Sunday, March 25, 2007

Things that go boom in the night, part 3

Image credit: NASA/Dana Berry, Sky Works Digital

This update is a few days later than promised; I happened to get involved in some discussions during breaks in our conference, and by the evening, I was too tired to go online. So, although the conference is over, I'll continue to try to bring things up to date.

So, remember that we were talking about Type Ia supernovae, some of the most intriguing explosions in the Universe. There are two suspects for objects that form these particular explosions. The first, discussed last time, involves a white dwarf pulling mass from a companion star until it reaches the all-critical "Chandrasekhar mass," about 40% larger than the sun's mass, at which the white dwarf would undergo a thermonuclear explosion. Today we talk about the second mechanism, the double degenerate model, so called because it involves two white dwarfs, and white dwarfs are also known as "degenerate stars," because when you pack the mass of the sun into something the size of the Earth, the matter enters a state called degeneracy.

The way the double degenerate model works is fairly simple. If you have two white dwarfs orbiting each other, over time they will get closer and closer. This is because under Einstein's Theory of General Relativity, the objects radiate "gravitational waves," which carry energy away from the system. When the white dwarfs get close enough, they will merge together. If their total mass is over the Chandrasekhar mass, the combined white dwarf will explode!

Of course, the devil is in the details. As the white dwarfs get close, if one is more massive than the other, the bigger one will rip the smaller one to shreds. We'd get the white dwarf version of Saturn, in which a white dwarf (with the mass of the sun squeezed into a ball the size of the Earth) would be surrounded by rings with a mass of half the sun or more, still in that weird state of degeneracy. This is really hard to study from theory, and we certainly have never observed a star in this state. Maybe the remnants of the second star will fall onto the bigger star quickly, which would cause an explosion. But if they don't fall on quickly enough, the system may cause big winds that will blow the smaller star's remnants away.

Perhaps a bigger problem is that we only know of a few white dwarfs in double degenerate systems, and only one of these is above the magic "Chandrasekhar limit." For these to cause Type Ia supernovae, there should be dozens in our Milky Way that we should be able to see. Maybe we are there and haven't found them yet. But maybe these aren't the parents of Type Ia supernovae.

So, in short, we really don't know what causes Type Ia supernovae. This is really disturbing, since we use these explosions to study the distant Universe. But it is a mystery I think we can solve, given some more time.

Over the next few days, I'll summarize a few other controversies from the conference that are quite interesting.

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