Charles H. Townes needs no introduction to OSA members for his groundbreaking work in laser physics. He won the Nobel Prize in 1964 and has been an OSA Honorary Member since 1970.
Perhaps less well known to the photonics community is Townes’ more recent career as an astrophysicist specializing in mid-infrared interferometry. The descendant of his early-1970s work with a few graduate students at the University of California, Berkeley (U.S.A.) is the Infrared Spatial Interferometer (ISI), a three-telescope array on Mount Wilson in California.
Operating between 9 and 12 µm, the ISI makes precise measurements of so-called “evolved stars” – stars that have reached the end of their hydrogen-burning middle age and are starting to shed material into interstellar space. With ISI, Berkeley scientists have been measuring the sizes of the dust shells around these ancient stars and, in a few cases, the diameters of the stars themselves.
On June 9, at a press conference during the American Astronomical Society (AAS) meeting in Pasadena, Calif. (U.S.A.), Townes and Berkeley research physicist Edward Wishnow said that Betelgeuse – that bright red supergiant star in the constellation Orion – has been mysteriously shrinking over the past 15 years. It’s still wider than the inner solar system, but its angular diameter has shrunk 15 percent, from 56 to 47 milli-arcseconds.
Because Betelgeuse is relatively close by – only 640 light-years from us – and so bloated, humans have been trying to figure out its size for decades. Indeed, A.A. Michelson (another OSA Honorary Member) and Charles Pease used Michelson’s own stellar interferometer to make the first such measurement in December 1920. They found the giant star to be 47 milli-arcseconds at visible wavelengths, but at shorter wavelengths, the apparent diameter varies because of absorption or emission by gas and dust near the star.
Townes and his colleagues observed Betelgeuse at 11.5 µm, a wavelength that gets through most of the dust and allows measurement of the width of the stellar photosphere, or light-radiating outer layer. Wishnow noted that the outer atmospheres of these evolved supergiants tend to be quite tenuous. “It’s somewhat difficult to ascertain the precise location of an edge,” he said.
The group is observing in a narrow spectral region that avoids spectral lines of gases. “We think we’re seeing the continuum of the star very precisely,” Townes said.
Betelgeuse is known to be a variable star. In fact, Wishnow said, it’s no longer the brightest star in Orion, as it was designated on a star chart from the year 1603. However, the star has not significantly dimmed overall during the 15 years of ISI observations.
Townes noted that his team found a red spot on Betelgeuse a few years ago, and the feature may or may not be related to the star’s instability. Only careful observing over time will reveal what activity is happening within the star.
“Basically, we hope to learn something new,” the Nobel laureate said with a chuckle. “That’s the great thing about science -- to keep trying, looking at things more and more carefully, getting better instrumentation so that you can measure more precisely. The more you watch things and the more carefully you look, the more surprises you’re likely to find.”
ISI, incidentally, is the only visible/infrared telescope array to use a heterodyne detection system as in radio interferometry. Incoming starlight is mixed with the beam from the master local oscillator (a CO2 laser) and downconverted to microwave frequencies for further processing. The Berkeley team published the technical details in this article nine years ago, shortly before upgrading ISI from two to three telescopes. Physics Today recently published more general information about optical and infrared stellar interferometry.
The Betelgeuse research by Townes, Wishnow and colleagues – which uses 15 years’ worth of ISI observations – appeared in the June 1 issue of Astrophysical Journal Letters.
In his remarks at the start of the news conference, longtime AAS press officer Steve Maran thanked Townes for several acts of assistance he performed over the years, especially setting up a special AAS meeting session in 1991 when the U.S. government first declassified the technology for laser-guide-star adaptive optics. The session featured “all kinds of military scientists whom we wouldn’t have known,” Maran said.
After the press conference, the AAS media team treated Townes to a birthday cake, about six weeks early. He’ll be 94 on July 28.