Bad Astronomy

Is Betelgeuse about to blow?

eso_betelgeuse   I was going to wait to write about this, but I’m getting a lot of emails about it, so I’ll say something now, and followup when I get more information. The story: BABloggee Alereon (and many others) sent me to an interesting site: Life After the Oil Crash Forum – a forum that apparently has a lot of doomsday-type scuttlebutt posted to it. An anonymous poster there says he has heard that the star Betelgeuse is about to go supernova, maybe as soon as a few weeks:

I was talking to my son last week (he works on Mauna Kea), and he mentioned some new observations (that will no doubt get published eventually) of “Beetlejuice”; it’s no longer round. This is a huge star, and when it goes, it will be at least as bright as that 1054 supernova…except that this one is 520 light years away, not 6,300 […] When it collapses, it will be at least as bright as the full moon, and maybe as bright as the sun. For six weeks. So the really lucky folks (for whom Betelgeuse is only visible at night) will get 24 hour days, everybody else will get at least some time with two suns in the sky. The extra hour of light from daylight savings time won’t burn the crops, but this might. Probably, all we’ll get is visible light (not gamma rays or X-rays), so it shouldn’t be an ELE. It’s sure gonna freak everyone out, though….. Then it will form a black hole, but we’re too far away for that to matter. The buzz is that this is weeks/months away, not the “any time in the next thousand years” that’s in all the books.
The basic takeaway: OK, folks, first: when news like this comes from an unnamed source on some random forum, and that source is not even a primary one, and that secondary source quoted is also unnamed, and that person heard it from a third party that is also unnamed… well, oddly enough my skeptic alarm bell in my head rings loudly enough that my eardrums explode outward in every direction at the speed of light. I hope I’m being clear here. The first important thing to note here is that if Betelgeuse explodes, we’re in no danger at all. It’s too far away to hurt us. Got that? It’s the most important thing to remember here, because I’m quite sure this story will get wildly exaggerated as it gets repeated. So, what’s the deal with Betelgeuse? What is it, will it explode, and if so, when? The details: Betelgeuse is one of the brightest stars in the sky. That’s because it’s an intrinsically luminous star, and one that’s relatively close by. By luminous, I mean something like 100,000 times that of the Sun, and by close I mean roughly 600 light years away if not more. That’s 6 quadrillion kilometers, or almost 4 quadrillion miles. In other words, quite a hike. Betelgeuse is a red supergiant. It has a mass of something like 20 times the Sun’s, and is near the end of its life. When it dies, it will explode as a supernova, a titanic event that is among the most violent in the Universe. For details on how this happens, read this essay I wrote about it. It’s hard to know just when a star will explode when you’re on the outside. Betelgeuse might go up tonight, or it might not be for 100,000 years. We’re just not sure. Betelgeuse isn’t round, and it’s shrinking! In the bulletin board post, he talks about the star not being round. It’s unclear, but it sounds like he’s referring to observations which show that there is a big plume coming from the surface of Betelgeuse. That was exciting news when it was released, but not hugely surprising; stars are active, and massive stars even more so. Also, note that those “new” observations are a year old! hst_betelgeuse That image above is from even earlier, and shows a Hubble observation of Betelgeuse taken in 2005. Note here that the star doesn’t look round, but that’s an illusion. The image shows a hot spot in Betelgeuse’s swollen atmosphere, and that makes it look like a bump is hanging of the side. In reality, that’s just because of the way the image is printed, and isn’t an actual physical bump. But the hot spot (probably due to a big ol’ bubble of hot gas rising near the surface) in itself shows that things on the star change all the time; just recently two such spots were found. The post also talks about Betelgeuse shrinking. That claim is from observations made over the course of many years. Those data indicate the star is shrinking, but it’s unclear what they mean. While it may mean the star is in fact shrinking, starspots (sunspots on another star) may be fooling us, for example. Also, red supergiants aren’t like marbles, with a clean, sharp surface. They are balls of gas, extended and bloated, so there is no real surface. It’s therefore entirely possible the astronomers aren’t even really measuring the surface of the star at all, and it’s just the highly extended atmosphere that’s changing. Surface tension, rotten to the core The point I’m making is that a lot of stuff can happen on the surface of the star that has nothing to do with the core. Since it’s the core that generates the star’s energy and eventually causes it to explode, what’s happening on the surface is not an indication of any impending explosion. Mind you, the surface and the core do “talk” to each other, though slowly. As the core changes, that information does leak to the surface, but it takes centuries. Until, that is, the core collapses. When that happens, the shock wave takes hours or days to get to the surface, and the star explodes. But that’s hardly a slow event taking decades! So any changes we see happening now probably have little to do with what’s happening hundreds of millions of kilometers deep in the star. Also, it’s been known for a long time that Betelgeuse is a variable star; its light output changes. This shrinking may just be a part of that natural cycle, and again no indication of an explosion. Having said all that, I’ll note that someday, Betelgeuse will explode. That’s for certain! But it’s also way too far away to hurt us. A supernova has to be no farther than about 25 light years away to be able to fry us with light or anything else, and Betelgeuse is 25 times that distance (which means its power to hurt us is weakened by over 600x). It’s the wrong kind of star to explode as a gamma-ray burst, so I’m not worried about that either. At that distance, it’ll get bright, about as bright as the full Moon. That’s pretty bright! It’ll hurt your eyes to look at it, but that’s about it. The original post says it may get as bright as the Sun, but that’s totally wrong. It won’t even get 1/100,000th that bright. Still bright, but it’s not going to cook us. Even if it were going to explode soon. Which it almost certainly isn’t. Conclusion: So my personal opinion is that this is just another breathless rumor of astronomical doomsday that we get every couple of years. Even if any of the science of it is right, it doesn’t mean Betelgeuse is about to explode any day now. And since this is a rumor three times removed, I don’t put any stock in it. I’ll wait until I hear from named scientists with published or publishable data before I start to wonder if the star is about to blow. And if and when it does explode, it can’t hurt us. Someday it will – maybe not for a hundred thousand years, but someday – and every astronomer on the planet hopes it happens in their lifetime! It will be a scientific bonanza unlike any ever seen. Image credits: NASA, ESA, ESO