For the first time, astronomers have reported observing the death of a Red supergiant star, in real time. And it did not behave as astronomers had believed it would. The observations were conducted by scientists at the Centre for Interdisciplinary and Exploratory Research in Astrophysics (CIERA) at Northwestern University in the US state of Illinois. (However, the research study lead author moved to the University of California, Berkeley, before their paper was published.)

The team were able to observe the last 130 days of the life of a Red supergiant star, during the northern summer and autumn of 2020. They used the University of Hawaii Institute for Astronomy’s Panoramic Survey Telescope and Rapid Response System, and the WM Keck Observatory’s Low Resolution Imaging Spectrometer, also on Hawaii, as well as other Keck instruments.

Red supergiants are elderly giant stars which have used up all their hydrogen (which serves as their fuel: stars are basically nuclear fusion reactors) in their cores. Instead, in their cores they use helium (created by the fusion of hydrogen atoms) as fuel. However, hydrogen fusion continues, but in the upper levels of the star, which causes the star to expand and cool (hence its red colour). The best known Red supergiant stars are probably Betelgeuse and Antares. 

This particular Red supergiant, located 120-million light years away in the NGC 5731 galaxy, had a mass ten times that of our Sun, putting it at the bottom end of the Supergiant size range. Red Supergiants were believed to fall dormant at the ends of their lives, before collapsing into white dwarfs, or (if bigger) exploding into supernovae and leaving remnant neutron stars or even black holes behind.   

But the astronomers observing it discovered that it was emanating bright radiation during its final months. This indicated that it was undergoing significant internal structural changes. The star ejected gas during these processes, which probably created the dense cloud of material that surrounded the star when it exploded into a Type II supernova (that is, a supernova involving a single star; Type I supernovae involve binary stars).

“This is a breakthrough in our understanding of what massive stars do moments before they die,” highlighted study lead author and PhD student 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 watched a Red supergiant star explode.

“It’s like watching a ticking time bomb,” said study senior author and CIERA adjunct Associate Professor Raffaella Margutti. “We’ve never confirmed such violent activity in a dying Red supergiant star where we see it produce such a luminous emission, then collapse and combust, until now.”

Following its explosion, the object was formally designated Supernova 2020 tlf, or SN 2020tlf for short.