|Supernova 2011dh in the Whirlpool Galaxy. Image Credit: Peter Edwards|
Thankfully, there are lots of galaxies in the universe. So, when astronomers want to study supernovae, they look at a lot of galaxies. Such surveys for supernovae are turning up new explosions in distant galaxies all the time. Still, many of these galaxies are fairly far away, and it is rare to find a supernova in our neck of the woods.
Enter the Whirlpool Galaxy, also called Messier 51. The Whirlpool is nearby, as far as galaxies go – "only" 26 million light-years away. It is also a favorite target of amateur astronomers, because it is a beautiful face-on spiral galaxy, and its spiral arms can be glimpsed by modest-sized telescopes in dark places. Last week, sometime before the evening of May 31, a star exploded in one of the spiral arms. The picture at the top of this post shows a picture with the supernova (the "new star" marked by white lines on the left picture) and a picture of the galaxy taken a couple of months ago, before the star exploded.
Although the new star may look faint to you, remember it is 26 million light-years away. The other stars in the picture are stars in our own galaxy and a few hundred light-years away. The supernova is 100,000 times more distant than these stars, but yet appears the same brightness! If one of the nearby Milky Way stars in this picture were to explode as a supernova, it might get as bright as the Full Moon, casting shadows at night and being easily visible at noon.
Because this supernova is nearby, it is fairly easy to study with modern equipment. We also are lucky that the Hubble Space Telescope took a detailed picture of the Whirlpool in January 2005. Astronomers at the University of California Berkeley and the California Institute of Technology were able to locate the star that exploded in the Hubble's image. This means that we know some details about the star before it exploded, which allows astronomers to determine some important parameters about the star. For example, before it exploded, the star was probably about 18 to 24 times the mass of our Sun. We expect stars with this much matter should end their lives in supernova explosions. (Some of my work is related to this very question – How big does a star have to be before it will die as a supernova instead of more gently becoming a white dwarf?)
With this new supernova, now christened with the unspectacular name of SN2011dh, the Whirlpool appears to be a veritable supernova factory. Since 1994, at least three stars have exploded in the Whirlpool: one in 1994, another one in 2005, and now this one in 2011. Prior to 1994, no supernovae were known to have occurred in the Whirlpool Galaxy, though astronomers had only been watching the galaxy for a century or so, and it's possible they missed one. But why does the Whirlpool get three in 17 years while our own Milky Way has not produced one visible from Earth for over 400 years?
It could be luck. Random, rare events, like supernovae, often seem to come clustered together. This is a property of statistics. Also, the Whirlpool's gorgeous spiral arms are visible because they are furiously producing new stars, perhaps at 5 times the rate that our Milky Way does. If the Whirlpool is producing stars five times faster than the Milky Way, then we should expect it to have supernova explosions five times faster (averaging one every 20 years). At this higher supernova rate, it is completely probable from statistics to see three explosions in 17 years after decades of not seeing any.
One thing we can be certain of: these three supernovae are not related. The stars that exploded are thousands of light-years apart. Each star exploded before any light from the other explosion reached them, and even if they were within a dozen light-years of each other, we know of no way for stars to cause one another to explode. (If the stars are really close, like a few light minutes apart, that could be different). Any aliens living on a planet near the 2011 supernova could have looked elsewhere in their sky and still have seen the stars that we on Earth saw explode in 1994 and 2005, and would have had to wait thousands of years to see those explosions. Of course, this all happened 28 million years ago by their reckoning. Time, space, and the order of events can be quite mind-blowing.
So, when will we see the next Milky Way supernova? It could be tomorrow. It could be a decade. It could be centuries. But I can guarantee that the light from that explosion (and many others) is already on its way – we just have to wait for it to get here.