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# Milky Way

A green and red Perseid meteor streaks across the sky just below the Milky Way in August 2007.
Infrared image of the core of the Milky Way Galaxy
The galactic center in the direction of Sagittarius. The primary stars of Sagittarius are indicated in red.
"Laser towards the Milky Way". Despite the first impression of a science-fiction movie, this extraordinary image is of recent cutting-edge research. It was taken at the high-altitude Paranal Observatory in Chile. The laser reaches 90 km into the Earth's mesosphere to form a reference point. It helps sharper observations of phenomena such as the giant black hole at the centre of the galaxy.

The Milky Way is our home galaxy. It contains over 200 billion stars,[1][2][3][4][5] including our Sun.[6]

The Milky Way has a diameter of 100,000 light years,[1] and is a barred spiral galaxy. The discovery of the Milky Way goes back to the Ancient Greek philosopher Democritus.[6] The Milky Way has three main parts: a disk, in which the Solar System resides, a bulge at the core, and an all encompassing halo.[7]

This galaxy belongs to the Local Group of three large galaxies and over 50 smaller galaxies. The Milky Way is one of the largest galaxies in the group, second to the Andromeda Galaxy.[1] Milky Way's closest neighbour is Canis Major Dwarf, which is about 25,000 light years away from the Earth. The Andromeda Galaxy moves towards the Milky Way Galaxy, and will meet it in about 3.75 billion years.[8] Andromeda Galaxy moves with a speed of about 1,800 kilometres per minute.[6]

## Size

The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years (9×1017 km) in diameter, and is considered to be, on average, about 1000 light years thick.[9]

It is estimated to contain at least 200 billion stars[10] and possibly up to 400 billion stars.[11] The figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially more than 300 light years from our sun. Therefore, present estimates of the total number are uncertain. This can be compared to the one trillion (1012) stars of the neighbouring Andromeda Galaxy.[12]

The stellar disc of the Milky Way does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the Galaxy. Beyond a radius of about 40,000 light years, the number of stars drops much faster, for reasons that are not understood.[13]

Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12000 light years–twice the previously accepted value.[14] As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a US quarter (about an inch in diameter), the Milky Way would have a diameter of 2,000 kilometers.[15] At 220 kilometers per second it takes the Solar System about 240 million years to complete one orbit of the Galaxy (a galactic year).[16]

The Galactic halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach is at about 180,000 light years.[17] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way.

## Galactic center

Observed spiral structure of the Milky Way galaxy.[18] The arrow points the direction of the solar system's motion relative to the spiral arms.
The Milky Way's spiral arms. Our Sun is in the Orion–Cygnus Arm.

The galactic disc, which bulges outward at the galactic center, has a diameter of 70–100,000 light years.[19]

The exact distance from the Sun to the galactic center is debated. The latest estimates give distances to the Galactic center of 25–28,000 light years.[20][21][22][23]

Movement of material around the galactic center shows that it has a compact object of very large mass.[24] The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is now confirmed to be a supermassive black hole.[25] Most galaxies are believed to have a supermassive black hole at their center.[26]

The nature of the galaxy's bar is also actively debated, with estimates for its half-length and orientation spanning from 3,300–16,000 light years (short or a long bar) and 10–50 degrees.[22][23][27] Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.[28]

## References

1. "The Milky Way Galaxy". seds.org. Retrieved 5 August 2010.
2. "The Milky Way Galaxy". cass.ucsd.edu. Retrieved 5 August 2010.
3. "NASA - Galaxy". nasa.gov. Retrieved 5 August 2010.
4. "01.09.2006 - Milky Way Galaxy is warped and vibrating like a drum". berkeley.edu. Retrieved 5 August 2010.
5. "How many stars are in the Milky Way?". universetoday.com. Retrieved 5 August 2010.
6. "Milky Way; Discovery of Milky Way; Milky Way Galaxy. SPACE.com". space.com. Retrieved 5 August 2010.
7. "Our own Galaxy - the Milky Way". University of Cambridge. 1996. Retrieved 2012-11-12.
8. NASA 2012. NASA's Hubble shows Milky Way is destined for head-on collision. [1]
9. Christian, Eric; Safi-Harb, Samar. "How large is the Milky Way?". NASA: Ask an Astrophysicist. Retrieved 2007-11-28.
10. Sanders, Robert (2006). "Milky Way Galaxy is warped and vibrating like a drum". UCBerkeley News. Retrieved 2006-05-24.
11. Frommert H; Kronberg C. (2005). "The Milky Way Galaxy". SEDS. Retrieved 2007-05-09.
12. Young, Kelly (2006). "Andromeda galaxy hosts a trillion stars". NewScientist. Retrieved 2006-06-08.
13. "[0909.3857] The structure of the outer galactic disc as revealed by IPHAS early A stars". Arxiv.org. 2009. Retrieved 2010-08-10.
14. "Milky Way fatter than first thought". The Sydney Morning Herald. Australian Associated Press. 2008. Retrieved 2008-04-24.
15. "How Big is Our Universe: How far is it across the Milky Way?". NASA-Smithsonian Education Forum on the Structure and Evolution of the Universe, at the Harvard Smithsonian Center for Astrophysics. Retrieved March 13, 2013.
16. Sparke, Linda S. & Gallagher, John S. 2007. Galaxies in the Universe: an introduction, p. 90. ISBN 9781139462389
17. Connors, et al.; Kawata, Daisuke; Gibson, Brad K. (2007). "N-body simulations of the Magellanic stream". Monthly Notices of the Royal Astronomical Society 371: 108. . Retrieved 2007-01-26.
18. Taylor J.H. & Cordes J.M 1993. Astrophysical Journal 411: 674-684
19. Grant J. & Lin B. (2000). "The stars of the Milky Way". Fairfax Public Access Corporation. Retrieved 2007-05-09.
20. Eisenhauer F. et al 2005. SINFONI in the Galactic Center: young stars and infrared flares in the central light-month. The Astrophysical Journal 628 (1): 246–259. doi:10.1086/430667. [2] Retrieved 2007-08-12.
21. Gillessen S. et al 2009. "Monitoring stellar orbits around the massive black hole in the Galactic center". The Astrophysical Journal. 692
22. Vanhollebeke E; Groenewegen M.A.T; Girardi L. 2009. "Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL". A&A. 498
23. Majaess D. 2010. "Concerning the distance to the center of the Milky Way and its structure". Acta A. 60
24. Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An introduction to galaxies and cosmology. Cambridge University Press. pp. 50–51. .
25. For a photo see Chandra X-ray Observatory; Jan. 6 2003
26. Blandford R.D. 1999. Origin and evolution of massive black holes in galactic nuclei. In Galaxy Dynamics, ASP Conference Series vol. 182 [3]
27. Cabrera-Lavers A. et al. 1998. "The long Galactic bar as seen by UKIDSS Galactic plane survey". A&A. 491
28. Staff (2005). "Introduction: Galactic Ring Survey". Boston University. Retrieved 2007-05-10.