Wednesday, May 27, 2009

Messier 82 conducts a covert nuclear test

Radio detection of a supernova in the galaxy Messier 82
Image Credit:
MPIfR (click on the picture for a full and extensive list of credits)

Supernovae are powerful explosions that end the lives of the most massive stars in the Universe. At their peak, supernovae can outshine an entire galaxy. Using the Hubble Space Telescope, we've seen supernovae in galaxies billions of light-years away.

Since massive stars end their lives as supernovae (those with masses at least 7 or 8 times the mass of the sun, depending on the results of my research), and since massive stars don't live very long (less than 50 million years, compared with our sun's lifetime of 10 billion years), we expect to see lots of supernovae in galaxies that are currently making loads of new stars, fewer in galaxies that are making just a few new stars, and no supernovae in galaxies not making stars. And this is what we tend to see.

But there has been one major exception. The "Cigar Galaxy," Messier 82, is a fairly nearby galaxy, and it is making new stars at a prodigious rate. So fast, in fact, that the galaxy appears to be exploding, with hot gas spewing into space. There are many gorgeous pictures of Messier 82, such as these from Hubble. We think that the hot gas is being blown out of the galaxy by dozens upon dozens of supernovae, which should be going off at a rate of one every few years or so. (Our Milky Way has a supernova about every 100 years, by comparison). But there's been a problem. Nobody's seen any supernovae in Messier 82.

Now, we astronomers weren't too concerned by this. A byproduct of star formation is lots of dust, and dust is very good at hiding optical light. Other wavelengths of light, like infrared, X-ray, and radio light can travel right through dust, to an extent, so we knew we either had to look for supernovae in other wavelengths than visible light, or we would just have to wait for one to happen near the edge of all the dust in Messier 82. But since most of the new stars in the galaxy are located deep inside the clouds of dust, we might have to wait a long time.

In April of this year, Dr. Andreas Brunthaler, an astronomer at Germany's Max Planck Institute for Radio Astronomy, was looking at Messier 82 with the VLA. The VLA is a large array of radio telescopes in New Mexico (if you saw the movie Contact, Jodie Fisher's character can be seen listening to the radio signals from these telescopes, something you can't actually do). Dr. Brunthaler was taking high-resolution radio images of Messier 82 in order to study the gas and forming stars in this galaxy, when he noticed something that didn't look right. There was a tiny circle of radio light, shining brighter than the rest of the galaxy, that hadn't been there in the past.

Dr. Brunthaler then looked at data they'd taken in early 2008, and he found the same radio source, only smaller. But there was no source on earlier data. He'd found an elusive Messier 82 supernova! You can see three radio pictures of Messier 82 in the photo at the top of this article. One from 2007 looks normal, the one from 2008 shows a tiny bright dot, and the one from 2009 shows the larger bright circle.

More work determined that the explosion happened around the start of February 2008. Lots of optical supernova searches had pictures of Messier 82 from that time, and they didn't see a thing. Neither did infrared or X-ray observations. This supernova is so deeply buried in the dust and gas at the center of Messier 82 that only radio waves can escape!

Now that we know there are supernovae in Messier 82, and not something weird, we can keep watching for more supernovae in the radio regime. If we can get a good count on how often supernovae occur in Messier 82, we might be able to get a good handle on how fast stars are being made deep in the dust enshrouded center of this galaxy.

Lastly, now the Universe should know that it cannot detect nuclear explosions without us knowing. It is unclear if the United Nations is going to act on this surreptitious explosion or not.

Note: In this article, when I talk about supernovae, I'm specifically talking about the explosions at the end of the lives of massive stars, not another type of supernova that is the explosion of white dwarf stars. That's a completely different kettle of fish. I just wanted to make that clear.

No comments:

Post a Comment