The Curious Case of the Shrinking Star

The red supergiant star Betelgeuse is undoubtedly enormous. But it’s shrinking, and astronomers aren’t sure why.

Researchers at the University of California at Berkeley have been monitoring the star by aiming the Infrared Spatial Interferometer, atop Mt. Wilson in Southern California, toward the star’s home in the constellation Orion. Since 1993, the Betelgeuse star (pictured in a NASA image at left) has shrunk in diameter by more than 15 percent.

Betelgeuse is so big that in our solar system it would reach to the orbit of Jupiter. Its radius is about five astronomical units, or five times the radius of Earth’s orbit. Its measured shrinkage means the star’s radius has shrunk 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 and his colleague, Edward Wishnow, a research physicist at UC Berkeley, presented their findings at a press conference on Tuesday during the Pasadena meeting of the American Astronomical Society. The results also appeared June 1 in The Astrophysical Journal Letters.

Despite Betelgeuse’s diminished size, Wishnow pointed out that its visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers, has shown no significant dimming over the past 15 years.

The ISI has been focusing on Betelgeuse for more than 15 years in an attempt to learn more about these giant massive stars and to discern features on the star’s surface, Wishnow said. He speculated that giant convection cells on the star’s surface might affect the measurements. Like convection granules on the Sun, the cells are so large that they bulge out from the surface. Townes and a former graduate student observed a bright spot on the surface of Betelgeuse in recent years, although at the moment, the star appears spherically symmetrical.

“But we do not know why the star is shrinking,” Wishnow said. “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.”

Betelgeuse 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. In 1921, Francis G. Pease and Albert Michelson used optical interferometry to estimate its diameter was equivalent to the orbit of Mars. Last year, new measurements of the distance to Betelgeuse raised it from 430 light-years to 640, which increased the star’s diameter from about 3.7 to about 5.5 AU.

“Since the 1921 measurement, its size has been re-measured by many different interferometer systems over a range of wavelengths where the diameter measured varies by about 30 percent,” Wishnow said. “At a given wavelength, however, the star has not varied in size much beyond the measurement uncertainties.”

The measurements cannot be compared anyway, because the star’s size depends on the wavelength of light used to measure it, Townes said. This is because the tenuous gas in the outer regions of the star emits light as well as absorbs it, which makes it difficult to determine the edge of the star.

The Infrared Spatial Interferometer, which Townes and his colleagues first built in the early 1990s, sidesteps these confounding emission and absorption lines by observing in the mid-infrared with a narrow bandwidth that can be tuned between spectral lines. The technique of stellar interferometry is highlighted in the June 2009 issue of Physics Today magazine.

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,” he said, “and uncover very fundamental and important new things.”

Sources: AAS and UC Berkeley. The paper is available here.