nebula
A nebula is a cloud of gas and dust in space. They are mainly full of hydrogen and some helium. Gravity can pull clumps of gas together to form a ...
protostar
A protostar is a clump of gas that has been pulled together by gravity. As gas is attracted into the protostar, it heats up. If the core of the protostar gets hot enough, hydrogen can start to fuse into helium. The protostar has now become a...
main sequence star
Main sequence stars fuse hydrogen into helium. This releases energy, so they shine. Stars contain lots of hydrogen, so they can fuse hydrogen into helium for billions of years before they run out.
The stars remain stable, because inward forces due to gravity are balanced by outward forces due to gas pressure and radiation pressure.
When the star runs out of hydrogen to fuse, it starts to collapse. The core gets hotter until it can start to fuse helium.
red giant or red supergiant
These stars fuse helium into heavier elements. Although the core is hotter, the rest of the star expands to hundreds of times its previous size, and the surface is cooler than it was as a main sequence star.
When red giants run out of helium, they contract and heat up, but not get hot enough to start further fusion. They become white dwarfs and gradually cool down until they are black dwarfs.
When red supergiants run out of helium, they collapse and get hot enough to start fusing heavier elements, so they expand again, etc etc. This continues until they make iron - after this no more fusion occurs, and the star collapses for a final time. This causes a supernova explosion, which is hot enough to fuse together elements heavier than iron. The remains of the star form a neutron star or a black hole.