The life of a star starts in huge clouds of dust and gas. The clouds swirl around and form clumps which shrink together and become very hot. A fully-formed star is made up of many tiny nuclear reactions. When you put the energy from all those reactions together, they form a star that gives off heat and light. You can tell how big and how hot a star is by looking at its colour and brightness.
Red stars are the coolest, yellow stars are in the middle and blue stars are the hottest.
Often blue stars are very young stars. There are many different kinds of stars. Our sun is a star - the nearest star to the earth. A white dwarf is a dying star that has collapsed to something around the size of a planet. A supernova is a star that has exploded for the last time.