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Betelgeuse red supergiant star

Betelgeuse brightness fading after outbreak of ‘star spots’

Image credit: Pitris | Dreamstime

The red supergiant Betelgeuse is cooling down and fading in brightness as it nears the end of its life.

Betelgeuse is the bright red star at the top left of the constellation of Orion, approximately 500 light years from Earth. It is discernible in the night sky by the naked human eye.

As a red supergiant, the star is 900 times larger than the Sun and nearing the end of its life. If Betelgeuse replaced the Sun at the centre of the Solar System, Earth and Mars would be inside the star and Jupiter would be skimming its surface.

In late 2019, the star began to rapidly fade in brightness and by February 2020 it was only half as bright as before.

Initially thought to be a sign that the star was about to go supernova or that it experienced a dust storm, scientists have now found that it was caused by a gigantic star spot 10 per cent colder than the rest of the star’s surface.

An international team of astronomers used the James Clerk Maxwell Telescope (JCMT) in Hawaii - a radio telescope - to determine the true cause of the fading. This telescope, built by the UK and operated by the East Asian Observatory in Taiwan, detects the light coming from dust grains that have formed around the star. The astronomers discovered that this emission had also decreased.

The global collaborative effort involved astronomers in Germany, Taiwan, the US and Chile and including Albert Zijlstra and Iain McDonald from The University of Manchester.

Publishing their findings today in The Astrophysical Journal Letters, the team confirmed that the fading was not the result of dust particles but instead by the formation of star spots now covering up to two-thirds of Betelgeuse. The observed star spots have a temperature of 3,500°C: around 10 per cent cooler than the rest of the star’s surface.

The University of Manchester’s Professor Albert Zijlstra, an author of the study, said: “For comparison, a typical sunspot is the size of the Earth. The Betelgeuse star spot would be a hundred times larger than the Sun. The sudden fading of Betelgeuse does not mean it is going supernova. It is a supergiant star growing a super-sized star spot.”

Following this outbreak of star spots, Betelgeuse slowly recovered and by May 2020 it was back at its original brightness.

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