The bright red star Betelgeuse in the constellation Orion continues to exhibit unusual behavior.
In late 2019 and 2020 it became weaker than we’ve ever seen. In the past it became fainter (almost) than Bellatrix, the third brightest star in Orion. This event became known as the “Great Blackout”. But since then Betelgeuse has been bright again. For a few days this year it was Orion’s brightest star, brighter than we’ve ever seen it. Both events have led to speculation whether its end 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, on the whole, remarkably stable. They shine with the same brightness year after year. But there are exceptions and some stars, dubbed variable stars, change in brightness. The most famous is Mira, the “star of wonders”, discovered as a variable star by the German shepherd David Fabricius 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 variable stars visible to the naked eye, although you have to pay close attention to notice their variation in brightness. Betelgeuse, the seventh brightest star in the sky (excluding the Sun), is the brightest of the variable stars. Sometimes Betelgeuse becomes nearly as bright as Rigel (the fourth brightest blue star in the constellation), while at other times it is noticeably dimmer. The variation is caused by pulsations, similar to Mira’s though not as large or regular. Sometimes, however, a star can become extremely bright for a short time. The brightest and rarest of these are supernovae, which form when an entire star ends its life in a powerful explosion. Supernovas can be bright enough to be visible during the day, though that’s only happened a few times in the last 1,000 years. A nearby, bright supernova is the kind of event astronomers live for, but that few of us will ever get to see. Although Betelgeuse is a variable star, the great blackout in 2021 was extreme. In just a few months, it had in fact subsided by about 60%. This was eventually shown to be caused by a cloud. Stars like Betelgeuse are continuously spewing gas and dust. A blob of gas in the wind, as large as the star itself, obscured half of it. Indeed, images of the star showed that the southern half was missing. That said, we still don’t know what caused the sudden change — it’s now 50% brighter than usual. But an impending supernova doesn’t seem that likely. In this type of star, a supernova explosion is triggered in the core. The variations in brightness, on the other hand, are a surface phenomenon. The extreme brightening could in fact be due to the same dust cloud that caused the dimming, now reflecting the star’s light back at us and making it appear brighter. 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 shows that it is a red supergiant, meaning that it is already nearing the end of its life. But that end may still be a million years away. Stars like Betelgeuse can live for over 10 million years, a very short time for astronomers, but a very long time for everyone else. Despite this, new models have been run, with some suggesting that 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 do not know its precise mass, and its distance is also disputed. The star is said to have recently merged with a smaller companion, which would explain why it rotates faster than expected – usually large stars do.
If Betelgeuse went supernova, what would happen? 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 brighten rapidly. After a week or two it would shine with about the same brightness as the full moon. Betelgeuse would then vanish over the next few months, but remain visible during the day for 6 to 12 months. At night, you should be able to see it with the naked eye for another year or two. 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, which is believed to have formed in a supernova explosion between 2 and 3 million years ago. That supernova was perhaps 300 light-years away, closer than Betelgeuse, but far enough away not to cause major problems for life on Earth. A very close supernova, less than 30 light-years away, could cause big problems: Cosmic rays could cause ozone destruction and dangerous UV levels on Earth. It could halve ozone over a period of one hundred to thousands of years: this level is considered capable of causing an extinction event. But a supernova this close would be very rare and could only occur once every billion years.