The Origin of Type Ia Supernovae

As you may have seen, last week a major discovery concerning the nature of Type Ia supernovae which are so important for cosmology. First, why are these supernovae so important? It is because this class of stellar explosions are believed to all be essentially the same - i.e., they all produce the same amount of energy. This means that, if you see one on the sky, you can use its brightness to get its distance (its "luminosity distance" to be overly technical). From its spectrum, you can get its redshift, and by comparing the distance vs. redshift for many of these, one derives the expansion history of the universe. This is pretty much how astronomers first inferred the existence of "dark energy" in the universe, a measurement that is supported by completely separate techniques which I'm not going to discuss now.

Okay, so what are these explosions? That has been the tricky part. Based on their similarity to each other, their optical spectra, how often they occur, and the type of galaxies in which they occur, type Ia supernovae have long been associated with the thermonuclear explosion created by the gravitational collapse of white dwarf. What's that? Well, white dwarfs are "stars" which are supported not by fusion in their core like our Sun, but by the fact that electrons can't get too close to each other ("electron degeneracy pressure"). However, if the mass of the white dwarf is too high, its own gravity is too strong to be balanced by this pressure and it collapses on its self, triggering an explosion which envelopes, burns, and then blows up the entire white dwarf. Since this maximum mass is pretty much the same for all white dwarfs (1.4 Solar Masses, the so-called Chandrasekhar limit since it was first derived by Prof. Chandrasekhar - easily one of the most brilliant astrophysicists of the 20th century and the person that Chandra is named after), it isn't too surprising that Ia supernovae are so similar.

Okay, but how does the white dwarf get so much material? There are two possibilities. The first is that a normal star is in a close orbit around the white dwarf. In some cases, the gravitational attraction of the white dwarf will be so strong that is will rip off the outer layers of the normal star and cause this material to fall onto the white dwarf, gaining mass. (This process is generically called "accretion") The second possibility is that two white dwarfs are orbiting each other, merge for whatever reason, and then the combined mass of the two white dwarfs is so high that it explodes. How can you distinguish them? Well, in the first scenario, the material which accretes onto the white dwarf is hot and shines brightly in the X-rays, while in the second scenario you would get no such emission before a Ia supernova. Is this difference actually observable? Looks like it is. To read more, go here and here among other places. The actual scientific article can be read here, but you might need a library or university IP address to read it for free (any decent public library will have the print version, which is easier to read anyway). Hope this made sense. Please leave questions below.