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.

Here’s our friend, the H-R diagram.

Several…ahem…months ago, we explored the life cycles of main-sequence stars. Those are the stars on that band that crosses the H-R diagram from top left to bottom right.

We know that stars begin as dense cores of material that condense out of giant molecular clouds. When they begin to burn bright enough to peek out of their dusty cocoon, their temperature and luminosity (total brightness) place them on the lower edge of the main sequence.

Relatively cool stars, like Proxima Centauri, form with very little mass. They remain cool and faint for as long as they fuse atomic nuclei for fuel and, as a result, are stable—which, for them, is a very long time.

Very hot stars, on the other hand—like Spica—form with high mass and begin their lives on the high-mass, high-temperature, and high-luminosity end of the main sequence. They, too, remain that way for as long as they are stable.

Eventually, all stars lose stability. Internally, they constantly wage a battle between the force of gravity due to their own mass that drives them to collapse, and the outward pressure due to energy production that drives them to expand.

Stars are violent and chaotic, but they’re also pretty good at maintaining their own hydrostatic equilibrium. That’s a fancy word for the balance between pressure and gravity, and it’s basically a star’s version of homeostasis.

That is…until they exhaust their fuel.

The pressure-temperature thermostat, a natural process within a star that maintains hydrostatic equilibrium, can only work as long as nuclei are fusing to produce energy in the core of a star. And, simply put, that can’t last forever.

Stars are quite literally made of their fuel. They are enormous balls of plasma, mostly hydrogen and helium, and this is what they fuse to generate energy. That means they don’t have an infinite supply of fuel—they can only use the stuff that they formed with.

And not only that…they can really only use the stuff inside their cores.

But why?

Remember what stars are like on the inside?

If you said hot, well, yeah, I’ll grant that. But specifically, I mean that the inside of a star is made up of a central core, a radiative zone, and a convective zone.

For all that stars are hot throughout, only the core can fuse hydrogen nuclei. This is because nuclei are naturally repulsive and will only fuse in significant amounts under extreme pressure—a condition which only exists in the core.

The core is ridiculously hot because its internal pressure leads to nuclear fusion, which generates energy. That energy escapes outward through the star and eventually makes its way to the surface, where it is radiated away.

The important thing is, the outer layers of the star are warmed by the core—because energy generated in the core radiates toward the surface.

Now take another look at the diagram above. See that radiative zone?

Stars could live so much longer if they could just use all their mass for fuel. But they can’t. Their interiors aren’t mixed. The stuff in the radiative zone will never reach the core, so it can’t be fused.

Wait a second…what about massive stars? Don’t they have a different internal structure?

Well, yeah—their convective and radiative zones are reversed. Their convective zone is near their center, instead of their radiative zone. But compared to their overall mass, this convective region honestly isn’t that big. So in general—enough to matter to us—their interiors aren’t mixed, either.

But what does this mean for the star?

To understand this better, let’s imagine a campfire.

Campfires can burn for a pretty long time, just as long as they’re stirred. Like stars, it’s best to mix up the fuel so that the firewood on the outside can be burned, too. If this isn’t done, ashes will accumulate in the center of the fire, and the fuel outside the center is never used.

In a star, these ashes are the helium nuclei that accumulate in the core as hydrogen nuclei are fused into helium. And the helium ashes stay right where they are.

So…how come the star can’t just fuse the helium ashes?

The pressure in the core isn’t high enough. It’s high enough to ignite hydrogen fusion—meaning, it’s high enough to force two positively-charged protons to collide. But it’s twice as hard to force the four total protons in two helium nuclei to collide. Right now, the core can’t do that.

What it can do…is contract.

The core’s equilibrium is sustained by its nuclear reactions. But as it exhausts its fuel, leaving only helium ashes behind, it no longer produces enough energy to support its mass. So gravity will force it to contract, and gravitational energy is converted into thermal energy.

In plain English, as stuff in a star’s core contracts, the pressure is driven up again, which increases the overall temperature of the core.

The core still doesn’t have a high enough pressure to fuse the helium ashes. But it does act like a stovetop, heating the hydrogen that immediately surrounds the core. This hydrogen has never before been under enough pressure to fuse…but now it can.

The hydrogen-fusing “shell” that ignites acts like a brush fire—it spreads outward through the star, its energy igniting hydrogen fusion in layers of the star above it as its helium ashes are left behind in the star’s center.

The helium core continues to contract. Some of its energy heats it up, and some of it heats the hydrogen shell above. The hydrogen shell itself continues to fuse hydrogen and produce energy.

The core of helium ash began contracting under its own gravity because the star couldn’t produce enough energy to balance its weight. Now, though, the star is producing too much energy.

The result? A flood of energy outward forces the star’s outer layers to puff up and expand.

So…what exactly does all this mean for the star?

We can safely say that the star has now left the main sequence. It has expanded to become a giant—or, if it was a high-mass star to begin with, a supergiant. And here’s the extra cool part. Its location on the H-R diagram changes too.

Remember that the H-R diagram shows a relationship between temperature—that’s the spectral classes labeled on the horizontal axis here—and luminosity or absolute magnitude, which are both measures of the star’s intrinsic brightness.

That means that if a star’s location on the H-R diagram changes, either its temperature is changing or its luminosity—or both. So what’s changing in this case?

The star is expanding, which means it’s getting bigger. Bigger things are brighter, no matter how much energy they produce or, if they’re not stars, how much light they reflect. Why? Click on back to my post on luminosity.

So, the star is getting more luminous, which means it’s going to move up on the H-R diagram. But that’s not all. It’s also getting cooler, because as the star’s gases are lifted and expanded, the floods of energy being produced within are absorbed.

The result is that the star moves up and quickly to the right on the H-R diagram.

Well…I say quickly, but we’re talking less than a million years to about 150 million years, depending on the star’s mass. Still, that’s pretty fast in the grand scheme of things. All of these changes take place over a relatively short span of a star’s life cycle.

Let’s come back to Betelgeuse. Why is it so bright, and so distinctively red?

It’s bright because it’s so big. It started out as a high-mass star and after it exhausted the fuel in its core, floods of energy from inside forced it to expand. But it’s red because it’s actually relatively cool. Like any other giant or supergiant, it has expanded and cooled.

We’ve seen how stars form from dense cores of giant molecular clouds, evolve across the main sequence for the majority of their lifespans, and eventually lose stability and leave the main sequence to become giants. But what happens next?

Well…I’ll try not to leave you hanging for months this time! 😉

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