What's Going To Happen To Betelgeuse?

BBC Stargazing Live is great TV - but watching #bbcstargazing on Twitter at the same time makes it even more fun. A few of us astronomers have been answering viewers' questions in real time, as they wonder about the Universe with Brian Cox and company. But here's one that had me stumped: what's going to happen to Betelgeuse? Time to call SPI-MAX...

Here's @KaatieT's question in full:

"How long will it be until Betelgeuse becomes a Supernova and at this point what will it look like to the earth? #BBCStargazing"

This is a great question. Betelgeuse is a red giant star, on one of Orion's shoulders: these bloated stars are nearing the end of their lives, using up the last of their fuel, and becoming unstable to collapse due to their own gravity. But how much time do they have left? I took a guess and answered @KaatieT with "a billion years", but like someone who's gone out and left the gas on, I wasn't quite sure... So I emailed the stellar evolution experts of the SPI-MAX group for a more accurate estimate.

Here's Mark Sullivan getting a thousand times closer:

"Maybe within a million years or so. Of course, it might already have become a supernova.. would be a spectacular one to see!"

Spectacular indeed: Andrew Gosling recalled reading about the impending death of Betelgeuse over at Bad Astronomy, and summarises:

In a nutshell, we don't understand those very end stages well enough to give an accurate prediction, it could go up in the next days/weeks, but far more likely is that it'll be many 1000s of years before it goes, and that we'll see a lot of activity from it before that. As to how bright, you'll definitely be able to see it during the day. I haven't done the calcs myself, but bad astronomy suggests full moon bright, which seems pretty sensible. Other close-by supernovae that have been recorded in ancient writings would agree with this."

That's amazing: in astronomical terms, a thousand years is like the blink of an eye. Mark and I guessed much longer times than this because we know that stars live a really long time, billions of years in most cases - so if you see one, it's probably been around a long time, and will probably carry on for a good while too. But Betelgeuse? Seems like it's really on the brink. Here's Mark again:

"Yes, somewhere around -11 apparent magnitude, so similar to the full moon and about 10,000 times brighter than Sirius... It'll be a point source though, unlike the moon..."

Now that really would be something to see live.