Constituents of Universe
The limitless expanse of space around us consisting of the solar system, stars, galaxies etc., is called the universe.
Important constituents of the universe as given below-
1. Solar system. It is the name given to the family of the sun and is part of the universe in which we live. The sun is the centre of the solar system around which nine heavenly bodies (called planets) revolve in elliptical orbits. The velocity of the planets and their distances from the sun keep on changing during rotation. The speed is the highest when they are closest to the sun and slowest when farthest away. The gravitational pull of the sun on the planets controls their motion.
These planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto* in the order of their increasing distance from the sun. There are many other heavenly bodies, which revolve around the planets called satellites (or moons) of the planets. The moon, which we seen at night is the satellite of the earth. The solar system also includes asteroids, comets and meteors.
2. Stars. A star is a fiery luminous heavenly body. The temperature of the star is very high and it emits light of its own. Our sun is one of the nearest stars that we can see. Its light reaches the earth in 8-3 minutes. Some of the stars are extremely large that they are equivalent to many suns put together. On a clear night, we can see about 2500 stars at the time through the total number is much more. These stars may occur as single star, in pairs or in clusters. After the sun, the nearest star is Alpha Centauri. It is at a distance 4-3 light years from the earth.
3. Galaxies. A galaxy is a large group of stars, dust and gas held together by mutual gravitational forces. Milky way or Akashganga is one such galaxy in which we live. It can be seen on a clear night as a hazy band of white light stretching across the sky. Our sun and other stars, which can be seen by naked eye, belong to this Milky Way, which contains more than a hundred billon stars. There are million of such galaxies in the universe which can be seen through large telescope.
We will now see each constituent of universe in detail starting with the solar system.