When stars go dim, astronomers take notice. Most stars, after all, shine with a steady light, including the sun. But the ones that are variable usually vary in predictable ways, so any unexpected dimming gets attention.

When one of the brightest stars in the sky abruptly went dim at the beginning of 2020, astronomers emphatically took notice. The Betelgeuse star, which marks the shoulder of the star-rich constellation of Orion the Hunter, is visibly bright in the winter night sky, largely due to its enormous size and luminosity.

According to Space.com, Betelgeuse is 900 times the size of the sun, and if it were put in the sun's place, it would extend past the orbit of Mars. It's so big that it's one of the relatively few stars you can actually image as more than a speck of light — a fact that plays a key role in resolving this mystery.

Betelgeuse Star on the Brink

These images show Betelgeuse as a turbulent place that seems to bubble and boil visibly on a time scale of months. This turbulence lends support to models of stellar structure and life cycle which suggest that Betelgeuse, a red supergiant, is a prime candidate to explode as a supernova. In cosmic terms, an explosion is imminent — which means it could take place at any time in the next few thousand years.

That is a long time in human terms, but any time really does mean any time, so when Betelgeuse suddenly went dim far beyond its usual fluctuations, astronomers couldn't help but wonder whether it was about to go out with a bang.

At the same time, astronomers understood that the same instability that will one day cause Betelgeuse to explode also makes the star prone to lesser outbursts and disruptions. Two alternative theories of what might be going on began to make the rounds.

One theory was that one of Betelgeuse's regular fluctuations in temperature and brightness happened to be steeper than usual, with dramatic effects on how it looked to distant observers. Another theory was that dust clouds known to be formed by stellar winds from Betelgeuse were the cause of the dimming.

If Betelgeuse were too far away for good observation, these alternative possibilities might have remained hypotheses, as there would be no way to test them. But because Betelgeuse is relatively close and easy to observe, it's a good object to study for lessons about what happens to these rare but spectacular stars.

A Serendipitous Snapshot Reveals the Mystery

As such, it was not a total surprise when it turned out that a team of astronomers was imaging Betelgeuse right through the period of its sudden dimming, beginning with what Science News calls an image "serendipitously" taken in January 2019 — not long before the mysterious dimming episode began.

These images, along with new computer models of the interior of Betelgeuse, allowed the research team to evaluate the two theories that had been proposed to explain the dimming and recovery. As it turns out, both theories were right, and the two phenomena put forth as explanations were actually working in tandem to produce the sudden dimming and subsequent recovery of Betelgeuse.

The episode began deep inside the star's tenuous outer layers, with a bubble of gas that erupted through the visible surface and billowed out into space — "a big burp," as Wired put it. This showed up clearly in Hubble Space Telescope imagery, notes SciTechDaily. "With Hubble," says Andrea Dupree, a member of the research team, "we could see the material as it left the star's surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim."

After the eruption, Betelgeuse entered one of its regular cycles of becoming slightly dimmer and cooler. This was enough to allow some of the gas from the eruption to condense into grains of dust that blocked much of the visible light still coming from the star. These combined effects produced the dimming observed from Earth. As the dust grains spread out and dissipated to the point where they no longer blocked Betelgeuse's starlight, Betelgeuse visibly brightened up again.

Betelgeuse is now shining as though the episode never happened, but sooner or later, it will explode. In the meantime, Universe Today notes that we are learning about new processes by which heavy elements produced in the cores of massive stars are released into space, where they may eventually contribute to the formation of Earthlike planets.

And we're learning more about the behavior of unstable red supergiants as well, which not only helps us understand why stars go dim but also may provide us with advance warning when Betelgeuse is ramping up for its grand finale.

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Betelgeuse Star Dims and Recovers — No Supernova This Time

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