Tuesday, May 08, 2007

The brightest supernova ever?

Photo credit: Lick/UC Berkeley/J.Bloom & C.Hansen

This morning, there are dozens of news stories on the internet about what is being called the brightest supernova ever seen. To be honest, I was a bit surprised about the hubbub, as the details of this supernova were published in December. Although the basic idea behind this explosion is relatively unchanged, new evidence has come in that supports the hypothesis, and, as always, NASA is hungry for press releases.

So, what's the big deal? The explosion we are seeing is the death of a very massive star. Typically, when a massive star dies, it has run out of fuel in its core, having burned all of its hydrogen gas through successive stages into iron. The iron tries to burn in a nuclear reaction, but that absorbs energy instead of releasing it, and the center of the star collapses into a neutron star or a black hole. The rebound from this explosion rips the star apart.

But that doesn't seem to be the case for Supernova 2006gy (the name astronomers use for this supernova). The supernova has stayed very bright far longer than expected for this type of supernova. The only explanation we can think of is a strange physics phenomenon called "pair instability." Pretty much, that means that the center of the star gets so amazingly hot that the light radiation in the core of the star (in the form of gamma rays) becomes so energetic that the gamma rays are converted into matter and antimatter. This is a direct result of Einstein's equation E=mc2, which means that energy can turn into matter and vice versa. It takes some special circumstances (very high temperatures and densities), but in these extreme stars, those conditions are met. And, as you may know, when the antimatter hits normal matter, it annihilates and releases energy, which then is turned back into matter and antimatter again because of the extreme conditions. This circumstance is unstable, and the entire stare is ripped apart in a huge explosion. We don't know if a black hole might be left behind or not, but most of the star (which is 120 times the mass of the sun or more!) is blasted into space.

This supernova mechanism has been proposed before, but astronomers didn't expect to see it. In most galaxies these days, the giant stars are polluted by lots of metals made in previous generations of stars. Before the star finishes its life cycle, these metals act almost like a sail, and light from the star pushes the metals away from the star. When the metals leave, they take a lot of hydrogen and helium with them. In this way, a star that started life with a mass 100 times that of the sun is whittled down to fewer than 10 times the mass of the sun in just a few million years. The resulting star still explodes, but in the "normal" way.

So, if supernova 2006gy is a pair instability supernova, astronomers will need to ask how such a thing could happen in the modern Universe. Can some monster stars somehow manage to hold on to their envelopes? Or did this star form in a galaxy or a part of a galaxy where there were far fewer metals than normal? Or have we mis-interpreted the data so far? These questions will keep supernova astronomers occupied for years to come!

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