Explosive Final Moments of Giant Star Observed by Astronomers for the First Time

It’s extremely difficult to observe a dying star. It’s a right-place-right-time, cross-your-fingers, and keep-scanning-the-night-sky-constantly kind of difficult. It’s a level of unimaginable difficulty that, up until recently, we hadn’t been able to fully crack. We’d come close, observing the explosive supernovas that brilliantly draw attention to a star’s final goodbye. But the last gasps, the death throes leading up to such a spectacular demise, had remained elusive.

No more. A team of astronomers led by researchers at Northwestern University and the University of California, Berkeley (UC Berkeley), have for the first time ever observed the final days of a red supergiant star. By the grace of good timing, they came across this star—which likely had been burning for tens of millions of years—a mere 130 days before it violently exploded into a supernova. 

“It’s like watching a ticking time bomb,” Raffaella Margutti, an adjunct associate professor at CIERA and senior author of the study of the historical event published in The Astrophysical Journal, said in a statement. “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.”

Right Place, Right Time

The dying giant star, officially known as “SN 2020tlf” and formerly located in galaxy NGC 5731, about 120 million light years from Earth, was spotted in the summer of 2020 by the University of Hawaii’s Pan-STARRS telescope. About ten times more massive than our own sun, it entered its red supergiant phase when the hydrogen fuel in its core became depleted. The core then transitioned to fusing helium, dramatically expanding the star’s radius and causing its temperature to plummet. For perhaps hundreds of thousands of years, it existed in this state. Over time, as the helium burned up and the star began burning carbon, fusion of heavier elements took place and an iron core began forming. 

In the fall of 2020, 130 days after it was first discovered, the core of the red supergiant collapsed and triggered what’s known as a Type II supernova. For the briefest of moments, based on data captured by the W. M. Keck Observatory’s Low Resolution Imaging Spectrometer on Mauna Kea, Hawai'i, the light generated by the supernova was brighter than all of the stars in its home galaxy—combined.

So what exactly did we learn from this event? For one, it was long theorized that red supergiants were quiet in the months and years ahead of their explosive endings. Instead, the team observed their supergiant emitting bright, luminous radiation in its final year.

“This suggests at least some of these stars must undergo significant changes in their internal structure, which then result in the tumultuous ejection of gas moments before they collapse,” they write. 

For the first time ever, the researchers were also able to capture the full spectrum of light created by the powerful supernova. It’s hoped that the observations of SN 2020tlf’s final moments will potentially provide a kind of roadmap for discovering other impending supernovas in the universe. 

“I am most excited by all of the new ‘unknowns’ that have been unlocked by this discovery,” astrophysicist and lead author of the study, Wynn Jacobson-Galán, said. “Detecting more events like SN 2020tlf will dramatically impact how we define the final months of stellar evolution, uniting observers and theorists in the quest to solve the mystery on how massive stars spend the final moments of their lives.”

Will Our Own Sun Eventually Explode? 

While discoveries like the final moments of SN 2020tlf are exciting, researchers believe that its explosive fate will not be shared by our own sun. For one thing, it’s too small. You need at least the mass of SN 2020tlf (ten times larger) to generate a supernova and an estimated ten times larger than that to create a black hole.

While the sun will eventually follow a similar path, burning through its hydrogen and helium and expanding into a red giant, it will go out with a hiss rather than a bang. After engulfing Mercury, Venus, and possibly even Earth, the sun will simply collapse into what’s known as a white dwarf, a remnant of its former self roughly the size of our own planet.

The good news? Because our sun is small, its lifespan is actually much longer than stars like SN 2020tlf. Giant stars burn through their fuel supply at a much faster rate, with the largest lasting only a few million years. Our sun, classified as a yellow dwarf star, has been burning bright for 4.5 billion years and will not run out of fuel for at least another 5 billion years.

So rest easy—there’s still plenty of time to unlock a few more secrets of the universe.