While a star is on the main sequence it is turning hydrogen into helium at its center. That goes on until the hydrogen there is gone, and what is left in the middle is a ball of helium that is not hot enough to burn.
A core that makes no energy makes no pressure to hold itself up, so it contracts. Contraction releases gravitational energy, so the core gets hotter even though it is no longer burning.
That heat reaches the shell of unburnt hydrogen just outside the core, and the hydrogen there lights. Burning in a shell is fiercer than burning in the center was, because the shell sits on a hotter, denser core.
The extra output pushes the outer layers away, and the star swells to tens or hundreds of times its old size. Spread over so much more area, the surface cools and turns red. This is a red giant, and it is the move up and to the right on the HR diagram.
For a star around the mass of the sun this is nearly the end. The helium core does eventually light and burn to carbon, but the center never gets hot enough to take carbon any further.
The swollen outer layers are only loosely held. They drift off, a shell at a time, until the star has no envelope left. The glowing gas around the exposed core is called a planetary nebula, a name from the eighteenth century that only records what it looked like in a small telescope. It has nothing to do with planets.
Within tens of thousands of years the shell disperses into space. What is left behind is the core alone, about the size of the earth and holding something like half the mass of the sun.
It has no fusion and no source of new energy. This is a white dwarf, and everything that happens to it from here is cooling. It will fade for longer than the universe has so far existed.
Something must hold a white dwarf against its own gravity, and it is not heat. A white dwarf goes on cooling without shrinking, so whatever supports it does not weaken as the temperature drops.
The support comes from the electrons, and from a rule that has nothing to do with temperature: no two of them may occupy the same state. Pack them tightly and they are forced into states of higher and higher momentum, and that motion is a pressure. It is called electron degeneracy pressure.
It has an odd consequence. Adding mass makes the star smaller, since it has to be squeezed further before the pressure can match the extra weight. The relation is R \propto M^{-1/3}.
Squeeze further still and the electrons approach the speed of light, and past that point the pressure stops keeping up. Working it through, the radius falls to zero at 1.44 M_\odot. This is the Chandrasekhar limit, and no white dwarf heavier than it can exist.
A star above about 8 M_\odot has a center hot enough to carry on past carbon. It burns to neon, to oxygen, to silicon, each stage faster than the last, and finishes by making iron.
Iron is where fusion stops paying. Fusing it takes energy in rather than giving it out, as the next article sets out. So the iron core makes no heat and stands on degeneracy pressure alone, while the shells around it keep adding to its weight.
When the core crosses the Chandrasekhar limit, it falls. The collapse takes less than a second and brings the core down to the density of an atomic nucleus, where it stiffens abruptly and stops.
The rest of the star is still falling onto it. It meets that wall, rebounds, and the outer layers are blown clear. For weeks the wreck shines as brightly as the whole galaxy around it. This is a supernova.
Three endings, and which one a star gets is settled by the mass it was born with. Up to about 8 M_\odot it becomes a white dwarf. From there to roughly 25 M_\odot it leaves a neutron star. Above that, a black hole. The boundaries are approximate and depend on composition as well.
A neutron star is what the collapsed core becomes when the electrons are pressed into the protons and neutrons are left. It holds more than a solar mass inside a radius of about 10 km. A piece of it the size of a sugar cube would weigh a billion tonnes.
If even the neutrons cannot hold, nothing else will, and the collapse has no stopping point. What remains is a region from which light cannot escape, bounded at R_{\mathrm{s}} = \dfrac{2GM}{c^2}. For a mass of one sun that radius is about 3 km.
The material blown off does not vanish. It returns to the space between the stars, carrying the elements that were built inside, and it is what the next generation of stars is made from.