Thread by @emsque

Looking for a primer on stars like Betelgeuse? There's an ebook for that😁iopscience.iop.org/book/978-0-750…

In short:
1) Betelgeuse probably won't explode tomorrow
2) (but if it did it'd be SO COOL)
3) we should observe the heck out of it anyway because it's a red supergiant and AWESOME

"But wait, why did it get fainter?? Does that mean it's about to die?!"

First of all, there are several not-because-it's-about-to-die explanations for why Betelgeuse might have gotten fainter...

Red supergiants are notoriously variable; even the normal ones will often change their visual brightness by about a magnitude, and we're still disentangling all the different things that cause this.

One reason has to do with the very bizarre appearance of these stars' surfaces.

Lots of stars have convection cells at their surfaces. Our sun is a great example! Think of these cells like standing over a pot of water & watching it boil. In the center of each cell we see hot gas rising to the surface; it then cools & falls back to lower layers at the edges.

Here's the North American continent for scale; each granule is ~900mi across.

So that's convection on the surface of a "normal" star like our sun.

A red supergiant is thousands of times bigger, very low-density, and much colder, with much weaker gravity at its surface.

Because of this a red supergiant's surface has ~3-4 ABSOLUTELY ENORMOUS CELLS.

This means a NORMAL red supergiant, just going about its day-to-day shiny star life, looks like a WACKY GIANT BOILING AMOEBA-STAR:

That GIF is a simulation w/ the timescale sped up, but we see these cells on the surfaces of red supergiants; they show up as bright/dark spots. You can see them in this image of red supergiant Antares (this is NOT BETELGEUSE, despite what some links on the internet are saying):

And this is Betelgeuse itself, clearly also looking a bit spotty:

So: spotty boil-y weird surfaces are one reason that red supergiants may change their brightness. There are others.

Red supergiants can puff off huge amounts of mass through processes we still don't fully understand, and that mass will condense into dust around the star. Sometimes that dust can block the light from the star and make it look dimmer.

(as an example, here's yet another red supergiant, VY Canis Majoris, photographed with Hubble. yes, there is a star in there, it's just buried in a massive pile of dust.)

We also expect red supergiants to pulsate, basically appearing to grow and shrink like weird beating hearts as their outer layers expand and contract.

Again, we're still figuring out exactly how and why they do this, but it will certainly change how bright they look.

Finally, plenty of red supergiants have binary companions (which @KathrynNeugent is studying!), and things like companions, rotation, and other weird physical processes all affect how these stars evolve and how their brightness changes with time (which @trevorzaylen is studying!)

@KathrynNeugent @trevorzaylen So, there are plenty of normal not-about-to-die explanations for why Betelgeuse is doing what it's doing.

Here's why I think we should keep an eye on it anyway...

@KathrynNeugent @trevorzaylen First of all, ALL OF THESE THINGS ARE RIDICULOUSLY COOL.

If Betelgeuse is doing one or more of these things - being wacky and boil-y, puffing off dust, pulsing, etc. - we should be watching and studying it to learn more about red supergiants as a whole.

@KathrynNeugent @trevorzaylen Second, while none of this behavior means "BETELGEUSE WILL EXPLODE TOMORROW", it's worth keeping in mind that:

a) Betelgeuse IS exactly the sort of star that dies as a supernova, and

b) we've rarely gotten a good look at a star right before it dies...

@KathrynNeugent @trevorzaylen Something a lot of people don't necessarily realize is that astronomers can't currently predict if or when a star is going to die and produce a supernova.

@KathrynNeugent @trevorzaylen In maybe a few dozen cases, we've spotted a supernova, dug back through old images, & found a star sitting precisely where the supernova happened. If new images, taken after the supernova fades ,show that the star is gone, then ta-da: we found the star that made the supernova!

@KathrynNeugent @trevorzaylen From a single image it's VERY hard to learn much about these stars, but occasionally we have enough data to measure brightness and temperature.

These stars almost always turn out to be red supergiants.

Check out this pre-image of the red supergiant that died as SN 2008bk:

@KathrynNeugent @trevorzaylen Still, a picture alone can only tell us so much. Ideally we'd like to have a spectrum of a star before it dies since it would tell us so much more about its physical properties, its chemistry, and how it's evolved.

@KathrynNeugent @trevorzaylen We have not - YET - seen a star that we've observed and studied closely die as a supernova. This is largely because we really only have detailed observations of stars in the Milky Way and a handful of neighbor galaxies.

@KathrynNeugent @trevorzaylen The last supernova that we observed in the Milky Way was in 1604, just before the telescope was invented:

@KathrynNeugent @trevorzaylen The last supernova that we observed in our local neighborhood was in the Large Magellanic Cloud in 1987. We actually DID get a spectrum of that star before it died and it STILL confuses the hell out of us (and behold the magnificent weirdness of that supernova remnant):

@KathrynNeugent @trevorzaylen So in short, we don't REALLY know what a star looks like right before it dies. We've got plenty of models and predictions and physics to help guide us, but nobody's ever observed an about-to-die star in as much detail as we've observed Betelgeuse.

@KathrynNeugent @trevorzaylen If Betelgeuse DID go supernova tomorrow it would be the best-studied dying star in human history by quite a wide margin. Even if it doesn't die for another year, or hundred years, or 100,000 years, studying it tells us more every day about how stars die.

@KathrynNeugent @trevorzaylen So go take a look at Betelgeuse tonight, and say hi to one of our favorite and most fascinating dying stars.

And check it out again tomorrow night. Just in case. 😝