Do you know why a star shines? A star shines due to the fusion reactions taking place in it. In these reactions, hydrogen is converted into helium at an extremly high temperature. The fusion reaction produces heat and light and keeps the star hot. When the amount of hydrogen in the star begins to deplete, the star begins to die. The life span of all stars is different. Small stars, dimmer than the Sun, use their fuel for a long time and live more than 10,000 million years. But very hot and brilliant stars, which have a large amount of hydrogen, may die out in a few million years. These stars explode as a supernova, leaving wreckage like the Crab Nebula as shown in the figure. Other stars gradually blow their surface outwards like a red-hot cloud and reach the ‘red giant’ stage. In the far future, therefore, our Sun will also probably become a red giant and gradually die out.