at least 100 billion planets[42] and between 200 and 400 billion stars.[43][44] The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[45] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[46] whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[47][48] Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way[23][49] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[50][51] The Milky Way Galaxy contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory.[24] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way Galaxy.[52] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way Galaxy.[53][54][55] 11 billion of these estimated planets may be orbiting sun-like stars.[56] The nearest such planet may be 12 light-years away, according to the scientists.[53][54] Such Earth-sized planets may be more numerous than gas giants.[23] Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way Galaxy.[52]

The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.[57] Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic Center is about 180,000 ly (55 kpc).[58] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be −20.9.

The Milky Way consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Milky Way closely resembles the type SBc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms.[1] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[61] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[62] that showed the Milky Way’s central bar to be larger than previously suspected.

Galactic quadrants
Main article: Galactic quadrant
A galactic quadrant, or quadrant of the galaxy, refers to one of four circular sectors in the division of the Milky Way. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the pole of the mapping system.

Quadrants are described using ordinals—for example, "1st galactic quadrant",[63] "second galactic quadrant",[64] or "third quadrant of the Galaxy".[65] Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are as follow:

1st galactic quadrant – 0° ≀ longitude (ℓ) ≀ 90°[66] 2nd galactic quadrant – 90° ≀ ℓ ≀ 180°[64] 3rd galactic quadrant – 180° ≀ ℓ ≀ 270°[65] 4th galactic quadrant – 270° ≀ ℓ ≀ 360° (0°)[63] The Sun is 26,000–28,000 ly (8.0–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.[10][67][68][69][70] In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[71] It has been proposed that the Milky Way lacks a bulge formed due to a collision and merger between previous galaxies and that instead has a pseudobulge formed by its central bar.[72]

The Galactic Center is marked by an intense radio source named Sagittarius A*. The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.[73] This concentration of mass is best explained as a supermassive black hole[nb 4][10][67] with an estimated mass of 4.1–4.5 million times the mass of the Sun.[67] Observations indicate that there are supermassive black holes located near the center of most normal galaxies.[74][75]

The nature of the Milky Way’s bar is actively debated, with estimates for its half-length and orientation spanning from 1–5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[69][70][76] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[77] In most galaxies, Wang et al. report, the rate of accretion of the supermassive black hole is slow, but the Milky Way seems to be an important exception. X-ray emission is aligned with the massive stars surrounding the central bar.[78] However, RR Lyr variables do not trace a prominent Galactic bar.[70][79][80] The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way’s star-formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[81]

In 2010, two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere.[82][83] Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Milky Way.[84]

Spiral arms
Outside the gravitational influence of the Galactic bars, astronomers generally organize the structure of the interstellar medium and stars in the disk of the Milky Way into four spiral arms.[85] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions[86][87] and molecular clouds.[88]

Maps of the Milky Way’s spiral structure are notoriously uncertain and exhibit striking differences.[60][85][87][89][90][91][92][93] Some 150 years after Alexander (1852)[94] first suggested that the Milky Way was a spiral, there is currently no consensus on the nature of the Milky Way’s spiral arms. Perfect logarithmic spiral patterns only crudely describe features near the Sun,[87][92] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[70][92][93] The possible scenario of the Sun within a spur / Local arm[87] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.[92]

As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from about 7° to 25°.[95][96] There are thought to be four spiral arms that all start near the Milky Way’s center. These are named as follows, with the positions of the arms shown in the image at right:
Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun’s orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm.[95][98] In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars.[60] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.[99][100][101] Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.[101]

The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen).[102][103] It was found to be expanding away from the center of the Milky Way at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the Galactic Center. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard-Smithsonian CfA). It’s located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center.[103][104]

A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.[105]

It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring[106] and the two patterns would be connected by the Cygnus arm.[107]

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.[108]

Halo
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[109] However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of the Milky Way’s clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[110] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.[111]

Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars.[11] Open clusters are also located primarily in the disk.[112]

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way’s structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[113] the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm[97][114] and of a similar extension of the Scutum-Centaurus Arm.[115] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[116]

Gaseous halo
In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself.[117][118][119] The temperature of this halo gas is between 1 million and 2.5 million kelvin, a few hundred times hotter than the surface of the sun.[120]

Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[121] If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.

The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center.[10][67][122] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.[123] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly).[124] The Sun, and thus the Solar System, is found in the Galactic habitable zone.

There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsec, or one star per 2,360 cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsec, or one per 284 cubic light-year (from List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14.[125]

The apex of the Sun’s way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun’s Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun’s orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth.[126] However, a reanalysis of the effects of the Sun’s transit through the spiral structure based on CO data has failed to find a correlation.[127]

It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a Galactic year),[11] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).

The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s.[131] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.

