Stars might change size and brightness over time but these are only minor changes. A major change happens when a star runs out of fuel. That is the end of its life. When the hydrogen in the middle of the star is gone, the nuclear reaction stops and the outward pressure from the star fades. When this happens, the inward force and the outward force no longer match, so the inward pressure shrinks the star. This happens to all stars, but what happens next depends on the star's mass and temperature.

White Dwarf Stars

Stars like the sun will cool at the center and shrink a little before an inward fall of material causes an explosion to take place. The small explosion will turn the star into a large, red star, or in other words, a red giant. When the center of the star contracts enough, the shrinking will stop. The star will brighten up again. A planetary nebula will burst from the surface and is then thrown outward. A planetary nebula is a sphere of gases. After that, the only thing remaining is a small star that will lose energy and become a white dwarf star. White dwarf stars are the size of a planet. Stars smaller than the sun will not create a planetary nebula, they will just shrink to a white dwarf star.

Supernovas

Stars larger than the sun will experience much different things when their nuclear reaction shuts down. These kinds of stars go through a quick shutdown of their nuclear reaction. If a star has more than eight to twelve times the mass of the sun, it will shrink like the Sun, but not for long. When the center of the star contracts to where it can't anymore, a shockwave happens throughout the star and throws out an irregular nebula. This is a supernova. The thing remaining is a neutron core, a very dense star that is as big as a city on Earth.

Black Holes

If a star has more than ten times the mass of the Sun, the remaining core from the supernova will shrink even smaller than a neutron core. The more mass the star has, the greater its gravity. The greater the gravity, the more force there is to pull the star's outer layer inward. The star will collapse so tightly that its gravity gets greater and greater. The gravity will become so strong that nothing can escape from it, even light can't escape from it. This is called a black hole. Scientists don't actually know if black holes do exist but a lot of evidence supports the theory.

Pulsars

The gases in the center of the neutron core left over from the supernova explosion spin thirty times per second. As it spins, the radiation beams out. The fast rotating star is a pulsar. We can learn about pulsars by studying light, heat, radio waves, and x-rays.