How does a star change over  time?

Medium stars

1. In medium size stars, after the nuclear fusion has used up all the fuel it has, gravity will pull the remaining material closer together.
2. The star will shrink. In fact, it may get to be only a few hundred km wide!
3. The star is then called a '' white dwarf''. It can stay like that for a long time.
4.  Eventually, it will stop producing light forever. Then it's called ''black dwarf''. 

Massive stars

1. In large size stars, nuclear fusion will continue until iron is formed.
2. In stars, iron acts like an energy sponge. It soaks up the star's energy.
3. This energy is eventually released in a big explosion called a supervona.
4. The little bit of matter that used to be at the centre of the star before the supervona will then be either a neutron star or a black hole. 
5. Which object it becomes depends on the size of the original star.
6. For it to be a black hole, it has to be way larger than our already-huge Sun!