Astronomers have successfully captured something that has been sought for a long time: the final days of a massive star before it explodes as a supernova. Surprisingly, the star in issue was quite active before the explosion, raising the question of whether such activity is normal and we missed it, or whether there is something odd about this star. Type II (or Ib/Ic) supernovas form when red supergiant stars with masses greater than eight times that of the Sun explode, leaving either a neutron star or a black hole behind. Although our galaxy has persistently failed to provide such an event in the last 400 years, we now see hundreds of similar explosions in other galaxies every year.
Regrettably, we only see them after the explosion has occurred. On astronomical timescales, the red giant phase may be brief, but it lasts hundreds of thousands to millions of years, and stars appear to offer us no notice before their final death throes. Alternatively, so we thought until one did, according to the Astrophysical Journal.
Astronomers commonly examine historical photos to locate a massive star known as the progenitor at the site of a supernova explosion. Our understanding is usually poor since we have no need to study it closely first, and these pre-explosion photographs do not show anything that indicates an explosion was near. Even though progenitors emitted a lot of light, they remained rather steady.
Astronomers discovered a supernova SN 2020tlf in the 120 million light-years distant galaxy NGC 5731 in September 2020, and examined it with telescopes across the spectrum. Prior photos of NGC 5731 show that the progenitor star’s brightness varied considerably from 128 to 51 before the explosion, giving anyone watching a heads-up that something was about to go off. Observations of the SN 2020tlf progenitor could not be very detailed at such a great distance. Nonetheless, Northwestern University graduate student Wynn Jacobson-Galán and co-authors have reconstructed the star’s final days in unparalleled detail using a combination of light captured before and after the explosion.
It had a mass 10-12 times that of the Sun when it began ejecting material, but it lost at least 0.01 solar masses every year in the year leading up to the explosion. It had a radius 1,100 times that of the Sun just before it exploded, and it was virtually out of Jupiter’s orbit. In a statement, Jacobson-Galán said, “This is a breakthrough in our knowledge of what huge stars do seconds before they die.” “In an ordinary type II supernova, direct observation of pre-supernova activity in a red supergiant star has never been observed before.” We saw a red supergiant starburst for the first time.” Examining the archive photographs, according to senior author Dr. Raffaella Margutti, is “like witnessing a ticking time bomb.”
The rapid mass loss and light emissions before the explosion are attributed to “deeply rooted in the stellar interior instabilities, most likely connected with the final nuclear burning stages,” according to the article. Despite this, it is unclear why this star was more unstable than others in its class were.
Other supernovas of this type, the authors believe, release similar but fainter emissions that have not been identified. There have been previous reports of pre-explosion emission spikes, such as for the supernova SN II iPTF14hls, the star that died twice. In other aspects, though, that event was so exceptional that it reveals little about normal supernova progenitors. SN 2020tlf, on the other hand, initially appeared to be an ordinary supernova, which makes the progenitor’s behavior even more intriguing.
