Supernovae are some of the brightest events in the entire Universe, outshining whole galaxies at their peak luminosity. However, they are rare events at the level of individual galaxies. Since the progenitors may be relatively dim, they are hard to spot, so monitoring the sky for future explosions is a daunting task. Based on observations, however, at least some stars shed a lot of mass prior to the explosion—an event that could be seen in advance.

Observers monitored one such star beginning 40 days before the explosion. By measuring the energy signature of the ejected matter and the spectrum of the subsequent explosion, E. O. Ofek and colleagues were able to connect the two events, showing they were not merely coincidental. Additionally, by studying the evolution of the system before, during, and after the supernova, the researchers found a coherent explanation for one type of supernova, and a means to spot them before they happen.

Supernovas are broadly divided into two categories: white dwarf supernovae (also known as type Ia) and core-collapse supernovae. The second category occurs in very massive stars (at least 8 times the mass of the Sun) that use up their fuel and explode because their cores can no longer withstand the pressure of their own gravity. Although core collapse supernovae have that in common, their progenitor stars can be quite diverse. The current study focused on type IIn supernovae (sometimes abbreviated as SNIIn), which are widely thought to be explosions of extremely rare stars known as luminous blue variables—some of the most massive and brightest stars known.

SNIIn explosions are characterized by relatively narrow hydrogen emission lines. The width of emission lines are related to the thickness of the gas surrounding the star, and how rapidly that gas is moving. The narrow hydrogen lines distinguishing SNIIn are from relatively thin shrouds of gas, ionized either by the intense radiation or the powerful shock waves produced during the supernova explosion. According to current explanations, this feature can be understood if the dying star shed a lot of mass before the explosion, which then interacted with the matter ejected during the supernova event, producing these emissions.

Supernova SN 2010mc was spotted by the Palomar Transient Factory (PTF), which looks for supernovae and other one-time (transient) events in a wide swath of the sky. As the name suggests, light from SN 2010mc arrived at Earth in 2010. (The first supernova of the year was 2010a; the 27th was 2010aa. Thus, there were a lot before 2010mc, but I'm too lazy to work out what number that was.) Going back in the PTF archives, the researchers discovered a precurser outburst, from the same region of the sky, which occurred 40 days earlier.

Because the non-supernova outburst and SN 2010mc were connected in time and space, it would be extremely unlikely they were coincidental. SN 2010mc was a SNIIn explosion based on its spectrum, so the relatively long period of observation allowed the astronomers to make detailed comparisons of the system's evolution to plausible models. (The research team also identified two other similar supernovae in the PTF data, but their total history was less clear.)

The story of SN 2010mc went like this: decades or even centuries before, a luminous blue variable star shed multiple layers of material into space. These began moving rapidly, but slowed as they interacted with gas in interstellar space. The final outburst before the supernova itself was particularly huge: about 10 Jupiters worth of gas was ejected at about 2,000 kilometers per second.

The supernova debris hit those previous ejected shells with dramatic effect, ionizing the gas and causing distinctive emission lines . However, as the shock front from the supernova expanded, it wiped those spectral lines out. The data even hinted that the whole system brightened again as the more distant ejected shells were engulfed.

Taken as a whole, the events leading up to and following SN 2010mc were consistent with the existing models of SNIIn explosions. As an added bonus, the researchers were able to compare the event to specific models of the precursor, focusing on the way the star shed mass. They concluded that similar supernovae should be predictable, provided we monitor luminous blue variable stars for telltale outbursts.

While these stars are rare, they include the dramatic binary star system Eta Carinae and up to 19 other relatively nearby stars. With a successful prediction scheme in place, we might be able to predict when they will explode with enough time to point our telescopes, prepare our observations, and make popcorn.

Nature, 2013. DOI: 10.1038/nature11877 (About DOIs).