Supernovae

Results from the Chandra telescope apparently say that Type Ia supernovae are the result of two white dwarfs spiraling in and colliding with each other:

Most scientists agree a Type 1a supernova occurs when a white dwarf star — a collapsed remnant of an elderly star — exceeds its weight limit, becomes unstable and explodes. Scientists have identified two main possibilities for pushing the white dwarf over the edge: two white dwarfs merging or accretion, a process in which the white dwarf pulls material from a sun-like companion star until it exceeds its weight limit.

“Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging,” said co-author Akos Bogdan, also of Max Planck. “This is probably not what many astronomers would expect.”

This is a really surprising result, at least to me.  I was under the impression that the second possibility (accretion) was generally agreed on.  I thought this was well-established, textbook stuff.  I didn’t know there was any significant uncertainty about it.  But I haven’t paid much attention to supernova physics, so I guess that’s just my ignorance.

Type Ia supernovae are very important in cosmology, because they’re pretty good “standard candles” — that is, they all have nearly the same intrinsic luminosity, meaning that by observing the apparent brightness of one you can figure out its distance.  Mapping out the distance versus redshift of these supernovae was the original way that people originally discovered the accelerating expansion of the Universe.

The accretion model provided a nice intuitive explanation of why these guys were standard candles.  Accretion occurs until the white dwarf exceeds a certain critical mass, at which point the star explodes.  So in this model all Type Ia supernovae come from stars of pretty much exactly the same mass.  Presumably there’s no such argument in the merger model.  That doesn’t mean that these supernovae aren’t standard candles: the fact that they are is well-established observationally and doesn’t depend on the theoretical model of where they come from.  But it does make the fact that they’re standard candles more surprising than before.

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