View source for Milky Way

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The '''Milky Way''' is the [[galaxy]] that contains the planet [[Earth]], and is therefore the home galaxy of the [[human race]]. It is a [[barred spiral galaxy]] and part of the [[Local Group]], which is in turn part of the [[Virgo Supercluster]]. At night it is visible from Earth as a concentrated band of stars, which was the inspiration for its name.
{{Image|Milkyway hi.jpg|right|350px|Wide Angle View of Mily Way taken from Cerro Tololo Inter-American Observatory in Chili, a US National Optical Astronomy Observatory of the US National Science Foundation (NOAO).}}
==Charactaristics==
===Size===
The disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and about 1,000 light years thick.<ref>{{cite web 
  | last = Christian
  | first = Eric
  | last2 = Samar
  | first2 = Safi-Harb
  | title = How large is the Milky Way?
  | url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html
  | accessdate = 2007-11-28 }}</ref>  It is estimated to contain 200 billion,<ref>{{cite news | first = Robert | last = Sanders | title=Milky Way galaxy is warped and vibrating like a drum | publisher=UCBerkeley News | date=January 9, 2006 | url=http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml | accessdate=2006-05-24 }}</ref> and up to 400 billion,<ref>{{cite web
 | author=Frommert, H.; Kronberg, C.
 | date =August 25, 2005 
 | url =http://www.seds.org/messier/more/mw.html
 | title =The Milky Way Galaxy 
 | publisher =SEDS
 | accessdate = 2007-05-09 }}</ref> stars (if small-mass stars predominate). As a guide to the relative physical [[scale (ratio)|scale]] of the Milky Way, if it were reduced to 130 km (80 mi) in diameter, the Solar System would be a mere 2 mm (0.08 inches) in width. The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small [[Magellanic Clouds]], whose [[Apsis|perigalacticon]] is at ~180,000 [[light-year]]s.<ref>{{cite news | first = et al. | last = Connors | title=N-body simulations of the Magellanic stream | publisher=Monthly Notices of the Royal Astronomical Society | date=January 26, 2007 | url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?bibcode=2006MNRAS.371..108C&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=45a3b56f2301128 | accessdate=2007-01-26 }}</ref> New discoveries indicate that the disk extends much farther than previously thought.


===Age===
It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest stars in the Galaxy is now estimated to be about 13.6 billion years, nearly as [[Age of the universe|old as the Universe]] itself.<ref name="age">{{cite news
 | author=Pasquini, L.; Galli, D.
 | title=How Old is the Milky Way?
 | publisher=European Southern Observatory
 | date=August 17, 2004
 | url=http://www.eso.org/outreach/press-rel/pr-2004/pr-20-04.html
 | accessdate=2007-05-09 }}</ref>

This estimate is based on research done in 2004 by astronomers Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. The team used the UV-Visual Echelle Spectrograph of the [[Very Large Telescope]] to [[Measurement|measure]], for the first time, the [[beryllium]] content of two stars in [[globular cluster]] [[NGC 6397]]. This allowed them to deduce the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster, at 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. (See also [[nucleocosmochronology]].)  Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago<ref>Eduardo F. del Peloso a1a, Licio da Silva a1, Gustavo F. Porto de Mello and Lilia I. Arany-Prado (2005),
"The age of the Galactic thin disk from Th/Eu nucleocosmochronology: extended sample" (Proceedings of the International Astronomical Union (2005), 1: 485-486 Cambridge University Press)</ref>.

