Astronomers watch a star die and then explode into a supernova – for the very first time


An artist’s rendition of a red supergiant star transforming into a Type II supernova, emitting a violent eruption of radiation and gas on its last breath before collapsing and exploding. Credit: WM Keck Observatory/Adam Makarenko

This is another first for astronomy.

For the first time, a team of astronomers has photographed in real time as a red supergiant star reaches the end of its life. They saw the star convulsing in its agony before finally exploding into a supernova.

And their observations contradict previous thinking about the behavior of red supergiants before they exploded.

red supergiant

Artist’s impression of a red supergiant star in the last year of its life emitting a tumultuous cloud of gas. This suggests that at least some of these stars undergo significant internal changes before going supernova. Credit: WM Keck Observatory/Adam Makarenko

A team of astronomers watched the drama unfold through the eyes of two observatories in Hawaii: Pan-STARRS in Haleakala, Maui, and the WM Keck Observatory in Maunakea, Hawaii. Their observations were part of the transient Young Supernova Experiment (YSE) survey. They observed the supernova explosion, named SN 2020tlf, in the last 130 days before its detonation.

“For the first time, we saw a red supergiant star explode!”
Wynn Jacobson-Galan, UC Berkeley

The title of the article presenting the discovery is “Final Moments. I. Precursor emission, envelope inflation and increased mass loss preceding the type II luminous supernova 2020tlf. The article is published in The Astrophysical Journal and the lead author is Wynn Jacobson-Galán, an NSF graduate researcher at UC Berkeley.

“This is a breakthrough in our understanding of what massive stars do moments before they die,” Jacobson-Galán said in a press release. “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!

“It’s like watching a time bomb.”
Raffaella Margutti, UC Berkeley

The discovery dates back to the summer of 2020. At that time, the progenitor star experienced a dramatic increase in brightness. Pan-STARRS detected this brightening, and when fall arrived, the star exploded as SN 2020tlf. Supernova is a type II supernova, where a massive star undergoes a rapid collapse and then explodes.

This video is an artist’s rendition of the red supergiant star transitioning into a Type II supernova, emitting a violent eruption of radiation and gas on its last gasp before collapsing and exploding. Credit: WM Keck Observatory/Adam Makarenko

The team used the Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) to capture the first spectrum of the supernova. LRIS data showed circumstellar material around the star as it exploded. This material is likely what Pan-STARRS saw the star eject the summer before it exploded.

“Keck was instrumental in providing direct evidence for the transition from a massive star to a supernova explosion,” said lead author Raffaella Margutti, associate professor of astronomy at UC Berkeley. “It’s like watching a time bomb. We have never confirmed such violent activity in a dying red supergiant star where we see it emit such a bright emission, then collapse and burn up, until now.

Supernova before and after the explosion

This figure from the study shows the supernova before and after the explosion. The top panel shows the total of all electromagnetic radiation emitted by the event on all wavelengths, in green. The middle panel shows black body temperatures in red and the bottom panel shows rays in blue. Image credit: Jacobson-Galán et al, 2022

After the explosion, the team turned to other Keck instruments to continue their observations. Data from the DEep Imaging and Multi-Object Spectrograph (DEIMOS) and the Near Infrared Echellette Spectrograph (NIRES) showed that the progenitor star was 10 times more massive than the Sun. The star is in the galaxy NGC 5731, about 120 million light-years away.

The team’s observations have led to a better understanding of type II supernovae and their progenitor stars. Prior to these observations, no one had seen a red supergiant display such a peak in brightness and undergo such powerful eruptions before exploding. They were much more placid in their final days as if accepting their fate.

Red supergiant stars eject material before the core collapses. But this material ejection takes place on much longer timescales than SN 2020tlf. This supernova emitted circumstellar material (CSM) for 130 days before collapsing, making it a bit of a puzzle. The bright flash before the star exploded is somehow related to the ejected CSM, but the research team isn’t sure how they all interacted.

Artist's impression of supernovae

Artist’s impression of a type II supernova explosion that involves the destruction of a massive supergiant star. Credit: ESO

The star’s significant variability leading to the collapse is puzzling. The powerful burst of light from the star before exploding suggests that something unknown is happening to its internal structure. Whatever these changes result in a gigantic ejection of gas before the star collapses and explodes.

In their article, the authors discuss what may have caused the gas ejection. One possibility is wave-induced mass loss, which occurs in the later stages of stellar evolution. It occurs when “…the excitement of gravitational waves by the combustion of oxygen or neon in the last years before the SN could allow the injection of energy into the outer stellar layers, resulting in an inflated envelope and/or eruptive episodes of mass loss,” write- they. But current wave-driven models don’t match the ejection of gas from the progenitor star. They are consistent with the radius of the progenitor star over its last 130 days, but not consistent with the burst of brightness.

In the conclusion of their article, the authors summarize things succinctly. “Given the progenitor mass range derived from nebular spectra, it is likely that the increased mass loss and precursor emission is the result of deeply rooted instabilities in the stellar interior, most likely associated with the final stages Energy deposition from gravitational waves generated in the neon/oxygen burn stages or a silicon flash during the last 130 days of the progenitor could have ejected stellar material which was later detected at both in the pre-explosion flow and in the early SN spectrum.

If there’s a supernova behaving like this, there must be more. The team’s findings mean that surveys like the Young Supernova Experiment transient survey now have a way of finding more in the future. If the survey finds more stars ejecting material like this, then they know to keep an eye on it to see if it collapses and explodes.

“I’m very excited about all the new ‘unknowns’ that have been unlocked by this discovery,” Jacobson-Galán said. “Detecting more events like SN 2020tlf will have a huge impact on how we define the final months of stellar evolution, uniting observers and theorists in the quest to solve the mystery of how massive stars pass last moments of their lives.

Originally published on Universe Today.

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