The sky — astronomy

Betelgeuse: The Red Supergiant in Orion

Betelgeuse is a red supergiant in the constellation Orion and will one day explode as a supernova, but astronomers do not expect that to happen soon in human terms: the usual estimate is within roughly 100,000 years. Its Great Dimming in 2019–2020 is mostly explained by a cloud of dust, not an imminent explosion. The star is far too distant to threaten Earth.

What is a red supergiant?

Betelgeuse (alpha Orionis) is one of the best-known stars of the winter sky. It marks the shoulder of the hunter in the constellation Orion and is easy to spot thanks to its orange-red colour, which contrasts with the blue-white Rigel at the opposite side of the figure.

A red supergiant is a massive star in a late stage of its life. After it has used up the hydrogen in its core, the star swells enormously while its surface cools and turns reddish. Betelgeuse is huge: depending on the estimate, its radius is several hundred times that of the Sun. Placed where the Sun is, its surface would reach well beyond Earth's orbit, probably beyond Mars as well. Its mass is estimated at roughly 15–20 solar masses, and its age at only about 8–10 million years, compared with about 4.6 billion years for the Sun. Massive stars burn through their fuel quickly.

The distance to Betelgeuse is not known with perfect precision. Estimates range from about 550 to 700 light-years, which also makes its exact size uncertain. The star is variable: its brightness changes over several cycles, the clearest lasting roughly a year, with a longer one of about six years. This is normal for a star at this stage.

The Great Dimming

In late 2019 and early 2020, observers noticed something unusual: Betelgeuse lost a large part of its brightness and became the faintest it had been in decades of observations. Even to the naked eye, Orion looked different. The event became known as the Great Dimming.

Naturally, people asked whether it was a sign of an imminent explosion. Observations with the Hubble Space Telescope, telescopes of the European Southern Observatory and other instruments pointed to a more ordinary explanation. According to published studies, the star ejected a large amount of material from its surface. As this gas moved away and cooled, it formed a cloud of dust that blocked part of the starlight toward us. A cooler patch on the surface also played a role. After 2020 the star returned to its usual pattern of fluctuations.

The episode is valuable for science, because it let astronomers watch in detail how a supergiant loses mass. This is how such stars enrich space with heavier elements.

Will it become a supernova, and when?

Yes. Betelgeuse is expected to end its life as a type II supernova, the explosion of a massive star. When nuclear fuel runs out in the core, the core will collapse and the outer layers will be blown away. A neutron star is the most likely remnant, though a black hole is not completely excluded.

The answer to “when” is: we do not know exactly. Astronomers think the star is burning helium in its core and that the explosion is probably tens of thousands of years away; estimates are often within about 100,000 years. On stellar timescales that is soon, but on human ones it is not. The 2019–2020 dimming was not a warning sign of an imminent end. Nobody can predict the day.

When it happens, we will notice in two ways. First, neutrino detectors should register a burst of particles before the light becomes visible. Second, the supernova will become very bright, visible even in daylight for weeks, and then fade gradually. Once it fades, Orion will look different, without the star that now marks the hunter’s shoulder. For comparison among sky landmarks, Sirius is the brightest star in the night sky, while Betelgeuse is among the brightest but varies in brightness.

Is it safe for Earth?

Yes. Betelgeuse is hundreds of light-years away, while dangerous distances for a supernova are on the order of tens of light-years. The explosion will not harm our planet, its atmosphere or people. There will be no weather changes and no dangerous radiation. The only effects will be a rare and beautiful sight and a wealth of new scientific data.

Betelgeuse is also a good gateway to the sky: it lies in Orion, one of the 88 constellations recognised by the International Astronomical Union (IAU). You can see it on winter evenings, and once you find it you will know what a star in the late stage of its life looks like. If it looks a bit fainter or brighter than last time, that is most likely ordinary variation, not a sign of anything unusual.

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