===Composition and structure===
Observations by the [[Spitzer Space Telescope]] in 2005 backed up previously collected evidence that suggested the Milky Way is a barred spiral galaxy. It consists of a bar-shaped core region surrounded by a disk of [[interstellar medium|gas, dust]] and stars. Within the disk region are several arm structures that spiral outward in a [[logarithmic spiral]] shape. The mass distribution within the Galaxy closely resembles the Sbc [[Galaxy morphological classification|Hubble classification]], which is a spiral galaxy with relatively loosely-wound arms.<ref>{{cite journal | last = Ortwin | first = Gerhard | title=Mass distribution in our Galaxy | journal=Space Science Reviews | year=2002 | volume=100 | issue=1/4 | pages=129–138 | url=http://adsabs.harvard.edu/abs/2002astro.ph..3110G | accessdate=2007-03-14 }}</ref> It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral<ref>{{cite journal | author=Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. | title=On the spiral arm interpretation of COMPTEL ^26^Al map features | journal=Space Science Reviews | year=1996 | volume=120 | pages=315–316 | url=http://adsabs.harvard.edu/abs/1996A&AS..120C.315C | accessdate=2007-03-14 }}</ref> rather than an ordinary [[spiral galaxy|spiral]], which observations in 2005 with the [[Spitzer Space Telescope]] have since confirmed, showing that the Galaxy's central bar is larger than previously suspected.<ref name="fn3">{{cite news
 | first=Maggie
 | last=McKee
 | title=Bar at Milky Way's heart revealed
 | publisher=New Scientist
 | date=August 16, 2005 
 | url=http://www.newscientistspace.com/article.ns?id=dn7854
 | accessdate=2007-05-09 }}</ref> This argues for a classification of type SBbc (loosely wound barred spiral). In 1970 [[Gérard de Vaucouleurs]] predicted that the Milky Way was of type SAB(rs)bc, where the "rs" indicates a broken ring structure around the core region.<ref>{{cite journal | author=López-Corredoira, M.; Cabrera-Lavers, A.; Mahoney, T. J.; Hammersley, P. L.; Garzón, F.; González-Fernández, C. | title=The Long Bar in the Milky Way: Corroboration of an Old Hypothesis | journal=The Astronomical Journal | year=2007 | volume=133 | issue=1 | pages=154–161 | url=http://adsabs.harvard.edu/abs/2006astro.ph..6201L | accessdate=2007-03-15 }}</ref>

As of 2006, the Milky Way's [[mass]] is thought to be about 5.8{{e|11}}&nbsp;[[solar mass|M<sub>☉</sub>]]<ref name="Karachentsevetal2006">{{cite journal | author=Karachentsev, I. D.; Kashibadze, O. G. | title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field | journal=Astrophysics | year=2006 | volume=49 | issue=1 | pages=3–18 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006Ap.....49....3K }}</ref><ref name="mass">{{cite web
 | last = Elert
 | first = Glenn
 | date = 2006-07-13
 | url =http://hypertextbook.com/facts/2000/AlinaVayntrub.shtml
 | title =Mass of the Milky Way
 | work=The Physics Factbook
 | publisher =Hypertextbook
 | accessdate = 2007-05-09 }}</ref><ref name="galmass">{{cite journal
 | author=Battaglia, G.; Helmi, A.; Morrison, H.; Harding, P.;  Olszewski, E. W.; Mateo, M.; Freeman, K. C.; Norris, J.; Shectman, S. A. 
 | title=The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way
 | journal=Monthly Notices of the Royal Astronomical Society 
 | year=2005
 | volume=364
 | pages=433–442
 | url=http://arxiv.org/abs/astro-ph/0506102
 | accessdate=2007-05-09 }}</ref> comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9. Most of the mass of the Galaxy is thought to be [[dark matter]], forming a [[dark matter halo]] of an estimated 600–3000 billion [[solar mass]]es (M<sub>☉</sub>) which is spread out relatively evenly.<ref name="galmass">filler</ref>

====Galactic center====
The galactic disk, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 [[light-year]]s.<ref name="Galactic bluge">{{cite news
 | author=Grant. J.; Lin, B.
 | title=The Stars of the Milky Way
 | year =2000
 | url =http://members.fcac.org/~sol/chview/chv5.htm
 | publisher=Fairfax Public Access Corporation
 | accessdate=2007-05-09 }}</ref> The distance from the [[Sun]] to the galactic center is now estimated at 26,000 ± 1400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge.

