BY DR EMILY BALDWIN
Posted: 10 June, 2009
Betelgeuse is so big that if placed in our Solar System it would extend to the orbit of Jupiter. But thanks to long term monitoring by Berkeley’s Infrared Spatial Interferometer (ISI) at Mt Wilson, measurements of the star’s diameter reveal the giant to be shrinking. Over the last 15 years, the star has shrunk by more than 15 percent, equivalent to the same diameter as the orbit of Venus.
“To see this change is very striking,” says 1964 Nobel Prize in Physics recipient Charles Townes. “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.”
Betelgeuse has been shrinking over the last 15 years. Image: A Dupree (CfA), NASA, ESA.
Despite its diminished size however, there is no evidence to suggest that the star is dimming. “But we do not know why the star is shrinking,” says Edward Wishnow, also of UC Berkeley. “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 to ever have its size measured. In 1921, Francis G. Pease and Albert Michelson used optical
“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,” says Wishnow. “At a given wavelength, however, the star has not varied in size much beyond the measurement uncertainties.”
Since the star’s size depends on the wavelength of light used to measure it, it is difficult to compare measurements. Tenuous gas in the outer regions of the star emits light as well as absorbs it, which presents a challenge in determining the edge of the star.
The ISI consists of three 1.65 metre diameter mirrors separated by distances that vary from 4-70 metres. 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. Image: David Hale 2006.
In the early 1990s, Townes and colleagues built the ISI that sidesteps these confounding emission and absorption lines by observing in the mid-infrared with a narrow bandwidth that can be tuned between spectral lines. “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,” says 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 first measurements showed a size quite close to Michelson’s result, but over 15 years, it has decreased in size about 15 percent, changing smoothly, but faster as the years progressed.”
Townes and colleagues intend to continue monitoring Betelgeuse in hopes of finding a pattern in the changing diameter. They will also 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,” adds Townes.
The results are published in the 1 June edition of The Astrophysical Journal Letters.