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
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.
Commenting on the research, Australian astronomer Peter Tuthill from the University of Sydney said that the diameter of large stars like Betelgeuse was difficult to define.
“Red giant stars like Betelgeuse are like fluffy, extended balls of gas – if you had a jar of Betelgeuse’s atmosphere on Earth it would form a very good vacuum; there’s almost nothing there.”
Since the star’s temperature and brightness hadn’t changed, it showed that the star wasn’t “shrinking away”. Instead something complicated was going on in the stellar atmosphere and dust halo around the star, he said. “[The research] shows that the weather on Betelgeuse is complicated and its [outer] layers seem to evolve and change.”
Townes said his team would be watching Betelgeuse very closely over the next five years to work out what’s going on. “It’s a star worth watching,” he said.
Townes won the Nobel Prize for Physics in 1964 for his work on creating the maser (an acronym for ‘microwave amplification by stimulated emission of radiation’) before turning to astrophysics.
