Two supernovae, no waiting

Contributed by
Nov 20, 2006

Sometimes, stars blow up.

The circumstances have to be just right. Sometimes, a star is really, really massive -- like 40 or more times the mass ofthe Sun -- and it ends its life with a bang. Its life was short, maybe a couple of million years, but it burned hot, and died hard. That kind is called a Type II supernova.

Other times, a star like the Sun lives a long time, blows off its outer layers, and becomes a dense, hot, white dwarf. If it orbits another star, that second star might lose matter which accumulates on the white dwarf. The immense gravity of the dwarf squeezes that gas, and if it piles up just right, BANG! It fuses all at once like an enormous bomb. The star rips itself to shreds. That's a Type I supernova. Note that the star has to be pretty old for it to happen: one of the two stars started out like the Sun, but had to go through its whole life first, maybe a billion years or more. Type I supernovae are from old stars.

Weirdly, both kinds of supernova put out about the same amount of energy, even though they are very different events. The energy is staggering: about a million million million times the energy the Sun puts out. Yikes.

Anyway, in a typical galaxy, you get roughly one supernova per century. It depends on a lot of things, but hey-- close enough. Some galaxies are underachievers (we haven't seen one on our own Galaxy since the 1600s!), and others are brown-nosers, blowing stars up left and right.

Meet NGC 1316.

By any standard, NGC 1316 is a weirdo. It's oddly shaped, sorta like a spiral, but sorta like an elliptical. It has a smaller galaxy nearby with which it appears to be interacting. It's a huge radio source. It has weird streamers of dust running through it, which you can see in this Hubble image:

It's weird for another reason. Up until this year, it is one of the very few galaxies which have been seen to host more than supernova. Then, in June 2006, another one went off, raising the total known exploding stars in the galaxy to three. All in just 26 years, which is quite a feat.

Then, in November of 2006, a fourth was discovered!

The Swift satellite, launched two years ago yesterday in 2004, has as part of its mission the task of observing new, bright supernovae. It took this image just the other day:

The two supernova are above and to the left and right of center (the object farther to the left is a star in our own Galaxy). The supernova on the right (SN2006dd) was the first one from June, and the one on the left (SN2006mr) is the newer one, first seen on November 5. The image from Swift is from its Ultraviolet/Optical Telescope. It has an X-ray telescope as well, and I expect we'll see some X-ray images eventually. That should be cool.

Two simultaneous supernovae is rare, but not unheard of. It's happened in NGC 772 and NGC 664, for example. But it's still interesting.

Even more interesting: both of the supernovae in NGC 1316 have been determined to be Type I! Sometimes, in peculiar galaxies, there can be a burst of star formation, which -- after a couple of million years -- leads to lots of Type II supernovae. But to see more than one Type I is a little weird, since they take so long to "cook". I wouldn't say there is anything cosmic in the timing (haha); more that it's just pretty nifty.

But it's also cool that this is a relatively nearby galaxy. Type I's are used to get the distance to very distant galaxies, and we need to understand them as well as we can to do that. The more that happen nearby the easier it is to study them. So this may turn out to be a real boon to astronomers.

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