Galaxies - Research Article from Macmillan Science Library: Space Sciences

This encyclopedia article consists of approximately 9 pages of information about Galaxies.
This section contains 2,401 words
(approx. 9 pages at 300 words per page)
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Galaxies

Galaxies are collections of stars, gas, and dust, combined with some unknown form of dark matter, all bound together by gravity. The visible parts come in a variety of sizes, ranging from a few thousand light years with a billion stars, to 100,000 light-years with a trillion stars. Our own Milky Way galaxy contains about 200 billion stars.

Types of Galaxies

The invisible parts of galaxies are known to exist only because of their influence on the motions of the visible parts. Stars and gas rotate around galaxy centers too fast to be gravitationally bound by their own mass, so dark matter has to be present to hold it together. Scientists do not yet know the size of the dark matter halos of galaxies; they might extend over ten times the extent of the visible galaxy. What we see in our telescopes as a giant galaxy of stars may be...

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This section contains 2,401 words
(approx. 9 pages at 300 words per page)
Buy the Galaxies Encyclopedia Article
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Macmillan
Galaxies from Macmillan. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.
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