Now let’s see how galaxies are classified and the characteristics of each of these types:
1) Spiral Galaxies: A spiral galaxy appears like a huge disk (bright spiral arms) emerging out from a central bulge. The Milky Way is a giant spiral galaxy, 100,000 light years in diameter and a mass of approximately a trillion times that of the sun. Spiral Galaxies rotate slowly. The Milky Way takes about 250 million years to make a complete rotation. In the spiral galaxies new stars are constantly born from dust and gas.
2) Elliptical Galaxies: It has been found that elliptical galaxies are formed when all the gas is made into stars before it has time to form a disk. They are oval shaped and are composed of older stars, since few new stars form in them. They rotate very slowly or sometimes do not rotate at all. They appear to be very bright at the center and grow faint at the outer edges.
3) Irregular Galaxies: They have no particular shape and are mostly chaotic in appearance. They make up about a quarter of all galaxies in the universe and consist of a number of bright young stars. Some of these galaxies might once have been spiral or elliptical in shape but were deformed due to gravitational action.
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