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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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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The Adult Life of a Star

Stars are like cars. They need fuel to go. And also like cars, they don’t have an infinite supply.

But here’s where the metaphor breaks down. They can never refuel.

Yup. That’s right. For their entire lives, stars are stuck with only the amount of fuel they formed with. They can’t get more.

What happens when you’re driving, and you run out of gas?

Well, if you can’t refuel, you’re gonna have to call a tow truck. But stars don’t have tow trucks, and for them, it’s not a matter of moving or not—it’s a matter of life and death, such as it is.

But how does that work?

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What is a “Normal” Star?

If we were talking about people, I’d say there’s no such thing as a “normal” person. We’re all weird in our own way—that’s what makes us unique and ourselves.

However, there’s such a thing as a functional human—a human with a combination of functional organ systems and/or prosthetics that makes daily life navigable. And just as no star is exactly alike, there are functional stars.

Nature makes mistakes all the time. It is not intelligent—it doesn’t know the best way to do anything. It doesn’t know the path of least resistance or least effort. It just tries everything at random, and we get to observe what happens.

A “normal” star is what happens when nature stumbles upon the right conditions. But…what does that mean?

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What Keeps a Star Stable?

animal_star.jpg

All life as we know it has to maintain homeostasis—that is, keep internal goings-on regulated. Body temperature is just one example. Mammals can maintain a stable body temperature with no trouble. Reptiles have to bask in the sun to keep warm.

You’re probably familiar with this idea. When you sweat, your body is trying to cool down. When you shiver, it’s trying to warm up. These are all examples of your own body maintaining its own homeostasis.

And then there’s blood pressure, heart rate, hormones, and pH—not that I have any real idea how all that works, but I know they’re all things that your body regulates on its own. Homeostasis is an important thing. Basically, when it fails, things go wrong.

And stars do the same thing. Continue reading