The galactic center harbors a compact object of very large mass (named [[Sagittarius A*]]), strongly suspected to be a [[supermassive black hole]]. Most galaxies are believed to have a supermassive black hole at their center.<ref>{{cite conference
 | first = R.D.
 | last = Blandford
 | authorlink = 
 | coauthors = 
 | title = Origin and evolution of massive black holes in galactic nuclei
 | booktitle = Galaxy Dynamics, proceedings of a conference held at Rutgers University, 8–12 Aug 1998,ASP Conference Series vol. 182
 | date = 1999
 | url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999ASPC..182...87B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=455327e36328623
}}</ref>

The Galaxy's bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the Galaxy. It is composed primarily of red stars, believed to be ancient (see [[red dwarf]], [[red giant]]). The bar is surrounded by a ring called the "5-[[Kiloparsec|kpc]] ring" that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way's [[star formation]] activity. Viewed from the [[Andromeda Galaxy]], it would be the brightest feature of our own galaxy.<ref name="fn14">{{cite web
 | author=Staff
 | date=[[September 12]] [[2005]]
 | url=http://www.bu.edu/galacticring/new_introduction.htm
 | title=Introduction: Galactic Ring Survey
 | publisher=Boston University
 | accessdate=2007-05-10 }}</ref>

====Spiral arms====
Each spiral arm describes a [[logarithmic spiral]] (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be two major spiral arms which start at both ends of the Galaxy's bar-shaped center and two minor arms located between the major arms. These are the Perseus Arm which is a major arm beginning from the top right of the bar, the Scutum-Centaurus Arm the other major arm which begins from the bottom left of the bar. The minor arm Sagittarius originates between the Perseus and the galactic center and then sweeps out to the right of the Scutum-Centaurus and the Norma Arm which originates between the galactic bar on the lower left and the Scutum-Centaurus. Around the galacitc bar are the Far-3 kiloparsec Arm on the left and the Near-3 kiloparsec Arm on the right. The Outer Arm is outside the Perseus on the lower left and the Orion Spur from bottom left between the Perseus and the Sun sweeping out toward the edges of the galaxy on the right.<ref>[http://www.nasa.gov/images/content/236084main_MilkyWay-full-annotated.jpg Milky Way Annotated] NASA/JPL-Cal Tech; [http://www.nasa.gov/mission_pages/spitzer/multimedia/20080603a.html Our Milky Way gets a makeover] NASA Spitzer, June 3, 2008</ref>
{{Image|236088main milkyway516.jpg|right|350px|New view of the Milky Way presented at the 212th American Astronomical Society meeting in St. Louis, Mo. Milky Way's spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms. For a much larger (3.6 MB) and annotated image go to [http://www.nasa.gov/images/content/236084main_MilkyWay-full-annotated.jpg Milky Way full annotated] Image Credit: NASA/JPL-Caltech June 3, 2008.}}
{|style="margin: 5px 0 5px 30px"
|-
!|color
!|arm(s)
|-
|style="background-color: #60FFFF"|cyan
||3-k[[parsec|pc]] and [[Perseus Arm]]
|-
|style="background-color: #c0c0FF"|sky-blue
||[[Norma Arm|Norma]] and [[Cygnus Arm]] (Along with a newly discovered extension)
|-
|style="background-color: #C0FFA0"|chlorine-green
||[[Scutum-Crux Arm|Crux]] and [[Scutum-Crux Arm|Scutum Arm]]
|-
|style="background-color: #FFC0C0"|pink
||[[Sagittarius Arm|Carina]] and [[Sagittarius Arm]]
|-
|colspan="2" style="padding: 0 0 0 30px;"|''There are at least two smaller arms or spurs, including:''
|-
|style="background-color: #FFC040"|orange
||[[Orion Arm]] (which contains the solar system and the [[Sun]])
|}

Outside of the major spiral arms is the [[Outer Ring]] or [[Monoceros Ring]], a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago.

