A galaxy is a massive collection of stars, gases, dust and dark matter held together by gravity. A theory of how galaxies formed says that after the big bang - - the original rapid expansion at the inception of the universe - - gas accumulated in protogalactic clouds. Gravity condensed the dust and gas in these clouds into rotating disks. These disks attracted more dust and gas, resulting in the formation of galactic disks. Stars formed in these disks, leaving an outside layer of gas and dust.
Galaxies come in many shapes and sizes, ranging from 10 million to 10 trillion stars. The shape of the galaxy can be elliptical or spiral. Elliptical galaxies, which probably account for 60 percent of the total number of galaxies, are rounded and have no gas or dust in their orbits, and no bright stars we can see. Spiral galaxies such as our Milky Way are disk-shaped and bright with gas, dust and visible stars.
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