A star on the edge of becoming a supernova may be metaphorically cool, but is literally very hot. Confusingly, however, a star that subsequently became a supernova has been revealed to be literally cool before it exploded, challenging much of what astronomers thought they knew about these mighty events.
Supernovas only occasionally reveal their backstory, since they almost always take us by surprise. Sometimes, however, photographs taken of a galaxy that later hosts a supernova can be searched to find the star before its dying gasp.
One such case was for the supernova 2019yvr, which occurred a cosmologically tiny 35-55 million light-years away in NGC 4666.
As a particularly active star-forming galaxy, NGC 4666 has been heavily studied by both amateur and professional astronomers, offering Dr Charles Kilpatrick of Northwestern University many opportunities to find the star responsible for 2019yvr in old photos.
In Monthly Notices of the Royal Astronomical Society (preprint ArXiv.org), Kilpatrick and co-authors report the star behind this explosion is yellow, like the Sun. Although plenty hot enough to power life on planets millions of kilometers away, yellow stars are much cooler than the blue stars that represent a lot of supernova progenitors. Some yellow stars have become supernovas, but in every case prior to this one that was because they had a shroud of hydrogen that prevented us from seeing the interior, which was believed to be exceptionally hot. The star in this case shows no sign of such a veil.
“We haven’t seen this scenario before,” Kilpatrick said in a statement. “If a star explodes without hydrogen, it should be extremely blue — really, really hot. It’s almost impossible for a star to be this cool without having hydrogen in its outer layer. We looked at every single stellar model that could explain a star like this, and every single model requires that the star had hydrogen, which, from its supernova, we know it did not. It stretches what’s physically possible.”
The images Kilpatrick used to reach this conclusion were taken by Hubble 2.6 years before the explosion.
2019yvr's spectrum gave no hint anything unusual was going on. “It seemed like a very normal hydrogen-free supernova,” Kilpatrick said. When studying the Hubble images he looked for the sort of blue star normally associated with explosions like this. None could be found anywhere near the location, with the yellow giant providing the only candidate.
Five months after the explosion, however, material thrown off by 2019yvr collided with a mass of hydrogen. Kilpatrick's working theory is the star once had a hydrogen layer but threw it off decades before exploding. Alternatively, a companion star may have stripped the hydrogen away.
NGC 4666 hosted another supernova, a Type Ia, whose light reached the Earth in 2014. The coincidence of two explosions so close together in time reflects the rate at which this galaxy is producing big, short-lived stars. Earth-based astronomers, frustrated by the lack of supernovas in our own galaxy since the telescope's invention can only sigh with envy. However, with the two events occurring thousands of light-years apart, any astronomers in NGC 4666 would have seen them as happening many years apart, depending on the observers' location relative to the two stars.