Life Cycle of Stars

Stars initially form as a cloud of dust and gas

Gravity pulls the gas and dust together, which forms a protostar.

As the temperature rises, the star gets denser and more particles collide. When the temperature is high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei which gives off lots of energy.

This keeps the core of the star hot and a star is born.

Stars then enter a long, stable period. During it, outward pressure caused by thermal expansion (energy produced by fusion trying to expand the star) balances gravity pulling everything inwards. This typically lasts for several billion years.

The heavier the star, the shorter this period.

As the hydrogen in the core begins to run out, the force of gravity is larger than the pressure of thermal expansion. The star is compressed until it's dense and hot enough that the energy (and pressure) created makes the outer layers of the star expand. 

If the star is small, it becomes a red giant. If the star is big, it becomes a red supergiant. It becomes red as the surface cools.

A red giant then becomes unstable and ejects it's outer layer of dust and gas which leaves behind a hot, dense, solid core.

Red supergiants, however, glow brighty as they undergo more fusion to make heavier elements. They expand and contract as the balance shifts between gravity and thermal expansion. They eventually explode in a supernova.

Exploding supernovas throw outer layers of gas and dust into space, which leaves a very dense core.

If the star is big enough, it collapses and becomes a black hole which is a very dense point in space that nothing, including light, can escape from.