[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.
He just sent me two more he took last night. He went to Yellowknife, in the Northwest Territories of Canada. The aurorae were active last night as the Sun’s recent hissy fit sparked a geomagnetic storm, but by the time Dave go his equipment set up, the Moon was up and the aurorae fading. But never one to waste an opportunity, he took this incredibly dramatic and moving picture:
Stunning. [Click to enannulenate.] Ice crystals suspended in the air refract (bend) light from the Moon, and due to their geometry they create a ring around it. This is common in winter, but it’s rare — at least in the lower 48 — to get one this bright. The bright "star" on the edge of the ring at the top is actually Mars, which is terribly bright and ruddy in the night skies right now. The fainter star inside the halo is Denebola, the tail of Leo.
He also took this more upbeat picture (click to embiggen) which is another fantastic shot of the halo. You can still see Mars, with the bright Regulus (the heart of Leo) to the right, and just to the left of his hand is either Saturn or the bright blue star Spica in Virgo; I’m not sure which since they’re close to each other in the sky right now. Given how far it’s outside the halo, I’m leaning toward it being Saturn with his hand blocking the view of Spica. As an added bonus, you can see a faint arc of light at the top of the halo, called an upper tangent arc; these are more rare. I’ve only seen them a handful of times near the Sun, and never from a Moon halo!
Having spent a lot of time — a lot — out in the cold waiting for that one great shot, that one great view through the telescope, I can sympathize with what Dave went through to get these… and know he agrees that it was absolutely worth it.
Image credits: Dave Brosha, used with permission.