Betelgeuse: Nearby supergiant star's dimming explained

James Marshall
July 2, 2020

Betelgeuse is the bright red star at the top left of the constellation of Orion around 500 light years from Earth.

Researchers now say this was caused by big cool areas similar to the sunspots seen on our own parent star.

If Betelgeuse replaced the Sun at the centre of the Solar System, the Earth and Mars would be inside the star and Jupiter would be skimming its surface.

By April 2020, the star had returned to its normal range of brightness, which varies between 0 and +1.6 magnitude.

'Towards the end of their lives, stars become red giants, ' Dharmawardena said, adding that 'as their fuel supply runs out, the processes change by which the stars release energy'.

Scientists have developed various scenarios to explain this change in the brightness of the star, which is visible to the naked eye and nearly 500 light years away. This difference was enough to be seen with the naked eye even at a distance of 500 light years. Pulsations can eject the outer layers of the star with relative ease. Due to the small mass at the huge size of the weak gravity is sometimes not able to hold atmosphere, forming a region of cold gas and dust with different temperatures across the surface, which reduce the brightness of the stars in General. They used the James Clerk Maxwell Telescope (JCMT) in Hawaii. Seen through these telescopes, the otherwise invisible interstellar dust would emit a glow in submillimetre waves. The wavelength of this radiation is a thousand times greater than that of visible light.

As this part of the spectrum enables one to observe the distribution of cosmic dust, astronomers were able to test one of the earlier suggested reasons for the star's dimming.

'What surprised us was that Betelgeuse turned 20 per cent darker even in the submillimetre wave range, ' reports Steve Mairs from the East Asian Observatory.

A study published Monday in the journal Astrophysical Journal Letters.

Instead, the astronomers say, temperature variations in the photosphere - the luminous surface of the star - most likely caused the brightness to drop.

"Corresponding high-resolution images of Betelgeuse from December 2019 show areas of varying brightness. Together with our result, this is a clear indication of huge star spots covering between 50% and 70% of the visible surface and having a lower temperature than the brighter photosphere", said co-author Peter Scicluna from the European Southern Observatory (Eso). "For comparison, a typical sunspot is the size of the Earth".

A study, "Betelgeuse fainter in the sub-millimetre too: an analysis of JCMT and APEX monitoring during the recent optical minimum", carried out by an worldwide team of astronomers led by Thavisha Dharmawardena of the Max Planck Institute for Astronomy in Heidelberg, dismisses previous conjectures that it was dust, recently ejected by supernova Betelgeuse, which caused the star to dim.

Betelgeuse began to brighten again from April, ruling out a supernova, and so astronomers set out to find a new theory to explain the unusual level of dimming. "It is a supergiant star growing a super-sized star spot." said Prof Zijlstra. The Betelgeuse star spot would be a hundred times larger than the Sun. In the case of our Sun, sunspots tend to increase and decrease in an 11-year cycle; but it is unclear whether it is the same with Betelgeuse, which is so massive that it would nearly reach Jupiter if placed at the center of our solar system.

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