Betelgeuse (pronounced "beetle juice") is a massive red supergiant star 640 light-years from Earth. Recent observations suggest it is shrinking.

Credit: A. Dupree (CfA), R. Gilliland (STScI), FOC, HST, NASA

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SYDNEY: A 15-year, continuous observation of the red supergiant Betelgeuse has found that the star, one of the largest known, is shrinking – but astronomers don’t understand why.

“We don't know what is causing the shrinking of Betelgeuse. This is part of the surprise and puzzle,” astrophysicist and Nobel laureate Charles Townes told Cosmos Online.

Betelgeuse is a red supergiant red star about 20 times as massive as the Sun. It sits in the western shoulder of the constellation Orion, and is one of the brightest stars in the sky.

Shrinking starlet

Betelgeuse was first measured in 1921 – the first time anyone had measured the diameter of a star. The star is so big if it were centred on the Sun, its outer surface would reach halfway to the orbit of Jupiter.

A team led by Townes, from the Space Sciences Laboratory of the University of California, Berkeley, measured Betelgeuse’s diameter from 1994 to 2009 using the university’s Spatial Interferometer Array.

The researchers observed the star over narrow frequencies using lasers to reduce the effects of the ‘twinkle’ in the night sky. This provided them with an accurate measurement that of the star itself rather than its dusty halo, Townes said.

They found the diameter of Betelgeuse diminished from 56 to 47 milliarcseconds (one milliarcsecond is less than three millionths of a degree or 1/3,600,000 degree), which suggests it has shrunk by around 15%.

Not so stable

Townes will present his research today at a meeting of the American Astronomical Society’s in Pasadena, California. The findings have also been published in a recent issue of the Astrophysical Journal.

“Everyone has [previously] thought that Betelgeuse is quite constant in size, and it has been measured since 1921 without any clear changes previously seen,” he said.

Townes added that the shrinking was “surprising” because while Betelgeuse is nearing the end of its life, astronomers thought it was still stable. Though red supergiants burn their fuel much faster than the Sun, they still have a typical lifetime of a few tens of millions of years.