[NOTE: Whenever I write about actual cosmic events that might possibly affect us on Earth, I get scared emails from some folks. So let me be up front: there are no stars close enough to Earth to hurt us should they explode. Nothing I write in this post changes that; I'm talking about a star that can go supernova that's closer than I thought any was, but still much too far away to do much to us. So don't panic. But do please enjoy the over-the-topness of what happens when a star explodes. Because it's cool.]
On May 13 I tweeted this one: BAFact: A supernova has to be less than about 75 light years away to hurt us. No star that close can explode, so we’re OK. The distance may actually be somewhere between 50 – 100 light years, and it depends on the kind of exploding star, but I have to keep these factoids to about 110 characters to tweet them. Nuance is at a premium.
I got so many replies about that one that I decided to do a theme week, and stick with supernovae. The next day I tweeted this: BAFact: The nearest star that can go supernova is Spica – it’s 260 light years away, so we’re safe, and I linked to a video I did a few years back this.
A few minutes later I got a tweet from Nyrath, saying that he thought the nearest star that could explode was IK Pegasi, 150 light years away.
I looked this up, and here’s the thing: he’s right! I had never heard of IK Peg, so I didn’t even know it existed. And it turns out it is the nearest star that can explode, though technically it probably isn’t.
And you know when I say something weirdly oxymoronic like that there must be a good story here, right? Mwuhahahaha. Yes. yes, there is. Stick with me; this is long, but also awesome.
It’s been known for a while that IK Peg is a weird star (you can read quite a bit about it on the ESO website, though the formatting is a bit messed up). It looks like an A-type star — that is, more massive, hotter, and bigger than the Sun. It’s not nearly enough to explode — stars need to be at least 8 times the Sun’s mass to do that, and this star is only about 1.7 times heftier than the Sun.
It pulsates, getting brighter and dimmer on a pretty rapid timescale: each cycle only takes about an hour. A lot of stars do this, but typically when one does it means it’s nearing the end of its life. In a few dozen million years it’ll swell up into a red giant, blow out a strong wind that’ll strip its outer layers away (creating a gorgeous planetary nebula), and eventually retire as a white dwarf; small, dense, and hot, cooling slowly over billions of years.
Except… there’s a monkey in the wrench. The star isn’t alone.
It has a companion. And this is where things get interesting.