The vast space surrounding us is called universe. It is mostly empty space. 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. However, it is estimated that the Universe contains 100 billion galaxies, each of which comprises 100 billion stars. The sun which sustains all the life on our planet is only 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 eight planets, all of which revolve around a central star called sun. The billions stars which exists 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.