Betelgeuse: Will It or Won’t It?

The amateur astronomy world and the professionals who study giant stars have been watching as supergiant Betelgeuse grows dimmer and dimmer. It’s apparently a natural phenomenon, this “fainting” of alpha Orionis (the “formal” name for this star). And, it’s giving new insight into how Betelgeuse is doing.

Of course, the whole thing has caught the attention of non-astronomers, particularly in the media. It’s been good for a whole lotta clickbait that looks flashy but doesn’t say very much about the star. But hey, they’ve figured out that at some point Betelgeuse will go supernova, and that’s good for headlines. The whole phenomenon generates lots of speculation about how this thing is about to blow.

So is it going to go “ka-blam” soon? Well, the answer isn’t clear. Let’s look at what we DO know about this star.

What’s Happening with Betelgeuse?

I find the whole thing about Betelgeuse fascinating and I’ve written about its general characteristics before. It’s interesting enough that I’ve asked a speaker at an upcoming event that I’m putting together to spend some time talking about the science behind the “fainting”. The star has been the subject of a lot of studies over the years, and yes, it is going to go supernova. But, whether that’s connected to the future explosion (whether it’s tonight or 100,000 years from now), astronomers don’t really know. It’s sort of like forecasting a snowstorm centuries ahead of time.

Here’s what we know about Betelgeuse. It’s a variable star—a red pulsating supergiant. That means it dims on a fairly regular schedule and then brightens up again. According to the American Association of Variable Star Observers page about this star, its brightness can drop quite a bit on a regular basis, roughly every 425 days. However, that’s not the only periodicity its brightness shows. There’s also a 100-180 day variation it can undergo, as well as one that takes nearly six years.

What causes these variations? Good question. The star shows dark patches on its “surface” that look like huge sunspots. Astronomers think that upwelling gas blobs could also affect the star’s brightness. It’s not a quiet star; it’s a dying one. But, this star, which sort of looks like a bubbling, boiling mass of plasma in simulations based on observational data, isn’t dead yet. Although the supernova explosion it will eventually suffer will spell the end of its stellar existence, there’s a lot that is happening and will happen before that final catastrophe hits.

When a Giant Star Dies

The life cycle of a star like Betelgeuse is pretty similar to the one our Sun has been experiencing, up to a certain point. That is, it assembles from a cloud of gas and dust. At some point, hydrogen fusion begins to take place in the center of the star, which causes it to shine. When the hydrogen runs out, the star starts fusing heavier elements. Our Sun will do this until it reaches the oxygen-burning phase. Supergiant stars fuse elements until they reach iron.

During the later stages of fusion of other elements, the aging supergiant starts shedding parts of its outer layers. This mass loss creates clouds and/or shells of material around the aging star.

At the point when the supergiant starts fusing iron, it takes more energy to do that than the star has available. So, the whole thing comes to a skidding halt. At that point, the star is about a second away from collapsing in on itself. When that happens, all the layers of the star fall inward because the core doesn’t have enough energy to “hold them at bay”. However, the collapse does send temperatures in the core soaring, and that releases huge amounts of energy. A shock wave reverberates through what’s left of the star and that sends the infalling material back out to space.

When the final explosion occurs, the material rushing out collides with that previously lost stellar “stuff”. In the case of Supernova 1987a, for example, astronomers tracked that collision and collected data and images of the ongoing catastrophe. This is roughly the process Betelgeuse has gone through and will experience when its final death throes occur.

Betelgeuse’s Dimming and End-of-Life Scenario

So, in the case of Betelgeuse, what’s causing the dimming? It’s very likely that the outer layers of the star expand slowly for a few years at a time. Then, they shrink down again. This steady pulsation means that the star’s surface area increases and decreases. It also means that the temperature rises and falls, and that causes it to look brighter at some times and dimmer at others.

This is a known characteristic of stars like Betelgeuse (red supergiants) and generally means their atmospheres are unstable. The rising and falling blobs of gas also affect surface brightness and color. Also, the existence of spots on the surface complicates the dimming and brightening. Astronomers are studying Betelgeuse and other stars like it to understand what those spots do and how they originate.

Is the periodic dimming related to its aging process? Very likely so. But, it doesn’t predict when Betelgeuse is going to do its final death dance. Part of the reason for that is that astronomers aren’t completely sure what is happening inside the star. They know that it’s not fusing hydrogen any more, and that is likely well into what they call the “helium-burning” phase. (If you want to read more detail about that, check out Ethan Siegel’s blog entry about Betelgeuse here. He goes into a bit more detail.

The End is Near… or Not

Now that it’s in its helium-burning stage, Betelgeuse still has a ways to go before the final act. Once it finishes its helium off (which could take a few hundred thousand years), it will start fusing carbon. It’ll only take a few hundred years to run through all its carbon supply. Each element it goes through will take less and less time to “burn through”.

If it’s still in helium fusion state, those estimates of a hundred thousand years or so before the beginning of the end make sense. But, maybe it’s farther along than scientists think. We know it is losing mass, we see that it’s pulsating, and its brightness is changing. So, it could be closer to death. But, it’s not likely an imminent death. None of those characteristics automatically means Betelgeuse is heading to Supernova City.

But they are intriguing. We need to know a LOT more about what’s going on inside this star before we can say with any confidence when its big, splashy death is coming.

Check Out Betelgeuse

In the meantime, though, we can all go outside and look at Betelgeuse. It’s noticeably dimmer than it used to be. It’s a good time to start watching it and tracking how it brightens up over time. And, maybe daydream a little about what it will look like when the supernova finally occurs. Sure, it would be fun to see it in our lifetimes, but aging supergiant stars are like cats: they do what they want when they want. And, all the rest of us can do is wait and see what’s next.

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