oh one of my favorite subjects! Stellar evolution! So, ok. A star, as you probably know, is just a great big fusion factory, where it's turning one element to another via fusion. Well, that fusion isn't taking place EVERYWHERE. It's really just occurring at the center. At the star's core.

So at the center of a star is the core. A giant fusion reaction where hydrogen is being converted into helium. And around the core of the star is the star's shell. Fusion is NOT taking place in the shell. It's just a ball of hydrogen that is very very very very hot. So hot it GLOWS. And the star has two oppositional forces on it. The gravity of all the mass pulling it IN and the heat of the star pushing it OUT. And those two forces largely meet an equilibrium.

But EVENTUALLY the star's core runs out of fusible "stuff". And when that is and what's left in that core really depends on the star's size. Small stars don't have enough gravitational mass and pressure to keep fusion going below the hydrogen>helium stage. Others might be able to fuse helium and beyond, building up cores of carbon, nitrogen, oxygen or iron (this stops at iron for more complex reasons). So bigger stars might have enough density to keep the fusion process going but EVENTUALLY the fuel runs out. There's nothing left in the core to fuse.

And remember, stars are a balance of gravity pulling in and heat pushing out. And when a core exhausts its fuel, it cools. And as it cools, that balance of forces causes the star core to contract. And through some, honestly, pretty damn complex processes, the core's collapse causes a LOT of radiation in the form of heat to eject from the core and into the shell.

And that radiation of heat causes fusion reactions to ignite in the shell. now the SHELL. The layer of the star around its core, now that's causing fusion reactions, which is giving off MORE heat. And when the star's shell starts to heat up, it expands.

And that's the growing. Eventually the start either sorta just..fizzles away into space, or through a various and varied processes ejects its shell in an electromagnetic pulse that results in what's called a planetary nebula. And in either event all that's left is the core. A white dwarf. A solid ball of "something". Helium, carbon, oxygen, possible silicon. Or iron. Some base element between helium and iron. Dense, compact, about the size of a planet. No more fusion is occurring. The white dwarf is all that remains, cooling down to the ambient temperature of the universe.

Our own star will PROBABLY end with a carbon core. A carbon white dwarf. A planet size diamond floating in the abyss. Glittering in the starlight, a tombstone for the human race.

a Star is effectively a constant tug of war between gravity(trying to collapse it) and nuclear fusion( who's energy causing its outer layers to expand).

for a star like our Sun it spends most of its life cycle fusing Hydrogen into Helium , when it eventually runs out of hydrogen to fuse this tug of war goes in favor of gravity whihc will cause its core to collapse further(becoming denser).

However this collapse caused the now helium core ot be under so much pressure it once against become able ot enter nuclear fusion(as now gravity is strong enough to force helium atoms together.).

the fusion of Helium into carbon outputs more energy than the previous reaction cuasing the star's outlayers ot expand further, but because these layers are now further apart the star " cools off" a bit causing its visible light emissions to shift from yellow towards the red part of the spectrum. the Star became a Red giant.

if you go further than this as in the star exhausts is helium supply, the same process happens again, the core will collapse further but for a star this small gravity isnt capable of collapsing it enough to fuse carbon into oxygen, which causes it to release its outer layers(Fusion won the tug of war mentioned above) in the attempt: the star " died" and it just created a planetary nebula, what's left behind is the now dead core of the star a white dwarf which slowly bleed out whatever energy remains thru radiance.

During a star's main sequence, which is what the sun is in the middle of, there is a balancing act taking place between the crush of gravity pulling the star in, and the outward pressure of fusion pushing the star apart. As the star reaches the end of the main sequencing, the elements undergoing fusion change as all the hydrogen is converted to helium, and then the helium into carbon. This increases the outward pressure of the fusion, causing the star to swell

The sun right is powered now by fusing two hydrogen atoms and creating helium. The fusion creates a lot of heat and pressure outwards, which is balanced by the pressure from the mass of the star, keeping it from collapsing in on itself.

That hydrogen is finite, and it will eventually run out. “The end of its life” is the or the end of our suns main sequence, when it can no longer keep fusing hydrogen. It will begin collapsing in on itself and the buildup of pressure will begin another reaction — for a very short while it will be fusing helium. This reaction, at the very edge of the suns core will push outwards and overcome the gravity, enlarging the diameter of the sun quite drastically. It is expected that the earth will be eaten up bu the sun. But all the while, as the sun grows, it also cools. Eventually it will shrink and form a white dwarf that will cool and become ever so dimmer over billions of years.