Any star starts as a nebula, which is a big area of gas and dust.
When the dust gathers in one spot to create a mass the star is born.
As the high mass star lives its life, gravity pushes in, and keeps pushing in. Once the star can't take anymore it expands, and pushes gravity further. It is now a Red Giant.
As a few million more years pass, the star implodes, going supernova!
The star can become a Neutron star, or a black hole that has a gravitational force so strong, not even light can escape.
After the Supernova is over what's left of the star will be one of two things.
