Friday, December 11, 2009

Galaxy

What is a Galaxy?
Galaxy is a collection of stars held together by their mutual galaxy. A group of billions of stars and their planets, gas, and dust that extends over many thousands of light-years and forms a unit within the universe. Held together by gravitational forces. Most of the estimated 50 billion galaxies are shaped as spirals and ellipses, with the remainder being asymmetric.
We are in Milky way galaxy and it is spiral galaxy. It has a bright central core with high density of stars and then a flattened disk surrounding it – like a spinning record. The Milky Way measures about 100,000 light-years across, and is thought to contain 200-400 billion stars. But the stars we can see are just a tiny fraction of the complete galaxy. It's also surrounded by a vast halo of dark matter. This material is invisible, and doesn't interact with regular matter or give off any kind of radiation that we can detect. But astronomers can measure its effects because it does exert a gravitational force on other matter. In fact, the Milky Way is made up of mostly dark matter. The stars account for about 580 billion solar masses, and the dark matter could be another 6 trillion solar masses.
A good example for elliptical shaped galaxy is the galaxy M87. It's thought to have 2.7 trillion stars.
Stars are collected together into galaxies. Galaxies are collected together into groups of galaxies, and these groups are collected into clusters. The largest structures in the Universe are galaxy super clusters, which contain millions of galaxies and can measure hundreds of millions of light-years across.
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Did you know the moon of Pluto?
CHARON
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Did you know the moons of Mars?
Phobos and Deimos
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Source: http://www.universetoday.com/guide-to-space/galaxies/what-is-a-galaxy/

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