Despite the massive number of stars in the sky, spotting one in the throes of a supernova is still an incredibly rare event. Now, astronomers have captured a red supergiant before, during and after a supernova explosion for the first time, gathering crucial new information about these dramatic events.
"This is a breakthrough in our understanding of what massive stars do moments before they die," said lead author Wynn Jacobson-Galán (UC Berkeley). "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!"
Using the Pan-STARRS telescope in Maui, Hawai'i, scientists detected the doomed red supergiant star in the summer of 2020 thanks to the huge amount of light it was emitting. Later in the fall when it went supernova, the team captured the powerful flash using the Hawai'i-based Keck Observatory's Low Resolution Imaging Spectrometer (LRIS). They also captured the very first spectrum of the supernova, known as SN 2020tlf.
The observations showed that the star likely ejected massive amounts of dense circumstellar material just ahead of the explosion. Previous observations showed that red giants were relatively calm before going supernova, so the new data suggests that some may change their internal structure significantly before exploding. That could then result in tumultuous gas ejections moments before collapse.
SN 2020tlf is located in the NGC 5731 galaxy about 120 million light-years from Earth and was about 10 times more massive than the Sun. Stars go supernova when they run out of fuel and collapse on their own gravity, fueling a massive carbon fusion explosion. For that to happen, they must be large enough (8 to 15 solar masses) or they'll simply collapse into white dwarf star like our Sun eventually will. Any larger than that and they could collapse into a black hole.
The discovery will now allow scientists to survey red supergiant stars looking for similar types of luminous radiation that could signal another supernova. "Detecting more events like SN 2020tlf will dramatically impact how we define the final months of stellar evolution... in the quest to solve the mystery on how massive stars spend the final moments of their lives," said Jacobson-Galán.