Stars and how elements are created

This is our Sun. A giant ball of glowing hot plasma.

As you might already have heard, our Sun gets the energy to shine from nuclear processes in its core, where it turns Hydrogen into Helium.
The Sun is (or was) made mostly of Hydrogen, as are all stars as far as we know.

The Sun is so big and massive, that it has a tremendous amount of gravitational pull on itself.
The Sun compresses itself, and it would be a lot smaller than it is if it weren't for nuclear fusion.

In the Sun's core, atoms are pushed together very very hard.
Some of them are pushed so close together, that they become one - they fusion together into heavier elements, and release a lot of energy in that process.

This energy radiates out from the Sun's core.
It is so immense that it pushes on the outer layers, releaving a bit of the gravitational pressure and thereby keeping the Sun from collapsing in on itself.

This is a Brown Dwarf, a failed star.

It is just big enough to create sufficient heat for turning it into a ball of plasma, but not enough to maintain nuclear fusion.
A brown dwarf will slowly radiate away whatever energy it has, and will eventually have lost enough mass to not even qualify as a star. It will be a huge rocky/gas planet of some kind.

After it has stopped all processes and lost all it's heat, it will slowly - very very slowly, via quantum tunnelling - decay into balls of iron floating in the universe.
This process will happen so slowly that calling it "forever" is okay.

This is a Red Dwarf. Slightly larger than a brown dwarf. It can shine.

Red dwarfs can sustain low amounts of nuclear fusion. They will shine for a very very very long time, and will probably be the last stars in the universe.

They don't shine very bright. In fact, their light is almost entirely in the red end of the spectrum.
These stars do have habitable zones, an orbit around the star where liquid water is possible.
These orbits will be very close to the stars, though, and will tidally lock any planet - just like the Moon is tidally locked to Earth: We ever only see one side.

More information about red dwarfs:
https://www.youtube.com/watch?v=LS-VPyLaJFM

Our Sun is barely visible to the left in this picture. There are some really big stars out there!

The larger a star is, the shorter lifespand it will have.
The small stars will shine almost forever, whereas the biggest stars in the universe will only shine for a few million years.

This is due to fusion processes and mass:
The heavier a star is, the more it will compress itself with gravity. And the more outgoing energy it needs to have, to keep it from collapsing.
The biggest stars will use up most of their Hydrogen within a few hundred thousand years.

When that happens, the energy-level in the core drops and the star will begin to feel heavier again. There is less energy pushing out from the center.

This means the core is now crushed even more from the entire mass of the star, and at some point it has enough pressure and heat to begin fusioning the Helium into Carbon.

Our Sun will do this too, in about 5 billion years time.
When our Sun has used up all of its Helium, the nuclear processes in the core will stop and the Sun will collapse into a very dense, very hot ball of coal. Or diamond.

The biggest stars don't stop here.
When most of the Helium is used, the star will once again undergo a transformation where it puts more and more pressure on the core until fusion begins again.

Depending on how large the star is, this process will repeat itself and create heavier and heavier elements:
-Carbon plus Helium produces Oxygen.
-Oxygen plus Helium produces Neon.
-Neon plus Helium produces Magnesium.
-Magnesium plus Helium produces Silicon.
-Silicon plus Helium produces Sulfur.
-Sulfur plus Helium produces Argon.
-Argon plus Helium produces Calcium.
-Calcium plus Helium produces Titanium.
-Titanium plus Helium produces Chromium.
-Chromium plus Helium produces Iron.

Exactly how big are some stars:
https://www.youtube.com/watch?v=GoW8Tf7hTGA

When a star creates Iron, it is doomed. It will end its life in an epic explosion: A supernova.

Iron absorbs pressure. And it takes insane amounts of energy to fusion it into heavier elements.
No matter how much the star compresses the core, no further fusion will happen.
The star loses all its power, and gravity takes over.
There is no more radiation coming from the core, and therefor nothing to withstand the inwards pull of gravity.

The star collapses, the outer layers begin to fall towards the center of the star.
The core will be compressed more and more, and at some point it will have enough built-up energy to explode:
The individual atoms are ripped apart, repelling each other with so much energy that the core literally explodes and send all the atoms out in all directions with almost the speed of light.

The outer layers are still falling in, and have by now they have gained some momentum.
These 2 fronts collide, and right here in that instant all elements heavier than Iron is created.

Some of the outer layers are blown away from the star, but on the really big stars the vast majority of the outer layers is still on its way to the star center,These gasses are moving increasingly faster all the time because gravity gets stronger and stronger.
The more the star is compressed, the heavier gravitational pull it will have.
This causes the star to compress even more, increasing the pull even more, causing even more compression, etc....
The outer layers are heavy and they are moving very fast!

The outer layers are so massive that they contain the explosion of the core.
The core is pushed back in on itself, causing it to collapse with almost the same energy as it exploded with.
It will be compressed beyond anything we've ever been able to do here on Earth.

At this point, strange things happen:
The star is so compressed, that it enters a new state.
It no longer makes sense to talk about atoms or elements.
The entire star is made of only 1 type of particle: Neutrons.
This is a Neutron Star.

If the star is even bigger, it will not stop here:
If gravitational pull is strong enough, it will compress even the neutron star. Make it increasingly smaller and smaller, until eventually it no longer has any volume or surface.
It will disappear from our universe as a Singularity and in its place will be a black hole.

Nobody knows what goes on inside a black hole. Those are way beyond the reach of science. If you ever go into one, you will never be able to come out and tell anyone about it.

How big is the observable Universe exactly:
https://www.youtube.com/watch?v=mcBV-cXVWFw

--- EDIT ---
Bonus info:
The Crab Nebula, the real remains of a supernova:
https://imgur.com/gallery/gjquqiM