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| The sun and all the stars that we see are part of our Galaxy. A galaxy is a congregation of millions or billions of stars held together by gravity. Apart from the stars, a galaxy also contains dust and hydrogen gas. Galaxies are the building blocks of this universe. There are millions of galaxies besides ours in the universe. Some are huge while other, are dwarf galaxies. |
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| Galaxy |
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| Galaxies are shaped differently. The two common shapes of galaxies are: |
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 Spiral |
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 Elliptical |
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| Some of the galaxies, however, do not have a regular shape, so they are called irregular galaxies. |
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| We live on the outer edge of a spiral type of galaxy called the Milky Way, which is about 105 light years in diameter and is slowly rotating. |
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| Spiral galaxies are flattened in shape and have a bulge in the center consisting of old stars surrounded by a disk of younger stars, which are arranged in spiral arms |
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| Spiral Galaxies |
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| Elliptical galaxies are just flattened balls of old stars and very little gas. They include the most massive galaxies containing a trillion stars. |
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| Elliptical Galaxies |
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| Irregular galaxies have no particular shape at all. There are billions of galaxies in the universe. Huge quantities of heat, radiation, radio waves and X-rays pour out from the center of these galaxies. Galaxies are moving further and further apart as the universe continues expanding. |
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| Irregular Galaxies |
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| Milky way is the galaxy that includes the sun, the earth and the rest of our solar system. It also includes huge clouds of dust particles and gases that lie throughout it and depths of interstellar space. The term Milky Way also refers to the portion of the Milky Way galaxy that can be seen from the earth. This is the outer edge of the galaxy. On clear, dark nights, it appears to the naked eye as a broad, milky looking band of starlight stretching across the sky. |
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| Milky Way |
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| The Milky Way galaxy is shaped like a thin disk with a bulge in the center. Stars, dust and gases fan out from the central bulge in long, curving arms that form a spiral pattern. For this reason, astronomers classify the Milky Way galaxy as a spiral galaxy. |
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| The diameter of the Milky Way galaxy is about 105 light years. A light year is the distance that light travels in one year - about 9.4618km. The Milky Way is about 105 light years thick at the central bulge and much flatter towards the edges of the disk. The thick center represents a relatively high concentration of the stars at the center. Our solar system is located in the outskirts of the galaxy, about 25 x 103 light years away from the center. |
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| All stars and star clusters in the Milky Way orbit the center of the galaxy, much as the planets in our solar system orbit the sun. |
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 A galaxy is a collection of billions of stars whereas a constellation is a collection of only a very few stars. |
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 A galaxy does not form a definite pattern, which resembles the shape of an animal or a human being, but a constellation appears to resemble an animal or human being in its shape. |
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 There are billions of galaxies in this universe but only about 88 constellations are known at present. |
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