On clear nights I make a point of looking for Betelgeuse – Alpha Orionis, an M2Iab class red supergiant. It’s the bright red star in Orion, below the belt from the New Zealand perspective.
Betelgeuse in Orion from the Southern Hemisphere – a picture I made with my trusty Celestia installation. To see it from the northern hemisphere, turn the picture upside down. Click to expand.
It’s naked-eye bright – magnitude 0.42, which is duller than it was in the 1930s but still one of the top eight brightest stars in the sky besides the Sun. And it’s gonna blow – as in, explode as a Type II Supernova. Not today. Maybe tomorrow. Maybe not for a million years.
But blow it will. Vlabadaboom! Actually, given that Betelgeuse’s distance was recently revised to 425 light years (about 4,020,724,570,000,000 kilometres) it could have blown any time after 1588 and we wouldn’t know yet. Not until the light hits us. It’s too far off to be any threat - but it’ll be bright, it’ll be spectacular, and we’ll have a second sun for a while. Here’s a simulation on YouTube.
Why will it go bang? Stars shine by fusing hydrogen into helium. When hydrogen near the core runs out, the star fuses helium into lithium, and so on up the Periodic Table. Each element fuses at higher temperatures, causing the star’s outer layers to puff up, turning the star into a red supergiant.
That’s what Betelgeuse is, and if you were to look at its surface you’d see it roiling and boiling under the ferment at its core. The star also waxes and wanes, vigorously – it’s a Type SRc variable.
For Betelgeuse, life as a red supergiant is a one-way journey. Once the core starts turning silicon into nickel, which decays into iron, that’s the end. At that point, the core cools and is buoyed from gravitationally collapsing on itself by a curious outcome of quantum physics – electrons cannot be forced into the same energy states. But once the core exceeds a mass of 2.864 × 10<exp>30 kg, known as Chandrashekar’s Limit, it collapses in milliseconds. Lots of interesting extreme physics things happen, leading to one thing – vlabadaboom!
Betelgeuse is so huge that with the right telescope it can resolve into a disk. Here’s the Hubble view, via Wikipedia. Public domain, NASA, Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA
We don’t know exactly when this will happen, but I calculated that the odds of my seeing Betelgeuse explode during a five second glance are, statistically, about the same as anybody winning New Zealand’s lottery prize.
What’s more, Betelgeuse is pretty neat to look at. It’s got a diameter 1500 times that of the Sun – so if its centre was where our Sun’s centre is, the edge of its photosphere would be most of the way to Uranus. (Don’t laugh.) Sure, that makes most of the star a good simulation of high temperature vacuum – it’s 20 times the mass of the Sun, making the average density about 1/10,000th that of sea-level air pressure on Earth. But it’s still mind-boggling. At 0.05 arc-seconds, you can’t see the disk with the naked eye, but you can see the colour, and that’s unusual.
So I figure Betelgeuse is pretty cool and interesting all round – and, maybe, it might explode while I’m looking at it. Maybe.
Copyright © Matthew Wright 2013
Next week…secrets of Eta Carinae and how it might blitz Earth. Seriously – this is genuine science. And yes, you need to know.