As is typical for many galaxies, the distribution of mass in the Milky Way Galaxy is such that the [[orbital speed]] of most stars in the Galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240&nbsp;km/s.<ref name="fn4">{{cite web
 | last=Imamura
 | first=Jim
 | date=[[August 10]] [[2006]]
 | url=http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html
 | title=Mass of the Milky Way Galaxy
 | publisher=University of Oregon
 | accessdate=2007-05-10 }}</ref> Hence the [[orbital period]] of the typical star is directly proportional only to the length of the path traveled. This is unlike in the Solar System where different orbits are also expected to have significantly different velocities associated with them, and is one of the major pieces of evidence for the existence of [[dark matter]]. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If one believes that the inner parts of the arms rotate faster than the outer part, then the Galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral arms form as a result of a matter-density wave emanating from the galactic center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. Thus this results in several spiral arms where there are a lot of stars and gas. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular Hydrogen and protostars form as a result.
{{-}}

====Halo====
The galactic disk is surrounded by a [[Galactic spheroid|spheroid halo]] of old stars and [[globular cluster]]s, of which 90% lie within 100,000 light-years,<ref>{{cite web
 | first=William E.
 | last=Harris
 | date=February 2003
 | url=http://www.seds.org/messier/xtra/data/mwgc.dat.txt
 | format=text
 | title=Catalog of Parameters for Milky Way Globular Clusters: The Database
 | publisher=SEDS
 | accessdate=2007-05-10 }}</ref> suggesting a stellar halo diameter of 200,000 light-years. 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. While the disk contains gas and dust obscuring the view in some wavelengths, the spheroid component does not. Active [[star formation]] takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. [[Open cluster]]s also occur primarily in the disk.

Recent discoveries have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disc of the [[Andromeda Galaxy]] (M31) extends much further than previously thought,<ref name="fn5">{{cite journal
 | author=Ibata, R.; Chapman, S.; Ferguson, A. M. N.; Lewis, G.; Irwin, M.; Tanvir, N.
 | title=On the accretion origin of a vast extended stellar disk around the Andromeda galaxy
 | journal=Astrophysical Journal
 | year=2005
 | volume=634
 | issue=1
 | pages=287–313
 | url=http://adsabs.harvard.edu/abs/2005ApJ...634..287I
 | accessdate=2007-05-10 }}</ref> the possibility of the disk of the Galaxy extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the [[Cygnus Arm]].<ref name="fn6">{{cite web
 | url =http://www.solstation.com/x-objects/gal-ring.htm
 | title =Outer Disk Ring?
 | publisher =SolStation
 | accessdate = 2007-05-10 }}</ref> With the discovery of the [[Sagittarius Dwarf Elliptical Galaxy]] came the discovery of a ribbon of galactic debris as the polar orbit of [[Sagittarius (constellation)|Sagittarius]] 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 Juric]] 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 Galaxy. 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 (constellation)|Virgo]] about 30,000 light-years away.


==View from Earth==

Visible from [[Earth]] as a hazy band of [[white]] [[light]] that is seen in the [[night sky]], arching across the entire [[celestial sphere]], the visual [[phenomenon]] of the Milky Way (as seen in the [[night]] [[sky]]) originates from [[star]]s and other material which lies within the galactic plane.

The Milky Way appears brightest in the direction of the [[constellation]] of [[Sagittarius (constellation)|Sagittarius]], toward the [[galactic center]]. Relative to the [[celestial equator]], it passes as far north as the constellation of [[Cassiopeia (constellation)|Cassiopeia]] and as far south as the constellation of [[Crux]], indicating the high inclination of Earth's [[equator|equatorial plane]] and the plane of the [[ecliptic]] relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal [[celestial sphere|hemispheres]] indicates that our Solar System lies close to the [[galactic plane]]. The Milky Way has a relatively low [[surface brightness]], making it difficult to see from any [[urban]] or [[suburban]] location suffering from [[light pollution]].



==Location==
{{main|Local group|Andromeda-Milky Way collision}}
The Milky Way and the [[Andromeda Galaxy]] are a [[binary system (astronomy)|binary system]] of giant spiral galaxies. Together with their companion galaxies they form the [[Local Group]], a group of some 50 closely bound galaxies. The Local Group is part of the [[Virgo Supercluster]].

The Milky Way is orbited by two smaller galaxies and a number of [[dwarf galaxy|dwarf galaxies]] in the Local Group. The largest of these is the [[Large Magellanic Cloud]] with a diameter of 20,000 light-years. It has a close companion, the [[Small Magellanic Cloud]]. The [[Magellanic Stream]] is a peculiar streamer of neutral [[hydrogen]] gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Galaxy. 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 galaxy)|Leo II Dwarf]]. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way. Observations through the [[zone of avoidance]] are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far.

