The problem with studying supernovas is you never know when they are going to happen. Consequently, although we have observed thousands of exploding stars as they peak and cool, the crucial lead up to the main event has remained something of a mystery. Considering how vital supernovas are to the development of the universe, including our existence, that’s a big hole in our knowledge. Now, however, thanks to a stroke of luck, astronomers have witnessed a complete shock cooling curve, one of the lesser-known stages prior to core collapse supernovas.
Before giant stars explode they undergo a collapse, followed by a rebound shock. This produces two events, known as shock breakout and shock cooling, that occur in the days before the explosion, although Patrick Armstrong of the Australian National University told IFLScience not all have both. A handful of shock breakouts have been seen in recent years, but only very partial pictures of shock cooling curves have been obtained.
Now, however, Armstrong is first author of a paper in Monthly Notices of the Royal Astronomical Society announcing detailed observations of the shock cooling curve prior to SN 2017jgh, a Type IIb supernova that went off around a billion light-years away, and whose light reached Earth four years ago.
Fortunately, when it did the Kepler Space Telescope was keeping track of a star in almost exactly the same direction as SN 2017jgh’s parent galaxy. In its regular check-ups of the star’s brightness Kepler’s field of view also took in the galaxy, allowing it to observe the shock cooling curve at 30-minute intervals.
Kepler captured other supernova in the background of stars it was tracking. Most of the time, however, no one noticed until a fair while afterwards when the data was analyzed.
However, Armstrong told IFLScience, at the end of Kepler’s life emphasis shifted to looking out for “transients”, or sudden changes in brightness in its field of view. Astronomers kept close track of anything unusual Kepler detected, and had larger ground-based telescopes check out anything interesting. The shock cooling process takes around three days, and we now have observations of all of it from Kepler, and the latter half with other instruments.
"Until now, the data we had was incomplete and only included the dimming of the shock cooling curve and the subsequent explosion, but never the bright burst of light at the very start of the supernova,” Armstrong said in a statement.
“Because we have the complete curve we could identify what star exploded,” Armstrong told IFLScience. “That’s normally very hard.” He described this as the “Really cool part of the research.” The work allowed the team to distinguish between many competing models for supernova explosions, favouring one known as SW 17.
From this model selection, it follows the explosion occurred in a yellow supergiant 50-290 times the radius of the Sun. Moreover, according to co-author Dr Brad Tucker, "Astronomers across the world will be able to use SW 17 and be confident it is the best model to identify stars that turn into supernovas."