Is Betelgeuse about to go bang?

Betelgeuse – the biggest, reddest star in our neighbourhood – has been fading of late. It oscillates anyway, over a defined cycle; but this cycle of dimming is more pronounced than any seen. It’s dropped from being one of the top ten brightest stars in the sky to the 21st. And that, astronomers consider, might be a sign that it’s about to become supernova. In short, it could well go bang.

The word is ‘might’. Betelgeuse is highly variable – think of it as boiling – and although this amount of dimming is unusual, it’s not unlikely. Odds are on that it’s simply an extreme end of a cycle and the star will pick up again. It’s even possible that it’s dimming so unusually purely because it’s blown off a chunk of gas that’s now masking some of its light. Sure, Betelgeuse is near the end of it’s stellar life – but we’re talking stellar scales here. That end could come tomorrow. Or it could come in a hundred thousand years. Nobody knows. There are actually other stars, such as Eta Carinae, thought likely to ‘blow’ sooner (and that one partially did, in the 1840s).

The difference is that Betelgeuse is in our stellar back yard – again, relatively speaking. You can see the star if you look at the constellation Orion: Betelgeuse is the ‘shoulder’, and usually it’s bright red. It’s also is 642.5 light years distant, give or take a bit, which means that even light takes that long to get to us from its vicinity. But by astronomical standards, that’s right next door. Space, when you get down to it, is really, really big. As Robert Heinlein once quipped, astronomers – and, for that matter, physicists – keep buckets by their desks, filled with zeroes.

Betelgeuse is so enormous it can be directly imaged from Earth. Most stars appear as a point source. Photo: ESO.

Betelgeuse is what’s known as a red supergiant; an M1-M2-la-ab class supergiant to be specific. It’s a ‘semi-regular variable’ star, meaning it waxes and wanes in a pattern. But here’s the thing. Stars of that class are physically huge; Betelgeuse has an estimated mass around 11.2 times the mass of the Sun. Some estimates suggest around 19 times. However, either way, its diameter is vast relative to that mass. This was first directly measured in 1920 – making Betelgeuse the first star to have its diameter actually quantified – but this changes as the star pulses. Efforts since to refine the range of diameters that Betelgeuse develops as it waxes and wanes have been largely ongoing, and have varied from 3.2-5.2 to 10.3-11.1 astronomnical units – meaning, anything from 3.2 to 11.1 times the distance of Earth from the Sun. In part that uncertainty is because the star is pulsing and, in effect, ‘boiling’. But in part it’s also because even measuring something that large from the distance of Earth is extremely tricky.
In general it’s thought that, were Betelgeuse to be where the Sun is, its ‘surface’ would be around the orbit of Jupiter.

Betelgeuse to scale against our solar system. Credit: ESO.

This implies, of course, that the density of the outer layers is very low – really, not much above what we’d consider a vacuum – but the interior is very different. Betelgeuse is one of those stars that’s born big, lives fast, and dies hard. Their ‘lifespan’ is only measured in tens of millions of years at most, unlike the Sun which has one measured in billions. And they evolve fast. Betelgeuse started life as an O-class supergiant, but over a million years ago ran out of hydrogen to fuse into helium in its core and began fusing ever-heavier elements. It’s thought that Betelgeuse is, in effect, ‘boiling’ – throwing off clumps of gas asymmetrically.  More important is what’s happening in the core, which is ‘burning’ through its fuel at a huge rate.

Without going into that in great detail, the issue is that nuclear fusion generates energy by ‘fusing’ lighter elements into heavier ones – hydrogen into helium, then helium into lithium, and so on. But it has an end point; once the fusion reaches manganese, the next step is iron; and iron requires a net energy input to fuse. That’s where things stop. Of course the star’s core is always a mix-and-match of the elements up to that point, but the critical issue is having enough material to keep the energy output going and thus stop the core from imploding.

When Betelgeuse’s core stops producing the necessary energy, it’ll implode – and that’s the point where a supernova happens. Boom. Or perhaps ‘vlabadaboom’. Who knows? There’s no sound in space. The kind of supernova produced by stars such as Betelgeuse is classified as a ‘Type IIp’, and one was observed in 1987 in the Large Magellanic Cloud, SN 1987A. This tells us what will happen to Betelgeuse.

Of that, more next week.

Copyright © Matthew Wright 2020