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 circle the Galaxy, causing vibrations at certain frequencies when they pass through its edges.<ref>{{cite press release
  | title = Milky Way galaxy is warped and vibrating like a drum
  | publisher = [[University of California, Berkeley]]
  | date = 2006-01-09
  | url = http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml
  | accessdate = 2007-10-18 
}}</ref>  Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account [[dark matter]], the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twentyfold increase in mass for the Galaxy. This calculation is according to a computer model made by Martin Weinberg of the [[University of Massachusetts, Amherst]]. In this model, the dark matter is spreading out from the galactic disc with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the Galaxy.

Current measurements suggest the [[Andromeda Galaxy]] is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, it is thought that the Sun and the other stars in the Milky Way will probably not collide with the stars of the Andromeda Galaxy, but that the two galaxies will merge to form a single [[elliptical galaxy]] over the course of about a billion years.<ref>{{cite news | first=Janet | last=Wong | title=Astrophysicist maps out our own galaxy's end | publisher=University of Toronto | date=April 14, 2000 | url=http://www.news.utoronto.ca/bin/000414b.asp | accessdate=2007-01-11 }}</ref>

===Velocity===
In the general sense, the absolute velocity of any object through space is not a meaningful question according to [[Albert Einstein|Einstein]]'s [[Special Theory of Relativity]], which declares that there is no "preferred" [[inertial frame of reference]] in space with which to compare the Galaxy's motion. (Motion must always be specified with respect to another object.)

Many astronomers believe the Milky Way is moving at approximately 600&nbsp;km per second relative to the observed locations of other nearby galaxies. Most recent estimates range from 130&nbsp;km/s to 1,000&nbsp;km/s. If the Galaxy is moving at 600&nbsp;km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from [[Pluto (dwarf planet)|Pluto]]. The Galaxy is thought to be moving towards the constellation [[Hydra (constellation)|Hydra]], and may someday become a close-knit member of the [[Virgo cluster]] of galaxies.

Another reference frame is provided by the [[Cosmic microwave background]] (CMB). The Milky Way is moving at around 552&nbsp;km/s<ref name="dipole">{{cite journal
 | author=Kogut, A.; Lineweaver, C.; Smoot, G. F.; Bennett, C. L.; Banday, A.; Boggess, N. W.; Cheng, E. S.; de Amici, G.; Fixsen, D. J.; Hinshaw, G.; Jackson, P. D.; Janssen, M.; Keegstra, P.; Loewenstein, K.; Lubin, P.; Mather, J. C.; Tenorio, L.; Weiss, R.; Wilkinson, D. T.; Wright, E. L.
 | title=Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps
 | journal=Astrophysical Journal
 | year=1993
 | volume=419
 | pages=1
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...419....1K
 | accessdate=2007-05-10 }}</ref> with respect to the photons of the CMB. This can be observed by satellites such as [[COBE]] and [[WMAP]] as a dipole contribution to the CMB, as photons in equilibrium at the CMB frame get [[Doppler effect|blue-shifted]] in the direction of the motion and [[redshift|red-shifted]] in the opposite direction.

