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# Write a note on galaxy.

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* Stars are found in big clusters called galaxies.
* There are about 1011 galaxies in the universe and each galaxy contains about 1011 stars. Thus, there are in all about 1022 stars.
* Galaxies have generally two types of shapes: (i)Spiral (ii) Elliptical. Some galaxies have irregular shape.
* Most stars in elliptical galaxies are red indicating that they are very old. On the other hand the stars in spiral galaxies are bluish indicating that they are young. Thus, elliptical galaxies are older than the spiral galaxies.
* The number of spiral galaxies is greater than that of elliptical galaxies.
* The galaxy in which our solar system is situated is called the Milky Way. Milky Way appears as a milky belt stretching from North to South during dark night when the sky is clear.
* Milky Way is spiral in shape when viewed from the top. When viewed from the sides, it is seen bulging at the center and tapered towards the ends.
* The diameter of Milky Way is about 1 lakh (0.1 million) light years and the thickness of the middle (bulging) part is about 15000-20000 light years.
* Sun (and therefore the solar system)is located at about 30000 light years away from the galactic center.
* The sun completes one revolution around the galactic center in 22.5 crore (225 million) years at the speed of 250 km/s.