Type Ia in M101

Excitement is spreading through the Supernova community as a Type Ia supernova was detected in M101 (aka the Pinwheel Galaxy). This makes it one of the closest supernovae in recent years (21 Mpc), even closer than the recent one in M51 (aka the Whirlpool Galaxy). It was caught early in its evolution by the Palomar Transient Factory and has been designated as SN 2011fe (PTF11kly). It is expected to brighten to around 10th magnitude, which should be visible to anyone with a moderate sized telescope. M101 is just above of the handle of the Big Dipper and so should be visible for most of the night. Today's APOD shows a picture of the supernova in M101, which should make it easier to find (a smaller version in visible on this page).

Even more exciting for astronomers is it's type as a Ia supernova. Type Ia supernovae are a special type of supernovae that lack hydrogen and show a singly ionized silicon line in their spectra. The leading theory to explain these observations is that a Type Ia supernovae is really a white dwarf that exceeded the Chandrashekar limit for mass of 1.4 solar masses and detonated like a massive thermonuclear bomb. White dwarves don't have any surrounding hydrogen and are rich in silicon, explaining the spectra.

White dwarves when they form typically aren't close to the Chandrashekar limit so they must accrete the mass some how. Currently it is thought that this is done either by the white dwarf pulling mass off of a companion star that is in a close orbit or it merges with another white dwarf. It is thought that when the white dwarf has accumulated enough mass, it undergoes a thermonuclear fusion runaway which starts as a deflagration but then converts to a detonation completely obliterating the white dwarf. This releases a ridiculous amount of energy to the tune of 10^{51} ergs (one FOE), which interestingly is equal to the gravitational binding energy of the white dwarf.

The fact that the white dwarf explodes at a preset mass and has a predetermined energy output based on that mass makes Type Ia's useful as standard candles. Since Type Ia's are super-bright they can be seen for a very long distance, even out to cosmologically relevant distances. In fact Type Ia's are so useful as standard candles in cosmology there is an entire subsection of astronomy devoted to their detection. With that data cosmologists have been able to show that the universe is not only expanding but that the expansion is accelerating.

However, there is still quite a bit that is unknown about Type Ia's as astrophysical objects which puts limits on the precision of distance measurements. In order to learn more, astronomers investigate Type Ia's that are closer by or that have already exploded. This is why the Type Ia in M101 is so exciting, it is closer than any Type Ia in recent history and we can use all our current instruments to take a very close look at it.

Of course what would be even more exciting would be to have a Type Ia go off in our own galaxy. We expect one to go off roughly every 100 years in our own Milky Way and since the last one was spotted by Kepler back in 1604, we are long past due for one. In fact we are due for a supernova of any type as the last one that we know of to go off in our galaxy was in the 1860's.