===Relative to Earth===
The [[Sun]] (and therefore the [[Earth]] and [[Solar System]]) may be found close to the inner rim of the Galaxy's [[Orion Arm]], in the [[Local Fluff]], at a hypothesized distance of 7.62±0.32 [[Kiloparsec|kpc]] from the [[Galactic Center]].<ref name="distance1">{{cite journal
 | last = Reid
 | first = Mark J.
 | title=The distance to the center of the Galaxy
 | journal=Annual review of astronomy and astrophysics
 | year=1993
 | volume=31
 | pages=345–372
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ARA%26A..31..345R&amp
 | accessdate=2007-05-10 }}</ref><ref name="distance2">{{cite journal
 | author=Eisenhauer, F.; Schödel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T.
 | title=A Geometric Determination of the Distance to the Galactic Center
 | journal=The Astrophysical Journal
 | year=2003
 | volume=597
 | pages=L121–L124
 | url=http://adsabs.harvard.edu/abs/2003astro.ph..6220E
 | accessdate=2007-05-10 
}}</ref><ref name="distance3">{{cite journal
 | author=Horrobin, M.; Eisenhauer, F.; Tecza, M.; Thatte, N.; Genzel, R.; Abuter, R.; Iserlohe, C.; Schreiber, J.; Schegerer, A.; Lutz, D.; Ott, T.; Schödel, R.
 | title=First results from SPIFFI. I: The Galactic Center
 | journal=Astronomische Nachrichten
 | year=2004
 | volume=325
 | pages=120–123
 | url=http://www.mpe.mpg.de/SPIFFI/preprints/first_result_an1.pdf
 | format=PDF
 | accessdate=2007-05-10 
}}</ref><ref name="eisenhaueretal2005">{{cite journal 
| title = SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month 
| author = Eisenhauer, F. et al.
| journal = The Astrophysical Journal
| volume = 628
| issue = 1
| pages = 246-259
| year = 2005
| url = http://adsabs.harvard.edu/abs/2005ApJ...628..246E
| accessdate = 2007-08-12}}</ref>
The distance between the local arm and the next arm out, the [[Perseus Arm]], is about 6,500 light-years.<ref name="fn9">{{cite news
 | last = English
 | first = Jayanne
 | date = 2000-01-14
 | title = Exposing the Stuff Between the Stars
 | publisher = Hubble News Desk
 | date = 1991-07-24
 | url = http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html
 | accessdate = 2007-05-10 
}}</ref> The Sun, and thus the Solar System, is found in what scientists call the [[Habitable zone#The galactic habitable zone|galactic habitable zone]].

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 (constellation)|Hercules]], at an angle of roughly 60 sky degrees to the direction of the [[Galactic Center]]. The Sun's orbit around the Galaxy 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 (dampening) term.

It takes the Solar System about 225–250 million years to complete one orbit (a ''[[galactic year]]''),<ref name="fn10">{{cite web
 | last = Elert
 | first = Glenn
 | year=2002
 | url =http://hypertextbook.com/facts/2002/StacyLeong.shtml
 | title =Period of the Sun's Orbit around the Galaxy (Cosmic Year)
 | publisher =Hypertextbook
 | accessdate =2007-05-10 }}</ref> and so it is thought to have completed about 20–25 orbits during its lifetime or 0.0008 orbit since the [[origin of humans]]. The [[orbital speed]] of the solar system is 220 km/s, i.e., 1 light-year in ca. 1400 years, and 1&nbsp;[[Astronomical unit|AU]] in 8 days.<ref>{{cite book
  | last = Garlick
  | first = Mark Antony
  | title = The Story of the Solar System
  | publisher = [[Cambridge University]]
  | date = 2002
  | pages = 46
  | isbn = 0521803365 
}}</ref>



==History== 
===Etymology and beliefs===
There are many [[creation myth]]s around the world which explain the origin of the Milky Way and give it its name. The [[English language|English]] phrase is a translation from Greek Γαλαξίας, ''Galaxias'', which is derived from the word for milk (γάλα, ''gala''). This is also the origin of the word ''[[galaxy]]''. In [[Greek mythology|Greek myth]], the Milky Way was caused by milk spilt by [[Hera]] when suckled by [[Heracles]].

The term ''Milky Way'' first appeared in [[English literature]] in a poem by [[Geoffrey Chaucer|Chaucer]].
{{Quote|"See yonder, lo, the Galaxyë<br />&nbsp;Which men clepeth the Milky Wey,<br />&nbsp;For hit is whyt."|Geoffrey Chaucer|Geoffrey Chaucer ''[[The House of Fame]]'', ''c.'' 1380.<ref>{{cite web
| url = http://www.etymonline.com/index.php?term=galaxy
| title = Online Etymology Dictionary
| accessdate = 2007-01-03
}}</ref>}}

