For the first time in human history, scientists have observed a core-collapse supernova from start to finish, in real time.
The progenitor star was a red supergiant about ten solar masses, some 120 million light-years away. During its final months, the dying star’s luminosity flared up dramatically as it shed its shell of hydrogen and helium. All the while, a team of researchers leading the Young Supernova Experiment (YSE) were watching — and what they saw calls into question our entire narrative of how these stars died.
“This is a breakthrough in our understanding of what massive stars do moments before they die,” noted principal researcher Wynn Jacobson-Galán. “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 have seen a red supergiant star explode!
So far, all of the red supergiants seen before exploding had spent their final months quiet. We had seen no evidence of violent stellar behavior at the end of their lives. But this star had a note of grace, a light show seen before its grand finale. That’s what makes it so remarkable – and it’s also how the YSE team detected the impending explosion.
YSE researchers made the first observations of the star’s increasing brightness about 130 days before the star finally exploded. Using PAN-STARRS optical telescopes, they also recorded evidence that the star had begun to eject its outer layers. His agony had begun. Thus warned, they listened to the LRIS, DEIMOS and NIRES spectrographs at the Keck observatory. As a result, we now have detailed broad-spectrum measurements of a type II supernova from start to finish.
Disco at the end of the universe
This supernova, named “SN 2020tlf”, is a great demonstration of how the end of a red supergiant’s life can be a spectacular light show. These stars are really going out with a bang. Some undergo a series of color changes as the star’s internal fusion engine produces heavier and heavier elements. Many spend a period as Cepheid variable stars, whose repetitive flash is so regular that it is sometimes used in astronomy as a “standard candle”. Before finally exploding into a supernova, even the most luminous red supergiants are thought to evolve into Wolf-Rayet stars – a rare stellar super-hell that burns hotter than nearly any other known star type, with a roaring stellar wind .
SN 2020tlf, however, can belong to a single weight class. Its progenitor star was between ten and twelve solar masses. For progenitor stars between ten and twenty solar masses, after a core-collapse supernova, what remains is a neutron star. But if that neutron star is massive enough, even neutron degeneracy pressure can’t stop the star’s continued gravitational collapse. A neutron star larger than about two solar masses can collapse again, forming a black hole.