The spiral galaxy NGC 2397 is more than just a pretty face… though it is a very pretty face:
Spiral galaxies have lots of gas and dust in their spiral arms which actively form new stars. Some of these stars are very massive and bright, and can be seen as individuals in Hubble images like this one. Massive stars don’t live long, exploding as supernovae after only a few million years. Images like this one can prove invaluable when such a star explodes; we can go back and look for the star itself in older images and learn more about it.
As it happens, the astronomers taking this image of NGC 2397 caught a supernova just as it was starting to brighten! That’s a cool coincidence, and very helpful; observations of supernovae before they get really bright are hard to come by.
So where is it in the image? It’s not either of the two really bright stars you see; those are both nearby stars in our own Milky Way. Actually, the star would be impossible to find without already knowing where it was (click to embiggen, as usual):
It doesn’t look like much… but that’s because it was caught early. Supernovae explode with so much energy that they can actually equal the brightness of an entire galaxy! So no doubt a few days later this nondescript star located in a galaxy 60 million light years away became a heckuva lot more noticeable, at least to any aliens local to NGC 2397. From Earth, it never got terribly bright, only about magnitude 15.6, or 1/10,000th as bright as the faintest naked-eye star. Still, that’s bright enough to be well-studied.
One other thing: once a star explodes, it’s very hard to know much about what it was like before the titanic event. The astronomers who made this study of supernova host galaxies have made an amazing discovery: stars as lightweight as 7 times the mass of the Sun can explode! That’s lower than previously thought, and makes me wonder; perhaps the star in that case had erupted previously, losing most of its outer layers, exposing just its core. When that exploded, it would be an underluminous supernova coming from what looks like an undermassive star. In reality it started out very massive (20 or more times the Sun’s mass) but lost a lot of weight on its way to exploding. I couldn’t find a paper online with any specifics, so if anyone knows about one (the lead author is Stephen Smartt) please leave a comment! In the meantime, I’ve emailed him and I’ll post a response once I get one.
