Tycho's Supernova Classified

In 1572 Tycho Brahe observed a supernova. He, of course, didn't know it was a supernova at the time, but he did see a "new star" and coined the term nova for such stars. In the 400+ years since then it has been decided that what Tycho saw was a type Ia supernova - the thermonuclear detonation of a carbon/oxygen white dwarf of about 1.4 solar masses. The classification was based on the fact that there is no detectable central object left in Tycho's supernova's remnant and all of the other types of supernovae leave either a neutron star or a black hole.

The problem with that classification is that all other supernovae are classified differently because we often can't tell if they left behind a neutron star/black hole or not. Due to historical reasons, supernovae are classified solely by their spectrum. Sample spectra with identifiers are shown below (thanks to Daniel Kasen at Lawrence Berkley National Lab for the image). Basically type Ia supernova are identified by a complete lack of hydrogen and helium lines and strong silicon lines. Type Ia supernovae are, for the most part, extremely uniform events, which make them useful for things like cosmology. However they are theoretically very poorly understood as we're not even sure what causes them to explode or how that explosion proceeds. So while most type Ia's are remarkably consistent, a small fraction of observed type Ia's do really odd things and nobody really has any idea why. To help settle this it would be nice to take a nearby supernovae remnant (like Tycho's) and find out its type as well as what it did or didn't leave behind.

A new paper published in Science by Krause et al. provides, for the first time, a spectral classification of Tycho's supernova by using actual spectra from an explosion whose light passed us in the 16th century. There are two ways to get that light - send a modern spectrograph back in time to 1572 or use a light-echo (light going 200 light-years in the opposite direction before bouncing off interstellar dust and heading back our way). Bet you can't guess which Krause and his team used...

It turns out that Tycho's supernovae was just your ordinary, run-of-the-mill, brighter-than-the-rest-of-the-galaxy-combined explosion. It has all the right silicon lines and absolutely no hydrogen or helium lines. This means that when we study Tycho's supernova remnant, we're studying the same kind of supernova remnant as those used as cosmological probes, heavy element production mechanisms, and galactic dynamics.