What Happens After Helium Fusion?

Back in August—sorry I took so long!—we talked about the helium flash, an explosion that occurs within stars when helium nuclei begin to fuse within a degenerate core.

So…this is not what the helium flash would look like.

Even though it’s a powerful explosion, it happens in such a small region in the center of the star that we wouldn’t see it at all, and the star’s outer layers absorb most of the energy from the explosion. I just thought it was a cool picture 🙂

In any case…what happens after the helium flash?

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Helium Ignition in Stars

When I first began learning about stars, I expected them to be violent and chaotic places. And to an extent, they certainly are.

Pressures are unbelievably high in their cores—high enough to smash protons together, and this is no small feat. And near their surfaces, magnetic field loops twist and tangle and a number of eruptions disrupt satellite function on Earth from time to time.

Beyond the obvious, though, stars are actually surprisingly…peaceful.

While stable, they only produce enough energy to sustain their own mass. Their way of maintaining homeostasis is beautiful in its simplicity.

But this can’t last forever. Eventually, stars exhaust their hydrogen fuel. Their cores begin to contract and their outer envelope expands to enormous proportions.

What’s next for a star—and why?

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What Happens in an Expanding Star’s Core?

Depending on their mass, stars can remain stable for millions and even billions of years. The most massive stars live for “only” about 10 million years, but models predict that the least massive live for much longer—longer than cosmologists believe the universe has existed.

As long as stars are stable, they exist on the “main sequence.” That’s just a fancy word for the best balance between temperature and mass. For a while now, we’ve been exploring the main sequence in depth, and I’ve shown you how stars eventually lose stability and “leave” the main sequence.

As stars exhaust their fuel, their internal structures change drastically. Their cores contract, but their outer layers are forced to expand, and they become giants. You’d think the next thing we’d cover would be what happens to these giant stars, right?

Well…not quite! At this point, something downright weird is going on in their cores, and it’s well worth a closer look…

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How a Star Expands

Well, everyone, look who’s back!

For those of you who are not signed up for my newsletter, I’m sorry I’ve been away forever—life happened. It’s been a very rough three months. I hope you’re all doing well in light of the COVID-19 pandemic. I know it’s pretty tough right now, but we’ll pull through. Hang in there! 🙂

And now, for some long-awaited astronomy…

Meet Betelgeuse, a bright star in the winter constellation Orion.

Betelgeuse is a cool red supergiant that we’ll talk about a lot more in just a couple weeks, when we cover variable stars. Not too long ago, it was the height of excitement among astronomers. No one was sure why it…well…appeared to be dimming.

Yeah. Like a lightbulb. It was literally getting fainter—considerably fainter.

It’s pretty normal for Betelgeuse, like any other variable star, to fluctuate in brightness over time, but it was doing something downright weird. We’ll explore what was going on with it soon enough.

For now, let’s take a look at why Betelgeuse, as a supergiant, is so darn big.

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