Red giant Betelgeuse is mysteriously shrinking

Pasadena, CA — Betelgeuse, the bright red supergiant star in the constellation Orion, is shrinking, according to University of California researchers.

Long-term monitoring by UC Berkeley’s Infrared Spatial Interferometer (ISI) on the top of Mount Wilson in Southern California shows that Betelgeuse – which is so big that in our solar system it would reach to the orbit of Jupiter – has shrunk by more than 15 percent since 1993.

“To see this change is very striking,” said Charles Townes, the UC Berkeley professor emeritus of physics who won the 1964 Nobel Prize in Physics for inventing the laser and the maser. “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.”

Despite Betelgeuse’s diminished size, its visible brightness, or magnitude, has shown no significant dimming over the past 15 years.

The researchers say they do not know why the star is shrinking, but speculate that the measurements may be affected by giant convection cells on the star’s surface that bulge out of the surface.

Measuring a star is difficult, because the star’s size depends on the wavelength of light used to measure it. 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 ISI sidesteps these problems by observing in the mid-infrared with a narrow bandwidth that can be tuned between spectral lines. The ISI consists of three 1.65m mirrors separated by distances that vary from  four to 70m. Using a laser as a common frequency standard, the ISI interferometer combines signals from telescope pairs in order to determine path length differences between light that originates at the star’s center and light that originates at the star’s edge.

“We observe around 11 microns, the mid-infrared, where this long wavelength penetrates the dust and the narrow bandwidth avoids any spectral lines, and so we see the star relatively undistorted,” said Townes. “We have also had the good fortune to have an instrument that has operated in a very similar manner for some 15 years, providing a long and consistent series of measurements that no one else has.”

The results are published in The Astrophysical Journal Letters.