Bad Astronomy

Double dipped supernova

Generally speaking, once a star explodes, that’s pretty much it. It’s done.

But there are exceptions. In the 1870s, the star Eta Carina underwent a massive and violent outburst, releasing so much energy it was really a mini-supernova event. It ejected two monster blobs of gas – massing as much as the Sun! – at high velocity, and was temporarily the second brightest star in the sky, even though it’s 7000+ light years away.

Eta Car is one of the most massive stars in the sky, and one of the most massive stars possible, in fact. But how common are objects like Eta Car?

Maybe more common than we thought. In 2004, Japanese astronomer Koichi Itagaki discovered what he thought was a supernova in the galaxy UGC 4904, which is about 78 million light years away. The object faded rapidly, and was gone 10 days later.

Then, two years later, he saw another supernova in UGC 4904 – in the same spot! The image above shows the sequence of events.

That’s just too big a coincidence to be two separate stars, so they followed up with more observations. What Itagaki and his team found is that this was a single star that blew up, probably very much like Eta Car: it had a violent paroxysm, and then exploded two years later. Spectra revealed an overabundance of helium in the star, which is expected if you have a very high mass star. It was probably 50 - 100 times the mass of the Sun, and they don’t get much bigger.

Eta Car is a singular star in our Galaxy; we have not seen another like it (though there may be others on the other side of the Galaxy where they are hard to spot). Maybe every galaxy actively forming stars has one or two like it. But I doubt it – stars like this don’t live very long before exploding, so they are most likely rare (there is none in the Andromeda galaxy of which I am aware, or any other nearby spiral). So gaze upon that picture above, at that that little unassuming dot, and know that you are witnessing the passing of something rare and amazing, and violent and monstrous. And also know that while it’s almost 80 million light years away, we’ll have a front row seat to a similar catastrophe soon enough. Maybe tonight, maybe not for a thousand years, but in the life of a galaxy, it’ll be in the blink of an eye.