Wednesday, March 21, 2007

Things that go boom in the night part 2

Image credit: NASA/STSci

Yesterday I mentioned that I am at a conference discussing exploding stars known as "Type Ia Supernovae." I also mentioned that there are competing hypotheses about what causes these explosions. Today we'll briefly mention one of these: the so-called "single-degenerate" channel.

A degenerate star is a star made out of a type of matter that is packed so densely that it cannot be packed tighter. It is like taking the sun (which is nearly a million miles across) and squeezing it into a ball the size of the Earth. We also call such a star a "white dwarf," because it is small and glowing white-hot.

The vast majority of white dwarfs slowly fade and cool away. Yet if a white dwarf gets heavy enough, the material it is made of is squeezed until it stars a nuclear reaction that, in a matter of a few seconds, burns the entire star and causes a supernova explosion. The magic mass limit is known as the "Chandrasekhar mass" after famous astrophysicist Subrahmanyan Chandrasekhar, who calculated this magic level.

If a white dwarf is not at the Chandrasekhar mass, it will have to gain matter until it reaches the Chandrasekhar mass in order to cause a supernova. One way to gain matter would be if the white dwarf happened to be close to another star, the white dwarf's gravity could pull gas from the other star onto the white dwarf.

We see white dwarfs pulling matter from nearby companions. These are called "cataclysmic variables." Dozens of these systems are known. But what we don't know is if these cataclysmic variables will end up sending enough matter to the white dwarf to cause the white dwarf to explode. The reasons for this uncertainty involve very complex physics that we can only partially model on computers. So, we don't know if any of these systems can ever make a Type Ia supernova. Many astronomers at this conference believe this is the best way to make the supernovae, but others disagree. Tomorrow I'll discuss the other competing theory for making Type Ia supernovae.

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