Stars change over time. Stars like the Sun are known as red dwarfs. These are the most common class of star in our galaxy.
It will spend most of its life—about 100 billion years—fusing hydrogen into helium for energy. When it has used up its hydrogen, it will swell up into a red giant and burn its helium.
When its helium is gone, the star will begin to contract, through the force of self-gravity. It will then move on to become a white dwarf, which signals the beginning of its decline. Finally it dies out and becomes what is known as a black dwarf, a dead star.
The largest stars, with great mass, have shorter lives and more violent ends, sometimes becoming black holes. No one knows for sure what black holes are, but most scientists believe they occur as a result of the crush of gravity on the mass of a great star.
The star collapses on itself, essentially becoming nothing but gravity, which continues to pull on anything passing by, even light! That’s why it’s called a black hole.