The bright, red star Betelgeuse in the constellation Orion showed unexpected behavior. In late 2019 and 2020, it became weaker than we’ve ever seen it, at least in records going back more than a century. In short, it has grown dimmer (approximately) than Bellatrix, the third-brightest star in Orion. This event became known as the “great gradation.” But Betelgeuse has been brilliant again ever since. For a few days this year, it was the brightest star in Orion, brighter than we’ve ever seen. Both events have led to speculation that his demise in the form of an explosion is imminent. But is there any evidence to support this idea? And how would such an explosion affect us here on Earth?

Stars are, in general, remarkably stable. They shine with the same brilliance year after year. But there are exceptions and some stars, called variable stars, change in brightness. The most famous is Mira, “the star of wonder”, which was discovered as a variable star by the German shepherd David Fabrice in 1596 – it is a pulsating star that expands and contracts regularly. Algol is another well-known example: it is periodically eclipsed by a companion star. There are about 30 of these variable stars visible to the naked eye, although care must be taken to note their variation in brightness.

Betelgeuse, the seventh brightest star in the sky (not counting the Sun), is the brightest of the variable stars. Sometimes Betelgeuse becomes almost as bright as Rigel (the constellation’s fourth brightest blue star), while at other times it is noticeably dimmer. The variation is caused by pulsations, similar to Mira’s but not as large or as regular.

Sometimes, however, a star can briefly become extremely bright. The brightest and rarest of these are supernovae, which are formed when an entire star ends its life in a powerful explosion. Supernovae can be bright enough to be visible during the day, although this has only happened a few times in the last 1,000 years. A nearby bright supernova is the kind of event astronomers live for, but few of us will see it. We live in hope.

Although Betelgeuse is a variable star, the Great Dimming in 2021 has been extreme. Within a few months, it was actually reduced by 60%. It was eventually shown to be caused by a cloud. Stars like Betelgeuse continually expel gas and dust. An accumulation of gas in the wind, as big as the star itself, obscured half of the star. In fact, star photos showed the southern half was missing. It seems that some stars, like Betelgeuse, have time. Betelgeuse darkening over time, with a cloud seen in the last panel. NASA, ESA and E. Wheatley (STScI) That said, we still don’t know what caused the sudden brightness: it’s now 50% brighter than usual. But an impending supernova doesn’t seem all that likely. In this type of star a supernova explosion is triggered in the nucleus. Brightness variations, on the other hand, are a superficial phenomenon.

The extreme brightness may actually be due to the same dust cloud that caused the dimming, now reflecting starlight back at us, making it appear brighter.

But we can’t be sure, and astronomers are excited. Betelgeuse is 15 to 20 times more massive than the Sun, and stars of this mass are expected to end their lives in a powerful explosion known as a supernova. Betelgeuse’s red color indicates that it is a red supergiant, which means that it is nearing the end of its life. But that end may still be a million years away. Stars like Betelgeuse can live for more than 10 million years, a very short period for astronomers, but a very long time for others.

Despite this, new models were run, some suggesting a supernova could occur within a few thousand years, while others place this event 1.5 million years in the future. There are many mysteries surrounding Betelgeuse. We don’t know its precise mass, and even its distance is disputed. The star is said to have recently merged with a smaller companion: this would explain why it spins faster than expected, as large stars tend to do. Some ancient manuscripts refer to the star as similar to the yellow Saturn instead of the reddish Mars. Has the star changed color? This could indicate rapid evolution, meaning a supernova could occur sooner rather than later.

If Betelgeuse were to go supernova, what would it look like? The star is about 500 light-years away. After an explosion, we would first detect a shower of massless particles called neutrinos, which would be harmless to us. After that, the star would shine rapidly. After a week or two, it would shine as bright as the full moon. Betelgeuse would then fade over the next several months, but would remain visible during the day for six to 12 months. At night, you should be able to see it with the naked eye for another year or two. But after that, we would never see it again: Orion would lose its red glow forever.

Is there any danger for us? Supernovae produce high-energy particles called cosmic rays, which can penetrate Earth’s magnetic field shield. But the amounts would be small compared to the other radiation we get for all but the closest supernovae. A supernova explosion would also create radioactive iron. In fact, this substance has been found in the seabed of the Earth and on the Moon, it is believed that it was formed during a supernova explosion between 2 and 3 million years ago. This supernova was perhaps 300 light-years away, closer than Betelgeuse, but far enough away that it didn’t cause any major problems for life on Earth. A very close supernova, less than 30 light-years away, could cause major problems: cosmic rays could cause ozone layer destruction and dangerous UV levels on Earth. It could halve ozone over a period of hundreds to thousands of years: this level is considered capable of causing an extinction event. But such a close supernova would be very rare and could only happen once every billion years. Ultimately, Betelgeuse could still be around for quite some time. And that’s good, because it’s a fascinating and mysterious star. We still have a lot to learn.