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Shrinking giantThis ultraviolet image of the red supergiant star Betelgeuse was taken in 1996 with the Hubble Space Telescope. New infrared observations with a ground-based interferometer now reveal that the star's diameter has mysteriously shrunk by more than 15 percent over the past 15 years.Andrea Dupree/Harvard-Smithsonian CfA, Ronald Gilliland/STScI, NASA, ESA

PASADENA, Calif. — Betelgeuse, one of the brightest stars visible to the naked eye, has shrunk in diameter by more than 15 percent since 1993.

The star, a red supergiant, has a radius exceeding the distance between the sun and Jupiter. The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the sun, researchers reported June 9 at an American Astronomical Society meeting and in the June 1 Astrophysical Journal Letters.

No other red supergiant has shown such a dramatic contraction, though others stars known as Mira variables routinely contract or expand by an even greater amount in a single year.

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Star trackerPhysicist Charles Townes cleans one of the mirrors of the infrared spatial interferometer that he and colleagues have used to monitor Betelgeuse since 1993.C. Ryan

It’s unclear why Betelgeuse has shrunk and whether the star will later rebound, said Charles Townes, a retired professor from the University of California, Berkeley who won the 1964 Nobel Prize in physics.

Townes and collaborator Edward Wishnow, also of UC Berkeley, used an infrared interferometer, consisting of three telescopes acting as a single unit, atop Mt. Wilson in southern California to monitor the star. The team examined the star at an infrared wavelength of 11.1 micrometers, which can penetrate the tenuous gas and dust in the star’s outer regions and accurately determine the star’s visible edge.

In 1921, Betelgeuse became the first star for which astronomers measured a size. Over the years, different interferometers, observing Betelgeuse over a wide range of wavelengths, have recorded diameters for the star that disagree with each other by as much as 30 percent. That’s not surprising because stars often look considerably bigger or smaller at different wavelengths. But the star hasn’t previously been found to vary significantly in size at any one wavelength, Townes says.

As hefty as 15 suns, Betelgeuse is nearing the end of its life and could soon go supernova. Continued close monitoring of Betelgeuse could lead to a better understanding of the evolution of massive stars near the end of their lifetime. 

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