Astronomers observed the last 130 days leading up to its deadly blast.

Astronomers have long suspected that massive stars turn into a so-called red giant towards the end of their lives. After a few hiccups, they then explode in a classic supernova that can be seen all over the universe. However, no one had ever seen this scenario play out in real life. Until now. “This is a breakthrough in our understanding of the last days of massive stars just before they die,” said study leader Wynn Jacobson-Galán.

Doomed Red Giant

For the first time ever, astronomers have been able to follow live the dramatic end of a doomed red giant’s life. They witnessed the dying star’s final 130 days before exploding into a stunning type II supernova. “We’ve never seen what happens to a red giant just before it explodes in a supernova,” Jacobson-Galán said. “For the first time, we saw a red supergiant explode.”

Types of supernovae
Supernovae can arise in two ways. For example, a supernova can occur when white dwarfs explode in binary star systems (type Ia). In addition, types II, Ib and Ic occur when massive stars have reached the end of their lives and then form neutron stars or black holes.

Researchers tracked down the doomed red giant using the Pan-STARRS telescope, located on the Hawaiian shield volcano Haleakala. The star turned out to be emitting an enormous amount of light. A few months later, a supernova lit up the night sky.

Supernova 2020tlf

The team quickly captured the powerful flash and obtained the very first spectrum of the energetic explosion, dubbed supernova 2020tlf. The data showed that the star was surrounded by gas at the time of the explosion; probably the same gas that the Pan-STARRS telescope had already picked up a few months ago and that had been violently spewed out of the star at the time.

Supernova SN 2020tlf, photographed by the Pan-STARRS telescope. Image: UC Berkeley

Even after the deadly explosion, the researchers continued to monitor the star closely. Based on newly collected data, the researchers determined that the red supergiant resided in the spiral galaxy NGC 5731, a celestial object about 120 million light-years away from Earth. The star had about ten times the mass of our sun.

Quiet

The discovery of the red giant defies previous theories about how doomed red supergiants behave just before their deaths. Until now, the ‘death struggle’ of previously observed red supergiants has been relatively uneventful; no violent eruptions or luminous emissions had been spotted before. However, the new observations show that the red giant in the current study did shine brightly in its final days.


At the end of its life, a red supergiant becomes unstable and blows out a large amount of gas before exploding in a supernova.

It means, the researchers say, that at least some of the red giants undergo significant internal changes, which then result in the tumultuous release of gas just before collapsing. “It was like watching a ticking time bomb,” said researcher Raffaella Margutti. “We have never seen such violent activity on a dying red supergiant.”

The researchers plan to ‘capture’ more supernovae to better understand the process of star death. Because there is still much to learn. “I’m especially excited about all the new ‘unknowns’ that have been unlocked,” says Jacobson-Galán. “If we detect more events like SN 2020tlf, we may expand our understanding of the final months of stellar evolution. The ultimate goal is to unravel the mystery of how massive stars spend the last moments of their lives.”