{♫Intro♫}.

Since they can live for billions of years, stars might seem eternal. But like all things, their lives will eventually come to an end.

In general, a star's size will determine its final destiny. Less massive stars live longer and die kind of peacefully as white dwarfs, while more massive stars live fast and go out with a bang, in a brilliant supernova. But for those in the middle, things can be more uncertain.

Some of these stars fizzle out, while others explode. And what seals their fate may come down to a curious, cannibalistic process happening inside their cores. Stars spend the first part of their lives fusing hydrogen into heavier elements, and when they've fused all the hydrogen they've got, they start to die.

For main-sequence stars like our Sun, and those up to about eight times its mass, this process is relatively chill. These stars don't have enough pressure or heat in their cores to fuse anything besides hydrogen. So when that hydrogen runs low, fusion splutters to a halt.

Without the heat energy from fusion, the star's core collapses to form a white dwarf: a star the same mass as the original, but squished into a ball about the size of the Earth. Since it's so dense, this thing's inclination is to collapse even more, but it's prevented from doing that by fast-moving electrons, which repel each other and create enough outward pressure to support the star's outer layers. Thanks to this, the white dwarf stays stable and gradually cools down over billions of years.

But for bigger stars, with at least 10 solar masses, death is more complex and dramatic. Their extra mass puts more pressure on the core and creates higher temperatures, so they can fuse heavier elements. This starts with helium and moves up the periodic table, but once it reaches iron, everything changes.

All of the other fusion reactions release energy, but the fusion of iron actually absorbs it. That's because the atomic structure of iron is incredibly stable, and it takes a huge amount of energy to overcome that. Without energy production, the star's core collapses at almost the speed of light, going from 8000 kilometers in diameter to just 20 kilometers in a fraction of a second.

And that causes the star's temperature to soar to about 100 billion degrees Celsius. The outer layers of the star also collapse, but then, they rebound. And the result is an aggressively bright supernova.

But what about those intermediate stars, between eight and ten solar masses? For these, size isn't always a perfect predictor of fate. Like, instead of dying as white dwarfs, some of the less massive stars can meet a more destructive and exotic end.

This requires some weird chemical processes to happen inside their cores — like, say, if a star starts cannibalizing its own electrons. Which, apparently, can actually happen. See, stars just over eight solar masses aren't hot enough to make iron, but they can fuse enough elements to get cores rich in neon and magnesium.

At this point, there are also lots of free electrons in the core, whizzing around and supporting the outer layers, just like they do in white dwarfs. But once the core reaches about 1.4 times the mass of our Sun, or the Chandrasekhar limit, a new process begins. The neon and magnesium in the core capture and consume the free electrons that are whizzing around.

Now, that might come as a bit of a surprise, since neon especially is famous for not reacting with anything on Earth. The electron shells that surround its atomic nucleus are already full, so there's no need for neon to combine with any other elements to complete its structure. That makes it super stable.

But the neon in these dying stars isn't using the electrons to complete its atomic structure: It's using them to change its structure entirely. The intense temperatures and pressures in these stellar cores are enough to start a process called electron capture. It's a type of radioactive decay, where a proton in an atom's nucleus captures a nearby electron, and becomes a neutron.

So this neon atom, which starts out with 10 protons and 10 neutrons, captures an electron, and transforms into an atom with nine protons and 11 neutrons. It actually becomes a form of fluorine! Because, you know, nothing normal ever happens inside stars.

This electron capture in the star's core means there are fewer electrons whizzing around and supporting the star's outer layers. So, the core collapses further, creating pressure and providing the energy to fuse heavier elements. This creates more heat, which allows protons to escape from the newly-formed atoms, and they in turn devour more electrons.

The result is a chain reaction that leads to the star's core collapsing — and that's the tipping point. It turns this otherwise lightweight star into a dramatic supernova explosion. So, basically, these intermediate stars are exploding because they're being eaten away from the inside.

And the amazing thing is, we might have even seen this happen from Earth once upon a time. According to a few papers — including a 2019 one — this process could have led to the supernova people observed back in 1054, the one that created the now-famous Crab Nebula. Even though our lives literally revolve around a star, things like this really remind you that there's nothing ordinary about these objects.

But the more we learn about them, the better we can understand our galaxy and our universe as a whole. Thanks for watching this episode of SciShow Space! And as always, thanks to our patrons on Patreon.

We know there are lots of places you could be putting your dollars these days, and it means a lot that you're using them to help put more free, educational content on the internet. If you're not a patron but are interested in becoming one, you can learn more about what that all means at patreon.com/SciShow. {♫Outro♫}.