Left: Image of the supernova SN2016aps about one year after peak brightness. Middle: optical image of the host galaxy obtained with the Hubble Space Telescope after the supernova had faded. Right: Ultraviolet image from Hubble obtained at the same time. The supernova exploded at a UV hotspot, suggesting it came from a very massive star.

I’m a bit late with this post, as this supernova has been in the news a bit over the past week. In this new paper in Nature Astronomy, my colleagues and I presented a supernova that released so much energy that it seemingly had to have come from one of the most massive stars in the Universe. The supernova is SN2016aps, and we found it in the PanSTARRS survey 4 years ago, so I’m delighted to finally put these results out into the world. The study took a long time because it took so long for the supernova to fade enough that we could see its host galaxy!

The supernova spectrum showed narrow-ish hydrogen emission lines, telling us that the fast-expanding ejecta were colliding with slow-moving circumstellar gas, which must have been shed by the star in the final years before explosion. This intense collision allowed SN2016aps to convert its kinetic energy to radiation and become extremely luminous, but the total energy in radiation actually exceeded the kinetic energy of nearly all other supernovae! Thus the explosion itself, as well as the light released, must have been unusually powerful.

Artist’s impression of SN2016aps crashing into a surrounding shell of dense gas (credit Aaron Geller, Northwestern University)

By modelling the slow fading over two years, we found that the circumstellar shell must have included several tens of solar masses of material, and the supernova itself likely had several tens more, for a total mass of 50-100 solar masses in this star! For comparison, most supernovae come from stars with less than 20 solar masses. To make such a massive and hydrogen rich star, the progenitor system may have initially comprised two stars in a binary, which merged when the more massive star reached a red supergiant phase.

Stars this massive are predicted to encounter a very rare process called the pair instability, which can cause violent pulsations ejecting the stellar envelope or even fully disrupt the star. This has been predicted since the 1960s, but it is unclear whether we have seen a real-world example before. Given the need for a both a huge mass and the ejection of the envelope soon before explosion, SN2016aps might be the best contender yet for a (pulsational) pair instability supernova.