In general, astronomy is reactive—we spot something unusual by chance and point as many telescopes as we can manage to try to figure out what's going on. It's rare that we have something pointing in the right direction to catch an event right as it starts.

But the Kepler Observatory was designed to point at the same spot and stare for years and capture a constant stream of images, which is how the telescope was able to catch planets as they moved in front of their host stars. But interesting things went on behind the stars, and Kepler captured that data, too. Yesterday, astronomers announced that an analysis of the Kepler data captured the moments when a supernova burst through the surface of its host star—not once, but twice.

Most people think of supernovae as explosions that destroy a star. But type II supernovae are really the collapse and explosion of the core of the star. In cases where the star is a giant, this collapse and explosion happens so quickly that the outer layers of the star are unaffected by it and continue to look normal for nearly an hour, even as catastrophic events are occurring behind the star's surface. That seeming normalcy ends when the shockwave of the explosion reaches the surface and breaks out in a brilliant flash of light. This type of explosion is called a type II-P supernova.

Models of this process predict a rapid rise in light, followed by a gentle peak during the first weeks afterward. From there, there's a gradual dimming for several months, which ends with a relatively sudden drop in emissions. This sort of pattern, which requires repeated observations to detect, would be easy for Kepler to capture should a type II-P supernova occur within its field of view, given that the telescope is designed to capture the light curves of stars.

So, astronomers managed to get some time on Kepler focused on monitoring hundreds of galaxies within its field of view. Their analysis of this data has identified two type II-P supernovae, KSN 2011a and KSN 2011d. They're different distances (a is 700 million light years, d 1.2 billion), but both look like the predicted II-P light curve.

To find out details on the stars involved, the authors turned to computer models of this type of explosion. These models will produce slightly different light curves based on the properties of the star that's doing the exploding. By matching the peak amount of light emitted and the time it took to reach that peak, the authors were able to estimate the properties of the supergiant stars that exploded.

The data is consistent with a stellar mass of anywhere between 10 and 20 times that of the Sun. And the stars were truly giants, with KSN 2011a having a radius of about 280 times that of the Sun and KSN 2011d having one that's nearly 500 times the radius of our Sun.

A detailed comparison of KSN 2011a's light curve to that produced by the models shows that it brightened faster than predicted. The authors suggest that this is because the star had been losing matter to the surrounding space for some time. Thus, after the shock wave burst out of the surface of the star, it ran into the material surrounding the star, causing that to emit light as well. But this material also obscured a key feature they were able to spot in the light curve of KSN 2011d.

There, observations right at the time of the supernova show a sudden, small peak before the big rise in light of the supernova. The authors argue that this represents the very moment of breakout, when the shockwave first pushes through the outer layers of the dying star.

Since these are only two examples, it's likely they don't capture the full diversity of type II-P supernovae. There are probably more subtleties waiting to be discovered. But the authors are optimistic that the extended Kepler mission will let them observe even more, so we may be building a better picture of the death of stars with some time. Such observations are just a bonus of building hardware that revolutionized our perspective on other planets.

The arXiv. Abstract number: 1603.05657 (About the arXiv). To be published in The Astrophysical Journal.