If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[25] The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the strongest proof of the existence of dark matter in galaxies. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.
The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[133][134]

Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Milky Way’s growth) and accretion of gas directly from the Galactic halo.[134] The Milky Way is currently accreting material from two of its nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high-velocity clouds like the Smith Cloud.[135][136] However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[137][138]

According to recent studies, the Milky Way as well as Andromeda lie in what in the galaxy color–magnitude diagram is known as the green valley, a region populated by galaxies in transition from the blue cloud (galaxies actively forming new stars) to the red sequence (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy.[139] In fact, measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.[140]

Age[edit] The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 12.5 ± 3 billion years for CS 31082-001[141] and 13.8 ± 4 billion years for BD +17° 3248.[142] Once a white dwarf is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Globular clusters are among the oldest objects in the Milky Way Galaxy, which thus set a lower limit on the age of the Milky Way. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[143]

In 2007, a star in the galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, ≈0.5 billion years less than the age of the universe. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.[144] This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[144]

The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk.
The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster. The Virgo Supercluster forms part of a greater structure, called Laniakea.[146]

Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 14,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a stream of neutral hydrogen gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.[147] Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri.

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.[148]

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. In 3 to 4 billion years, there may be an Andromeda–Milky Way collision, depending on the importance of unknown lateral components to the galaxies’ relative motion. If they collide, the chance of individual stars colliding with each other is extremely low, but instead the two galaxies will merge to form a single elliptical galaxy or perhaps a large disk galaxy[149] over the course of about a billion years.[150]

Velocity
Although special relativity states that there is no "preferred" inertial frame of reference in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological frames of reference.

One such frame of reference is the Hubble flow, the apparent motions of galaxy clusters due to the expansion of space. Individual galaxies, including the Milky Way, have peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km per second with respect to this local co-moving frame of reference.[151] The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.[152] The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[153]

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km/s[13] with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[13]

Etymology and mythology
Main articles: List of names for the Milky Way and Milky Way (mythology)
In western culture the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚, short for ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚ ÎșύÎșÎ»ÎżÏ‚ (pr. galaktikos kyklos, "milky circle"). The Ancient Greek ÎłÎ±Î»Î±ÎŸÎŻÎ±Ï‚ (galaxias), from root γαλαÎșτ-, γΏλα (milk) + -ÎŻÎ±Ï‚ (forming adjectives), is also the root of "galaxy", the name for our, and later all such, collections of stars.[18][154][155][156] The Milky Way "milk circle" was just one of 11 circles the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.

In Meteorologica (DK 59 A80), Aristotle (384–322 BC) wrote that the Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."[158] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.[159]

The Persian astronomer AbĆ« Rayhān al-BÄ«rĆ«nÄ« (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars".[160] The Andalusian astronomer Avempace (d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth’s atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.[158] Ibn Qayyim Al-Jawziyya (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.[161]

According to Jamil Ragep, the Persian astronomer NaáčŁÄ«r al-DÄ«n al-áčŹĆ«sÄ« (1201–1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."[162]

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars.[163][164] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[165] speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.[166] The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[167][168][169]

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center.[170]

In 1845, Lord Rosse construct

Posted by !!! Painting with Light !!! #schauer on 2014-10-14 22:29:36

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Day Does Spring Break

Day Does Spring Break

Hwy. 50, Nevada ~ The Loneliest Highway in the US

I’ve just returned from an incredible trip through the southwest with my good friend and gal pal, Mel. Six days together with an additional three for me for traveling and photography. It was a jam packed adventure filled with good times and great people, as well as just about every weather condition encountered, except for a tornado, thank goodness! I have a few gigabytes to go through, but it’s all downloaded and backed up, the truck is unloaded, the laundry is half done, and I’ve even had time for dinner. I thought I’d post this now, one-because I really needed a new icon, and two-to let you know I’m back and will get to your streams soon. I hope everyone had a great week and a chance to find the good light. Thanks for stopping by!

Here’s a brief account of my trip which will be followed in more detail with each post…

Day 1: Valley of Fire State Park, Nevada with David "DBreezy" Thompson and Aaron Matney. Explored the Fire Wave with some killer evening light, then discovered a critter decided to join me in the back of my truck while I slept! No-not dangerous, but not funny until the next morning, either!

Day 2: Sunrise at VoF was a bust, but I had a nice time exploring the Fire Wave Slot Canyon (my name for it) and what a very sweet place. I believe I have quite a few good comps from it. Then off to Page, AZ to meet Mel. We had a good evening out for dinner then went to Horseshoe Bend which also turned out quite nicely.

Day 3: We hit Antelope Canyon in the morning and ran into (almost literally) Stephen and Terry Oachs who were enjoying some personal vacation time. Got some really nice light here. Next we were off to Monument Valley where I finally got a shot I’ve wanted for almost 30 years! I did end up paying for it (not a biggie), but got much more in return including a new friend. The light was perfect for my shot and later that night we had thunder showers and one big crack of lightning that had both Mel and I jumping.

