Dimly lit by a distant red star in the initial stages of going supernova, the rugged landscape of th...

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Astronomers decipher a few early warning signs for stars about to go supernova

Keep an eye on the sky.

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We know Betelgeuse isn’t about to explode anytime soon, but if it ever does, astronomers say they now have a way to give us a few weeks’ warning.

A recent study suggests that red supergiant stars like Betelgeuse undergo distinct changes in the final months of their lives, and that could give astronomers a heads-up to catch a supernova in action instead of just looking at the aftermath. The astrophysicists published their work in a recent paper in the Monthly Notices of the Royal Astronomical Society.

What’s New – If a red supergiant — a large star about 10 to 40 times more massive than our Sun — dims drastically and turns a deeper red, a supernova is probably imminent, according to Liverpool John Moores University astrophysicist Benjamin Davies and his colleagues. They simulated the process that causes that pre-mortem dimming, based on observations from the aftermath of dozens of red supergiant supernovae in nearby galaxies. And their results suggest that in the weeks or months just before a supernova, the star’s visible light should become about 100 times dimmer.

The dimming and reddening happen because the star shrouds itself in a dense cocoon of gas (mostly hydrogen, along with other elements) during its final months. The star ejects the gas from its surface — at least according to a still-untested idea — in a series of small explosions as the star burns through the last of its nuclear fuel. Think of it as a stellar version of the warning rumbles just before a volcano erupts here on Earth.

In the final year or two of its life, the star burns neon, then oxygen, then silicon in its core. As it burns out its supply of one element, the core contracts — a sort of partial collapse that stops when it ignites the next element.

“When these late-stage burning transitions burst into life, they send shockwaves through the star that are almost like mini-supernova explosions,” Davies tells Inverse. “They don’t blow the star up, but they do lift material off the surface,”

A portrait of Orion and the northern winter Milky Way, on a February night, 2020. Alan Dyer/Stocktrek Images/Stocktrek Images/Getty Images

Based on Davies and his colleagues’ observations and models, at the moment of its final core collapse, a red supergiant star is surrounded by a cocoon of gas “orders of magnitude more dense” than the faint cloud currently around Betelgeuse. Picture a cloud of gas about a tenth the mass of our Sun, surrounding a dim red star out to about a million kilometers from its surface; in other words, that’s a thick layer of gas about 5 percent as dense as our Sun.

Here’s the background — When a star explodes, astronomers can study its remnants — an expanding cloud of hot gas and radiation — for clues about what the star and its immediate environment were like just before the final cataclysm.

“The supernova radiation itself can give us hints about what the star looked like at the moment the core collapsed,” wrote Davies and his colleagues. And they noticed that the way light brightened and dimmed in the aftermath of dozens of red supergiant supernovae suggested that the doomed stars had been surrounded by a dense cloud of material in their final moments.

Telescopes that managed to catch a glimpse of supernova remnants within the first day or two after the explosion saw faint lines of radiation emitted from that cloud of material as the blast wave passed through it. However, no one had ever actually seen a red supergiant in that state, despite the dozens that had been imaged in the years prior to their fiery cataclysmic demise. That, in turn, suggested that the change must happen not just years, but mere months or even weeks, before the end of the star’s life.

So Davies and his colleagues programmed several computer simulations of different ways that cloud of material around the star could accumulate. One involved a much slower process, in which an unusually strong stellar wind lifted material from the star’s surface during about a century. Another involved the much faster, more dramatic process Davies describes for Inverse. The researchers compared the simulations to see which one produced a supernova that most closely resembled the real-world examples they’d studied.

It turned out that the sudden dimming pretty much has to happen within the last few months or even weeks of a star’s life as it jolts its way through its final stages, blasting material into space in the process.

Betelgeuse is a red supergiant and the tenth-brightest star in the night sky.Javier Zayas Photography/Moment/Getty Images

Why It Matters — That’s knowledge astronomers can use.

Even the hottest, brightest, fastest-living stars burn for millions of years, but when a massive star dies, it dies in an instant. Astronomers often spot the blazing remnants of a supernova within a few days, since it’s hard to miss something that’s bright enough to outshine an entire galaxy. But the actual moment of the explosion is a blink-and-you’ll-miss-it phenomenon.

“To get these very early data you basically have to point your biggest and best telescopes at anything that so much as flinches — if you wait to find out if it’s really a supernova, then it’s already too late! So, there are a lot of duds,” says Davies.

If astronomers can read the warning signs of an impending supernova, however, they’ll know which stars to watch, and they can catch a stellar cataclysm in action.

“There are a bunch of new missions in the pipeline (e.g. Vera Rubin Observatory) that plan to survey large fractions of the sky every few days or so,” says Davies. “If we saw a red supergiant start to ‘circle the drain,’ we would have enough advance notice to put other telescopes on it to watch what happens as this material builds up.”

What’s Next — Meanwhile, although it has been behaving weirdly for the last few years, Betelgeuse isn’t likely to go supernova anytime soon.

“Betelgeuse, despite its recent dimming, isn’t ejecting material on the scale that we see for stars in the last year of their life,” says Davies. “Also, we’ve known for a while that red supergiants that go on to blow up in the next ~10 years are redder than the average red supergiant, which Betelgeuse isn’t. So I think we are safe!”

What will it look like if Betelgeuse is about to blow?

“It might do a little ‘flash’ as the material is ejected, but will get 100 times fainter over the next few months as that material quickly cools and enshrouds the star,” says Davies.

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