Mass of the Milky Way

The Physics Factbook
Edited by Glenn Elert -- Written by his students
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Bibliographic Entry Result
(w/surrounding text)
Shipman, Harry L. The Restless Universe: An Introduction to Astronomy. Boston: Houghton Mifflin, 1978: 368. "Our own Milky Way is a spiral of average mass (200 billion solar masses)." 200 billion
solar masses
Lerner, Rita G. & George L. Trigg. Encyclopedia of Physics. 2nd Edition. New York: VCH, 1991: 736. "The total mass of the stars and gas in our galaxy is about 200 billion solar masses … its total mass may be 1000 billion solar masses." 200 billion
solar masses
(visible disk)

1000 billion
solar masses
(within halo)
 "A Macho Milky Way?" Sky & Telescope. 91, 6 (June 1996): 16. "Our galaxy contains 490 +/- 110 billion Suns' worth of mass." 380–600 billion
solar masses
Inglis, Stuart J. Planets, Stars, and Galaxies: An Introduction to Astronomy-3rd Edition. New York: Wiley, 1972: 433. "It is estimated that our galaxy has a mass of 200 billion suns!" 200 billion
solar masses
Imamura, James. Mass of the Milky Way. Astronomy 123. Cosmology. "The mass contained in the Milky Way (in the visible disk) is 2x1011 M(Sun). [T]he mass contained in the Milky Way galaxy (out to as far as we can see HI gas) is 6x1011 M(Sun)." 200 billion
solar masses
(visible disk)

600 billion
solar masses
(HI gas)

As we know, a galaxy is a huge collection of millions, billions, or trillions of stars. The galaxy we live in is called the Milky Way. The name comes from the literal translation of the Latin "Via Galactica", with via meaning "road"or "way"and the word galaxy from the Greek root "gala", meaning "milk". The Milky Way galaxy is composed of about 400 billion stars and is about 100,000 light years in diameter.

The Milky Way is classified as a spiral galaxy and is composed of three main regions:

  1. the disk, where the solar system is located,
  2. the central bulge at the core, which is densely packed with stars and presumed to have a black hole at its center, and
  3. a halo,a diffuse region with a low density of stars, that surrounds everything. The halo is believed to be composed mainly of dark matter that exceeds even beyond its ends.

The total mass of the Milky Way is assumed to be at least 600 billion times the mass of the sun, while the densely packed visible part is only 200 billion times the mass of the sun. This discrepancy in numbers is believed to be caused by the dark matter in the halo, since it seems to be taking up mass, but doesn't emit or radiate light. This "missing mass"accounts for almost 90% of the mass in the universe. Even though scientists don't know what it is, they know it's there because they can detect it by the gravitational effect it has on the surrounding visible objects.

Unfortunately, this unknown dark matter is also the determining factor in the evolutionary future of the universe. If there is too little of it to gravitationally bind the the universe together, it can continue expanding forever. If there is enough, though, the universe might slow down the expansion, come to a halt, and begin to contract and eventually collapse. This is why it is so important to find out just what dark matter is and how much of it there is.

Alina Vayntrub -- 2000

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