Back in 2016, an astronomy enthusiast named Víctor Buso decided it was a good night to test a new camera on his telescope. The test went well enough that hardware in space was redirected to image what he spotted, and Buso now has his name on a paper in Nature.

Lots of amateurs, like Buso, have spotted supernovae. That typically leads to a search of image archives to determine when the last time a specific location was imaged when the supernova wasn't present—this is often years earlier. Buso didn't have to search, because his first batch of images contained no sign of the supernova. Then 45 minutes later, it was there, and the supernova continued to brighten as he captured more pictures. Buso had essentially captured the moment when the explosion of a supernova burst out of the surface of a star, and the analysis of the follow-on observations was published on Wednesday.

It went boom

The odd thing about many supernova (specifically those in the category called type II) is that they're not explosions in the sense of the ones we experience on Earth. In a supernova, the core of the star collapses suddenly, triggering the explosion. But it happens so quickly that the outer layers of the star barely budge. The first overt sign of what's going on comes when the debris of the explosion reaches the surface of the star, a process called the breakout. This causes the star to suddenly brighten, a process that continues through some ups and downs for days afterward.

The dynamics of the brightening and ensuing changes could tell us something about the processes that are driving the supernova. But we rarely see them, because it takes an extreme bit of luck to have an instrument pointing at the right place at the right time. In fact, the team doing the new analysis estimates that there's only a one-in-one-million chance of catching something like this in general—and that's not considering the need for clear night-time skies and a supernova that's not buried too deeply in its host galaxy to be visible.

Víctor Buso and Gastón Folatelli

Víctor Buso and Gastón Folatelli

We have had some previous bits of serendipity, like a supernova that was caught only three hours after it exploded. And the Kepler mission, which was designed to watch a single patch of sky, caught the breakout of two supernovae, but they were only recognized years after the event, which eliminated the opportunity for much in the way of followup observations.

Here, followup started in less than a day and included the Swift Observatory, an orbiting platform that specializes in high-energy (UV and X-ray) astronomy. The observations include Buso's original imaging as the supernova ramped up toward the peak of breakout light. They miss a brief drop after the breakout is complete and a ramp up to the supernova's full intensity, but observations begin again in multiple wavelengths as the debris cools from this peak about a day later. They also cover the beginning of when the decay of radioactive isotopes start heating the debris a second time, about a week after the explosion.

Matching models to data

This gave the authors the chance to test different models of the exploding star to find out which properties they'd have to tweak to reproduce the light that was observed. The model that worked involved a supernova that released more than 1051 ergs (that's 2.4 x 1028 Megatons) and ejected nearly three and a half Suns' worth of materials. If the star was to replace the Sun, its outer edges would come close to engulfing Mars. The weak emission of hydrogen from the explosion also suggests the star had shed most of this element and instead was mostly composed of helium.

But it's really the initial observations by Buso that are key. They show the amount of light emitted rising much faster than any of the later data, and there's no way to account for both the early and later observations through a single mechanism. The authors conclude that's because the light is produced by two different processes: the breakout of the explosion early and the expansion of the debris later.

The new observations are an important validation of our models. We had predicted for some time that there would be a breakout event generated by this sort of supernova, but it took a great deal of luck (and a new camera) for us to finally catch a supernova early enough to test these models.

Nature, 2018. DOI: 10.1038/nature25151 (About DOIs).