Astronomers have got closer to one of the holy grails of stellar research – capturing a supernova as it explodes. Although the first moments of the explosion were not seen, the event was picked up three hours after it started, easily setting a new record. We have watched thousands of supernovas as they pass their peak brightness and decline, however the first stages remain something of a mystery, and such an early start to observations has been eagerly anticipated.

Although we can identify likely supernovae candidates and assess when they are getting close, we can't yet predict when stars are going to explode. Consequently, we usually only notice these enormous events after they have become bright enough to stand out. To address this, the Intermediate Palomar Transient Factory (iPTF) makes regular checks on red supergiants considered good prospects to become type II supernovae.

On October 6, 2013, iPTF hit paydirt, witnessing SN2013fs in the nearby galaxy NGC 7610 an estimated three hours after the explosion began. Other observatories were quickly alerted and a range of telescopes capable of viewing the event at different wavelengths of light were focused on the star within six hours of ignition.

The findings, both from SN2013fs' initial exciting rise in brightness and the long decline, have now been published in Nature Physics. The most important aspect is that SN2013fs was surrounded by circumstellar material to a distance roughly equal to the orbit of Jupiter.

Recent measurements of other type II supernova have led some astronomers to suspect that stars about to become supernovae throw off large amounts of gas. When the explosion occurs, its light must pass through this material, giving each supernova a unique signature.

Dr Ofer Yaron of the Weizmann Institute of Science, Israel, and co-authors concluded that SN2103fs supports this theory. Their estimates of the speed with which the material was expelled led them to calculate that most was emitted in the 500 days leading up to the explosion. This is consistent with some previous supernova observations, but these were all from fairly rare subcategories, whereas SN2013fs falls into the most common category, which makes understanding its nature particularly important. The finding will force a rethink of models of supernova behavior.

Although astronomers think they understand the broad outlines of how type II supernova occur, with an iron core growing and then collapsing, the details remain vague. “Stellar evolution in the final years [before explosion], which sets the initial conditions for the final collapse and explosion of such stars... is poorly understood,” the paper notes. “Statistically it is very likely that not even a single star that is within 1 year of explosion currently exists in our Galaxy.” SN2013fs could be our best view for a while.