The universe is a huge space which contains everything that exists. The vast expanse of space that contains everything in it is known as the universe. It includes the sun, the planets, the Milky Way galaxy and all the other galaxies. The universe is continually expanding. Nobody knows how big the universe is. It is impossible to even have shown that the universe is getting bigger and bigger, like a balloon being blown up. The branch of science that deals with the study of heavenly bodies is called astronomy. People who study the universe are called astronomers. The most important 
instrument used by the astronomers is the telescope.              


  •     The universe is so vast in relation to the matter it contains that it can be compared with a building 20 miles long, 20 miles wide and 20 miles high that contains 1 grain of sand. 
  •     The three most common elements in the universe are hydrogen, helium and oxygen. 


    During the daytime you can see only the sun, which appears to move from East to West. 
    After sunset, the night sky is dotted with bright stars. At night the moon and thousands of stars are seen twinkling in the sky. If you watch the sky through a telescope you may see millions of stars, some of     which are brightly coloured. You see certain groups of stars which appear to be forming figures in clusters. They are called constellations. The moon is a natural satellite of earth whose size changes every       day. Moon completes one revolution around the earth in 27 days, 7 hours and 43 minutes. 
    Furthermore, you see an occasional comet, which appears as a ball of fire, having a tail. Then, there are shooting stars or meteors. They appear to fall from the sky. In addition to all these there are some            bright heavenly bodies which do not twinkle. They are called planets. 
    The natural bodies in the sky are called celestial bodies or heavenly bodies. Example– The sun, the earth, the moon, planets, stars, meteors, comets are called heavenly bodies or celestial bodies. 


    The three main theories are :-
(1)    Big-Bang theory :- According to this theory the universe was born after a massive explosion called the Big-Bang.  A long time after the Big-Bang, stars like our sun were formed. At that time clouds of hot gases and particles revolved around the sun. Over time, many particles got stuck together to form large bodies. The bodies pulled in smaller objects near them by gravitational force. "This made them larger still. These bodies finally become the planets and this universe has been expanding ever since".
(2)    Steady state Theory :- This theory states that the universe appears the same at all times and at all points. It is unchanging without a beginning or an end.
(3)    Pulsating Theory  :-  This theory states that all matter is flying apart from a heavily compacted mass and will eventually slow down, begin to contract and become so condensed that it will explode again and the expansion will start again.


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