A rare glimpse of a star before it went supernova defies expectations

A rare glimpse of a star before it exploded in a fiery supernova looks nothing like astronomers expected, a new study suggests.

Images from the Hubble Space Telescope reveal that a relatively cool, puffy star ended its life in a hydrogen-free supernova. Until now, supernovas without hydrogen were thought to originate only from extremely hot, compact stars.

The discovery “is a very important test case for stellar evolution,” says Sung-Chul Yoon, an astrophysicist at Seoul National University in South Korea, who was not involved in the work. Theorists have some ideas about how massive stars behave right before they blow up, but such hefty stars are scant in the local universe and many are nowhere near ready to go supernova, Yoon says. Retroactively identifying the star responsible for a supernova provides an opportunity to test scenarios of how stars evolve right before exploding.

Finding those stars, however, is difficult, explains Charlie Kilpatrick, an astronomer at Northwestern University in Evanston, Ill. A telescope must have looked at that exact region of the sky in the years leading up to the supernova. And the explosion must have happened close enough for light from its much fainter source star to have reached a telescope.

Although both conditions are tricky to meet, Kilpatrick is undaunted by the hunt. After scientists discovered a supernova in December 2019, in a galaxy called NGC 4666 about 46 million light-years away, he and colleagues rushed to check old Hubble observations from the same region of the sky. They wanted to find the star behind the explosion, dubbed SN 2019yvr.

After poring over images and cross-checking observations with those from ground-based telescopes, the team found their quarry: a star at the same spot as the supernova, observed about 2.6 years before the explosion. It appeared to be a yellow star about 6,500° Celsius and about 320 times wider than the sun.

“I was kind of puzzled by all that,” Kilpatrick says. The supernova SN 2019yvr lacked hydrogen, so its progenitor was expected to be hydrogen-deficient, too. But “if a star lacks a hydrogen envelope, then you expect to be seeing deeper inside of the star to the hotter layers,” Kilpatrick says. That is, the star should have looked extremely hot and blue and compact — maybe 10,0000 to 50,000° C, and no more than 50 times wider than the sun. The cool, large, yellow progenitor of SN 2019yvr, on the other hand, appeared to be padded with lots of hydrogen. The researchers report the results May 5 in the Monthly Notices of the Royal Astronomical Society.

For this kind of star to have produced a supernova like SN 2019yvr, it must have shed much of its hydrogen before blowing up, Kilpatrick says. But how?

He and colleagues have come up with a couple scenarios. The star could have expelled much of its hydrogen into space through violent eruptions, possibly caused by some instability in the star’s core or interference from another star nearby. Or perhaps the star’s hydrogen could have been stripped off by another star that was in orbit around it.

To whittle these possibilities down, Jan Eldridge, an astrophysicist at the University of Auckland in New Zealand, suggests turning the Hubble telescope back on that area of the sky. Astronomers should first make sure that the star seen 2.6 years before SN 2019yvr really is gone now, says Eldridge, who was not involved in the work. Researchers could also check whether a star that once orbited SN 2019yvr’s progenitor still remains.

“They’ve found a mystery, and they’ve got some solutions,” Eldridge notes. Trying to figure out how such an unlikely star pulled off this particular supernova, she says, “is going to be fun.”