Astronomers witness the explosive death of a giant star

For the first time, astronomers have observed the final days and death throes of a red supergiant star before its final collapse and massive explosion into a supernova.

Supernovae are normally only detected after they occur, although some of the other types have been caught in the act of explosion. In this case, scientists detected the star in its final phase about 130 days before detonation and could see it get progressively brighter and finally explode.

“It’s like watching a ticking time bomb,” astrophysicist Raffaella Margutti, lead author of a study on the supernova published last week in the Astrophysical Journal, said in a statement. “We have never confirmed such violent activity in a dying red supergiant star where we saw such luminous emission produced, only to collapse and go into burnout, until now.”

Margutti is now an associate professor of astronomy and physics at the University of California, Berkeley, but conducted the study while at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) in Evanston, Illinois.  

The lead author of the new study is Margutti’s graduate student, astrophysicist Wynn Jacobson-Galán, who was also at Northwestern for the observations but has since joined UC Berkeley.

Jacobson-Galan said the star was detected in the summer of 2020 during a survey by the University of Hawaii’s Pan-STARRS telescope at the peak of the Haleakalā volcano on Maui.

Although the star was in a galaxy about 120 million light-years away-both the star and the galaxy are too faint to be seen with the naked eye-the Pan-STARRS data showed that the star had become much brighter than usual, he said.

Scientists continued to observe the star with the Pan-STARRS telescope, which showed that it was violently expelling large amounts of gas.

When the supernova’s final explosion occurred, they were able to capture the powerful flash it emitted – for a brief moment, it was brighter than all the other stars in that galaxy combined – thanks to CIERA’s ability to remotely drive the telescopes at the W.M Keck Observatory, located atop Mauna Kea in Hawaii.

The supernova flash and subsequent observations showed that the star was surrounded by gas shells when it exploded, probably the same gas it had emitted in the month before detonation.

Other supernovae have been seen before, but not of this type. Usually, they have occurred when a giant star has collided with its binary companion, Jacobson-Galán said.

But in this case, no other stars appeared to be involved in the explosion. 

“These types of supernovae come from a massive star, and it’s usually a red supergiant,” she said. “And that’s what we saw: they are one of the most common supernovae in the universe.”

Observations made after the explosion suggest that the star was about 10 times larger than our sun, near the lower end of the range of stars that go supernovae.

Stars like our sun are too small to become supernovae. They expand and then shrink to become a white dwarf at the end of their lives.

Smaller stars also last several billion years, Jacobson-Galan said, because they are not large enough to burn all their fusion fuel in a short time.

Red giants like the supernova in the study, however, can use up their fuel in only a few hundred million years and then collapse when they can no longer carry out fusion.

The final supernova is caused by the “bouncing” of the star’s outer layers against its core. 

“That propagates outward and unleashes the whole star,” he said. “It rips through the star and pushes all the layers outward rapidly.”

Supernovae are the final stages of many stars and are responsible for seeding interstellar clouds of gas and dust with “heavy” chemical elements. The clouds seeded by the explosions then coalesce into younger stars, such as the Sun, and the elements – such as carbon, oxygen, silicon, and iron – are incorporated into their planets.

Until now, the “old age” of massive stars was almost impossible to observe, says Matt Nicholl, professor of physics and astronomy at the University of Birmingham (UK). “Until now, we have never been able to directly study this crucial phase,” he said in an e-mail.

Nicholl was not involved in the latest study but led a team that discovered the brightest supernova ever seen.

He said dedicated robotic telescopes, such as Pan-STARRS, can now survey the sky for explosive events such as supernovae, and more are likely to be found as surveys become more effective.

 Although Jacobson-Galan proposes that the star’s brightness before it went supernova as a consequence of its final stages, Albert Zijlstra, a professor of astrophysics at the University of Manchester in the United Kingdom, said in an e-mail that the brightness may not have been related.

Zijlstra was not involved in the new study, but he is an expert on supernovae. He was part of the team that determined which star exploded to create the supernova seen over China in 1181.

He points to a similar star, Eta Carinae, about 7,500 light-years away, which emits vast clouds of gas and gets much brighter for many years in a row, but then fades away again without exploding.

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