Astronomers have finally caught something long sought – the last days of a giant star prior to its explosion of a supernova. Confounding expectations, the star in question was very active prior to the explosion, raising the question of whether such behavior is common and we have somehow missed it, or there was something very unusual about this star.
Red supergiant stars with masses more than eight times the Sun’s eventually become Type II (or Ib/Ic) supernovas, leaving either a neutron star or a black hole behind. Although our galaxy has stubbornly failed to provide us with such an event in the last 400 years, we now see hundreds of explosions like this in other galaxies each year.
Unfortunately, we only see them once the explosion happens. The red giant phase may be short on astronomical time-scales, but it still lasts hundreds of thousands to millions of years, and stars seem to give us no warning before their final death throes. Or so we thought until as the Astrophysical Journal reports, one did.
Astronomers frequently go back through archived images and identify a giant star, called the progenitor, at the location of a supernova explosion. However, with no reason to study it closely first, our knowledge is usually poor, and these pre-explosion images don’t show anything that indicates an explosion was imminent. Even when progenitors were emitting a lot of light they have been generally fairly stable.
However, in September 2020 astronomers detected a supernova SN 2020tlf in the 120 million light-years distant galaxy NGC 5731 and observed it with telescopes across the spectrum. Prior images of NGC 5731 reveal the progenitor star was varying dramatically in brightness from 128 to 51 before the explosion, offering a clue to anyone watching it was about to go off.
At such a distance, observations of the SN 2020tlf progenitor couldn’t be all that detailed. Nevertheless, using a combination of light collected before and after the explosion, Northwestern University graduate student Wynn Jacobson-Galán and co-authors have modeled the star’s last days in unprecedented detail. They think it had a mass 10-12 times greater than the Sun before it started throwing off material, but it lost at least 0.01 solar masses a year in the lead up to the explosion. Its radius was 1,100 times greater than the Sun shortly before it exploded – almost out to orbit of Jupiter.
“This is a breakthrough in our understanding of what massive stars do moments before they die,” Jacobson-Galán, said in a statement. “Direct detection of pre-supernova activity in a red supergiant star has never been observed before in an ordinary type II supernova. For the first time, we watched a red supergiant star explode.” Senior author Dr Raffaella Margutti described examining the archival images as “like watching a ticking time bomb.”
The paper attributes the rapid mass loss and bright emissions prior to the explosion to “instabilities deeply rooted in the stellar interior, most likely associated with the final nuclear burning stages”. Nevertheless, it’s not clear why this star should have been more unstable than others of its kind. The authors suspect other supernovas of this type produce similar but fainter emissions that have not been detected.
Pre-explosion emission spikes have been found before, for example for the supernova SN II iPTF14hls, the star that died twice. However, that was such an unusual event in other ways it tells us little about typical supernova progenitors. SN 2020tlf, on the other hand, looked like a fairly normal supernova once it happened, making the progenitor’s behavior particularly intriguing.