In a large area from [[Central Asia]] to [[Africa]], the name for the Milky Way is related to the word for [[straw]]. It has been claimed that this was spread by [[Arab]]s who in turn borrowed the word from [[Armenian language|Armenian]].<ref>{{cite journal
 | last = Harutyunyan
 | first =Hayk
 | title = The Armenian name of the Milky Way
 | journal = ArAS News
 | volume =6
 | publisher = Armenian Astronomical Society (ArAS)
 | date =[[2003-08-29]]
 | url = http://www.aras.am/ARASNEWS/arasnews06.html
 | accessdate = 2007-01-05 }}</ref> In several [[Uralic languages|Uralic]] and [[Turkic languages]] and in the [[Baltic languages]] the Milky Way is called the "Birds' Path". The [[Chinese language|Chinese]] name "Silver River" (銀河) is used throughout [[East Asia]], including [[Korea]]. In [[Japanese language|Japanese]], "Silver River" (銀河) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系) or the "River of Heaven" (天の川). In [[Swedish language|Swedish]], it is called ''Vintergatan'', or "Winter Street", because the stars in the belt were used to predict time of the approaching winter.

===Discovery===
:''See also:[[Galaxy#Observation history|Galaxy-Observation history]]''


The [[Greek philosophy|Greek philosopher]] [[Democritus]] (450–370 BC) was the first known person to propose that the Milky Way might consist of distant stars. Actual proof of this came in 1610 when [[Galileo Galilei]] used a [[Optical telescope|telescope]] to study the Milky Way and discovered that it was composed of a huge number of faint stars.<ref>{{cite web | author=J. J. O'Connor, E. F. Robertson |date = November 2002 | url = http://www-gap.dcs.st-and.ac.uk/~history/Biographies/Galileo.html | title = Galileo Galilei | publisher = University of St Andrews | accessdate = 2007-01-08 }}</ref> In a treatise in 1755, [[Immanuel Kant]], drawing on earlier work by [[Thomas Wright (astronomer)|Thomas Wright]], speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by [[gravitation|gravitational forces]] akin to the Solar System but on much larger scales. 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 [[nebula]]e visible in the night sky might be separate "galaxies" themselves, similar to our own.<ref name="our_galaxy">{{cite web | last = Evans | first = J. C. |date= [[November 24]] [[1998]] | url = http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm | title = Our Galaxy | publisher = George Mason University | accessdate = 2007-01-04 }}</ref>

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 sky. He produced a diagram of the shape of the Galaxy with the solar system close to the center.

In 1845, [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.<ref>{{cite web | last = Abbey | first = Lenny | url = http://labbey.com/Telescopes/Parsontown.html | title = The Earl of Rosse and the Leviathan of Parsontown | publisher = The Compleat Amateur Astronomer | accessdate = 2007-01-04 }}</ref>

In 1917, [[Heber Curtis]] had observed the nova [[S Andromedae]] within the "Great [[Andromeda (constellation)|Andromeda]] Nebula" ([[Messier object]] [[Andromeda Galaxy|M31]]). Searching the photographic record, he found 11 more [[nova]]e. Curtis noticed that these novae were, on average, 10 [[magnitude (astronomy)|magnitudes]] fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies.<ref>{{cite journal | author=Heber D. Curtis | authorlink=Heber Doust Curtis | title=Novae in Spiral Nebulae and the Island Universe Theory | journal=Publications of the Astronomical Society of the Pacific | year=1988 | volume=100 | pages=6 | url=http://adsabs.harvard.edu/abs/1988PASP..100....6C }}</ref> In 1920 the [[The Great Debate|Great Debate]] took place between [[Harlow Shapley]] and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant [[Doppler effect|Doppler shift]].<ref>{{cite web | first=Harold F. | last=Weaver | url = http://www.nap.edu/readingroom/books/biomems/rtrumpler.html | title = Robert Julius Trumpler | publisher = National Academy of Sciences | accessdate = 2007-01-05 }}</ref>

