Popular Giant Star Shrinks Mysteriously
UC Berkeley physicist Charles Townes cleans one of the large mirrors of the Infrared Spatial Interferometer (ISI). The ISI consists of three 5.4-foot (1.65-meter) diameter mirrors separated by distances that vary from 12 to 230 feet (4 to 70 meters).
Credit: Cristina Ryan (2008)

A massive red star in the constellation Orion has shrunk in the past 15 years and astronomers don't know why.

Called Betelgeuse, the star is considered a red supergiant. Such massive stars are nearing the ends of their lives and can swell to 100 times their original size before exploding as supernovae, or possibly just collapsing to form black holes without violent explosions (as one study suggested).

Betelgeuse, one of the top 10 brightest stars in our sky, is a popular target among backyard skywatchers and was the first star ever to have its size measured, and even today is one of only a handful of stars that appears through the Hubble Space Telescope as a disk rather than a point of light. It was the first star (besides our sun) to have its surface photographed (by Hubble).

The new finding, presented today at a meeting of the American Astronomical Society in Pasadena, Calif., was based on data collected by UC Berkeley's Infrared Spatial Interferometer (ISI) on the top of Mt. Wilson in Southern California.

In 1993, measurements put Betelgeuse's radius at about 5.5 astronomical units (AU), where one AU equals the average Earth-sun distance of 93 million miles, or about 150 million km. Since then it has shrunk in size by 15 percent. That means the star's radius has contracted by a distance equal to the orbit of Venus.

"To see this change is very striking," said Charles Townes, a UC Berkeley professor emeritus of physics. "We will be watching it carefully over the next few years to see if it will keep contracting or will go back up in size." (Townes won the 1964 Nobel Prize in physics for inventing the laser and the maser, a microwave laser.)

Though the star is shrinking, its visible brightness has not dimmed significantly over the past 15 years, the researchers say.

"But we do not know why the star is shrinking," said Edward Wishnow, a research physicist at UC Berkeley's Space Sciences Laboratory. "Considering all that we know about galaxies and the distant universe, there are still lots of things we don't know about stars, including what happens as red giants near the ends of their lives."

Townes, who turns 94 in July, plans to continue monitoring Betelgeuse in hopes of finding a pattern in the changing diameter, and to improve the ISI's capabilities by adding a spectrometer to the interferometer.

"Whenever you look at things with more precision, you are going to find some surprises and uncover very fundamental and important new things," he said.

The finding was also published June 1 in the Astrophysical Journal Letters.

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