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On a clear night, try observing the sky. What do you observe? You will be able to see numerous stars twinkling in the sky. The position of these stars has been of great use to our ancestors in determining the time. These days we seldom use it for that purpose, but their study will help us to know how different celestial bodies were formed and how the universe came into being.

The vast surrounding space is called the universe. The universe includes everything that exists: the most distant stars, planets, satellites, as well as our own earth and all the objects on it. Nobody knows how big the universe is or whether it has any limits. The Sun which sustains all the life on our planet is one of the billions and billions of stars that exist in this universe, whereas the planet earth on which we live is only a tiny speck in this vast space called universe. The earth is one of the nine planets, all of which revolve around a central star called Sun. The billions of stars which exist in the universe are not distributed uniformly in space. These stars occur in the form of clusters (or groups) of billions of stars called galaxies. Thus, in order to study the constitution of this universe we have to first discuss the objects like galaxies, stars, planets and satellites, etc., which are found in the universe.


 Did you know?


The Sun is nearly 150,000,000 kilometers (150 million km) away from the Earth.

The next nearest star is Alpha Centauri. It is at a distance of about 40,000,000,000 km from the Earth. Some stars are even further away.

Such large distances are expressed in another unit known as light year. It is the distance traveled by light in one year. Remember that the speed of light is about 300,000 km per second. Thus the distance of the Sun from the earth may be said to be 8 light minutes. The distance of Alpha Centauri is about 4.3 light years.


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