This is the process that causes the star to shine and basically what keeps it alive.

Main Sequence Stars (Sun) {a lot brighter and hotter} -> Red Giant -> White Dwarf + Planetery Nebula

Massive Stars -> Hypergiants (Blue and Yellow) -> Supernova + Neutron Star or (if big enough) Black Hole

At this moment the star explodes and then leaves behind a nebula from which a lot of different things can be formed.

A star shines because of the fusion of the elements (nuclear fusion) in its core, this fusion emits a lot of energy, consequently making the star bright and hot.

Hydrogen fusion generates Helium.

When the star runs out of Hydrogen to fuse it starts to use Helium (Carbon and Oxygen) and since it is a stronger energy the force of gravity gets weaker and that causes the core to swell and expand. Then because of that, the star becomes what we call a Red Giant.

Incredibly, the only thing that holds stars together is gravity and without it they wouldn't even be formed.

The core of this star is formed by protons and electrons that were forced together and because of that became neutrons.

Taking that into consideration one of the biggest stars currently known in our universe called WOH G64 is about 1,540 times the size of our Sun. This star is very interesting since around it there's a huge layer of dust that surrounds it.

Even though it's gigantic size this star is a lot cooler than our Sun, 3,100°C meanwhile the Sun has a surface temperature of around 5,226.85°C. This star when out of fuel will cause a huge supernova and can even become a black hole.

This shows how complex and unique stars are, and that proves that there isn't only one way of categorizing and defining stars.

Stars form from nebulas in the galaxy, as soon as the star is born some things can be concluded, depending on the size of the star it has different future fates.

Main sequence stars such as our Sun after a lot of nuclear fusion will become a red giant, then later slowly its outer layers will expand until all that's left is a white dwarf.

However, if the star is bigger than the first it will also become a red hypergiant. Still, two different things can happen: it will either end its life with a supernova explosion and leave behind only its core (neutron star) or become a black hole.