Monday, September 18, 2006

Overdue Supernovae

We're overdue a supernova or two. (A supernova is the violently explosive death of a (usually) massive star.) The Milky Way is believed to have a supernova rate of one or two per century, and yet, there has not been a supernova observed in our home galaxy since the advent of the modern telescope (in round numbers, let's say that was 100 years ago).

There isn't really a problem here, other than the fact that this is kind of annoying and we astronomers will fall over ourselves in a melee of sheer giddiness while pointing every telescope known to mankind at the thing when it finally happens. The reason it's not a problem is thanks to "small number statistics"; there have been about half a dozen supernovae in our Galaxy in the last millenium. We've simply been unlucky.

The supernova rate for a given galaxy is usually based on a measurement of the star formation rate. This may seem a little counterintuitive, but supernovae precursors are (usually!) massive stars. Massive stars have a much shorter lifetime than lighter ones; the lifetime of our own Sun is about 10 billion years, while the lifetime of a star ten times heavier is more like 10 million years. If we see a young star die, we know that it was born recently. Turning this around, if we can measure the rate of star formation, we can estimate a supernovae rate.

Star formation rate measurements are usually based on measurements of the density of ionized gas, since star formation and young hot stars will ionize the gas they are sitting in. More locally (and more simply, and more sexily) we can watch radioactive isotopes decay. Aluminum-26, which has a half-life of 746,000 years, emits a gamma ray with an energy of 1.81keV when it decays. So, by looking at the strength of this emission in the Milky Way, an estimate can be made of the Galactic star formation, and thus supernovae rate. This has been done, estimating a supernovae rate of about two per century.

So we know we're due. It will happen; we just have to be ready. And while we're pretty damn sure we'll notice it by the fact that it'll signal thousands of events in sitting happy neutrino detectors,* it's an interesting question as to whether or not we'll actually be able to get all of the information feasible from it. As I see it, there are two basic problems. The first is noticing it. The Milky Way takes up a rather large fraction of the sky, but there are all-sky monitoring surveys either already in place or on the drawing board. For many of these, however, "all-sky" really means "all of the sky we can see without looking in the plane of the Galaxy because it's really messy down there." Unless the supernova goes off nearly opposite the other side of the center of the galaxy from where we are situated and we can't see it becaues there's just so much other junk between us and it, I doubt that detection will be a problem. Historical supernovae are just that: they were bright and noticeable enough to sometimes even be noted by the casual observer.

No, no, what is much more likely to be the real kicker is the fact that telescopes and instrumentation have been designed for observing really faint objects, or really far away objects. Not incredibly bright nearby ones, which is exactly what a Galactic supernova would be. If it goes off while we've still got working space telescopes, will any of them even be able to take a snapshot of it without it saturating the detector or worse? The large telescopes that the majority of the astronomy community is gravitating towards are not ideally suited for studying bright nearby stars. If our nearest neighbor, Andromeda (also known as M31), gets around to having a supernova (Andromeda is also stalling on killing off stars lately, it seems), though, we might be better prepared to stare at it with all of our lids open.

* On the other hand, apparently dropping a wrench near the big neutrino detecting water tank can cause thousands of events as well...


Stephen said...

The Milky Way is saving up for a really nearby SN, perhaps RS Ophiuchi, or Eta Carina.

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