Day 4: Sunrise in Monument Valley and it did not disappoint. In fact I got something of a UFO in one of my shots, though I’m sure it was just sun flare. 😉 We left in the afternoon and just outside the valley, we stopped on the highway for the typical “Forrest Gump” shot of the Monuments from the middle of the highway (while dodging traffic). Stopped at the Mexican Hat Rock to shoot some cool cloud formations, and then a quick stopover at Goosenecks State Park. Next we hiked to Fallen Roof ruins in Cedar Mesa’s Road Canyon. Totally cool place with four more ruins in the area. Lots of hiking up and down slickrock and through a canyon. Next was Natural Bridges National Park to spend the night. It was full so we explored one of the bridges, neither of us being very impressed. We decided to get a hotel in nearby Blanding, UT and were met by a beautiful sunset on the highway, and more rain through the night.

Day 5: Beds and showers felt great, but we left the hotel early in order to get to our next location, House on Fire in Mule Canyon at Cedar Mesa. A fairly easy hike and no problem to find, this turned out to be a great morning. Next, we went to spend the night at Capital Reef National Park. We had time for a little exploring and good thing, because sunset was uneventful, but this place really needs to be hit up in a big way. It was absolutely gorgeous and the campground was lovely and we only saw a small piece of it. A little more rain during the night, but we both faired well.

Day 6: We left Capital Reef in the morning our next stop being Lower Calf Creek Falls in Grand Staircase-Escalante. Gorgeous drive along Hwy. 12 with tons of Aspen trees and snow! We arrived at the Calf Creek BLM campground and trailhead in late morning for a 3 mile hike. That place was a zoo, but it was a fairly easy hike, despite our aches and exhaustion. We actually shared part of the hike with Elliot Porter’s great grandson who, his companion attests, has many great talents though none of them being photography. Next stop was Bryce Canyon National Park, in time for the sunset. We found a great campsite at the Sunset campground and proceeded to get snowed on during our sunset shoot with the only really good light happening in one small area around the sun. May have a couple nice silhouettes from that. Temps got down in the 20s that night and a pot of hot coffee was indeed relished in the morning.

Day 7: Sunrise at Bryce was very nice with more beamage than color and still bitter cold. Later, we headed off for a hike down into Bryce Canyon which was very nice, but had these two old gals regretting almost every step back up. We had planned on stopping at either Coral Pink Sand Dunes or the Rim Rock hoodoos, but just getting back to Page, hot showers and a comfy bed were foremost on our minds. After showers and dinner out, I decided to try Horseshoe Bend one more time. Checked on Mel’s computer for a different route home then got to bed.

Day 8: Mel and I parted ways, she leaving around 5AM for her trip back to eastern New Mexico, and I got out of town around 8:30. I took a drive for about 15 miles on the Cottonwood Canyon Road, then the old Paria road. Showers were looming so I couldn’t spend too much time on these roads without risking getting stuck. In fact, as soon as I got back on Hwy. 89 it started pouring! I made a few more stops along the way for photos going up along Hwy. 14 to Cedar City, UT. I managed to get a bit of a sunset somewhere north of a town called Pioche, NV. I spent the night napping in two spots along Hwy. 50 just east of Ely and driving through a range in white out conditions.

Day 9: Started just west of the town of Eureka where the above photo was taken. In and out of more rain, wind, snow showers, and dust devils through the basin and range of Nevada. It was lovely and green in most places with some early wildflowers here and there. Returned home safely after driving through more snow on Donner Pass. I’ve never driven Hwy. 50 before and am glad I did, but wouldn’t try it in the summer. Go in the spring…you’ll love it! Only thing is…there are absolutely no Starbucks or any decent coffee between Cedar City, UT and Fernley, NV! But I’d do it all again in a heartbeat!~ Thanks for taking the time to read all of this. Photos with more in depth stories to come!

Posted by jeandayphotography.com on 2011-04-25 02:21:03

Tagged: , clouds , color , desert , highway , Hwy. 50 , JDay , Jean Day , landscape , Lonliest Highway in the Us , mountains , Nevada , NV , people , road , self portrait , snow , Spring , Spring Break , street , April , 2011

Rust & Dust

Rust & Dust

Posted by alexandriabrangwin on 2015-09-12 11:18:08

Tagged: , Sinjane , Second Life , 3D , CGI , Computer , Graphics , Virtual , world , road , warrior , goddess , independant , woman , Mad Max , Interceptor , V8 , supercharged , XB , Falcon , GT , desert , sand , post , apocalyptic , wasteland , shotgun , leather , armor , dust , storm , horizon , repose , contemplation , stand , pose , heroic , heroine