The matter was conclusively settled by [[Edwin Hubble]] in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some [[Cepheid variable]]s, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.<ref>{{cite journal | author=E. P. Hubble | authorlink=Edwin Hubble | title=A spiral nebula as a stellar system, Messier 31 | journal=Astrophysical JournalEngl | year=1929 | volume=69 | pages=103–158 | url=http://adsabs.harvard.edu/cgi-bin/bib_query?1929ApJ....69..103H}}</ref> In 1936 Hubble produced a classification system for galaxies that is used to this day, the [[Hubble sequence]].<ref>{{cite journal | last = Sandage | first = Allan | title=Edwin Hubble, 1889–1953 | journal=The Journal of the Royal Astronomical Society of Canada | year=1989 | volume=83 | issue=6 | url=http://antwrp.gsfc.nasa.gov/diamond_jubilee/1996/sandage_hubble.html | accessdate = 2007-01-08 }}</ref>

==References==
{{reflist|2}}

==External links==
*[http://www.spitzer.caltech.edu/Media/happenings/20070530/index.shtml Running Rings Around the Galaxy] Spitzer Space Telescope News
*[http://www.seds.org/messier/more/mw.html The Milky Way Galaxy], SEDS Messier pages
*[http://adc.gsfc.nasa.gov/mw/milkyway.html MultiWavelength Milky Way], NASA site with images and [[VRML]] models
*[http://ryutao.main.jp/english/film_milkyway_l.html The Milky Way at the Astro-Photography Site Of Mister T. Yoshida.]
*[http://www.perseus.gr/Astro-Milky-Way-20050710.htm Widefield Image of the Summer Milky Way]
*[http://www.atlasoftheuniverse.com/galaxy.html The Milky Way Galaxy] from ''An Atlas of the Universe''
*[http://www.solstation.com/x-objects/gal-ring.htm Proposed Ring around the Milky Way]
*[http://www.newscientist.com/news/news.jsp?id=ns99994959 Milky Way spiral gets an extra arm], New Scientist.com
*[http://skyandtelescope.com/news/archive/article_1254_1.asp Possible New Milky Way Spiral Arm], Sky and Telescope.com
*[http://www.sciencebits.com/ice-ages The Milky Way spiral arms and a possible climate connection]
*[http://www.spitzer.caltech.edu/Media/releases/ssc2006-02/ssc2006-02a.shtml Galactic center mosaic via sun-orbiting Spitzer infrared telescope]
*[http://www.ras.ucalgary.ca/CGPS/where/plan/ Milky Way Plan Views], The University of Calgary Radio Astronomy Laboratory
*[http://www.sciam.com/article.cfm?chanID=sa006&articleID=0000107A-6236-1FD5-A23683414B7F0000 Our Growing, Breathing Galaxy], Scientific American Magazine (January 2004 Issue)
*[http://www.skynightly.com/reports/Deriving_The_Shape_Of_The_Galactic_Stellar_Disc.html Deriving The Shape Of The Galactic Stellar Disc], SkyNightly (March 17, 2006)
*[http://www.digitalskyllc.com Digital Sky LLC], Digital Sky's Milky Way Panorama and other images
*[http://freeimages.reliable-facts.com/earth_and_space/pages/The_Milky_Way.htm A new view of the Milky Way galaxy] obtained by the Diffuse Infrared Background Experiment (DIRBE) on NASA's Cosmic Background Explorer satellite (COBE).
*[http://chandra.harvard.edu/photo/2006/w3/milkyway_ill.jpg Image of Milky Way galaxy arms], Chandra X-ray Observatory Center
*[http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate20.html The 1920 Shapley - Curtis Debate on the size of the Milky Way]
*Astronomy Picture of the Day:
**[http://antwrp.gsfc.nasa.gov/apod/ap051004.html Composite image of the Milky Way]
**[http://antwrp.gsfc.nasa.gov/apod/ap050104.html Milky Way Illustrated]
**[http://antwrp.gsfc.nasa.gov/apod/ap050825.html Barred Spiral Milky Way (Illustrated)]
**[http://antwrp.gsfc.nasa.gov/apod/ap951029.html Radioactive Clouds in the Milky Way]
**[http://antwrp.gsfc.nasa.gov/apod/ap970430.html Milky Way Molecule Map]
**[http://antwrp.gsfc.nasa.gov/apod/ap971105.html The Milky Way's Gamma-Ray Halo]