Mixing lasers and starlight
Townes and his colleagues built what's called an interferometer on Mount Wilson, a 5,700-foot (1,742-meter) peak just east of Los Angeles.
Interferometry is a method of combining radio waves or light from two or more telescopes separated by several feet or even miles to effectively create one large telescope. The technique has been widely used in radio astronomy, but Townes' team is the first to employ it to gather radiation in the mid-infrared portion of the electromagnetic spectrum. They use two 65-inch (1.65-meter) telescopes mounted on trailers so the distance between them can be easily changed.
But when gathering a wide range of frequencies, even within the infrared portion of the spectrum, an interferometer (or any telescope for that matter) will see radiation emitted by water vapor and other gas in the cloud around a star. So Townes turned to his expertise with lasers.
After the infrared radiation is collected by the two telescopes that make up the interferometer, it is mixed in a box, focused, and combined with a laser that cancels out all but one specific frequency. The technique, called heterodyne interferometry, is common in radio astronomy, but it had never been tried with light.
"Mixing starlight and laser light, when I first heard of the idea ... sounded almost like alchemy -- a mythic mixing of the pure and elemental in pursuit of gold," Tuthill says. "However, unlike alchemy, it is founded upon solid principles."
Tuthill, who was not involved in the new study, worked with Townes at Berkeley from 1995 to 1999 and is familiar with the technique. He said Townes has set a benchmark for other optical interferometers to measure up to, adding that the ten years of ISI measurements leading up to this latest study "has pioneered a new field and established its legitimacy to an often skeptical astronomical community."
On the fringe of science
Interferometers do not produce conventional images. Instead, they produce data that can be used to derive the size of objects, like stars, and the size and shape of features within those objects.
The physics behind an interferometer is complex, but in a nutshell it involves combining light from two or more telescopes and determining how the pair of light waves interfere with one another. If both waves are in step, or in phase, the crest of the waves coincide and add together to form a single wave whose size is doubled.
This results in an "interference fringe."
The fringes in Townes' new study reveal a phenomenon that was first predicted in the early 1920s by Albert Michelson, who made the initial attempts at interferometry, ironically, from Mount Wilson when Townes was just six years old.
Michelson predicted that the material surrounding a red giant would shade the star more on the edges than at the center, making the star appear smaller than it really is.
The effect, which Michelson called limb darkening, has been widely recognized, but no one has known exactly how much of it to factor into their measurements. Finding two stars that were 10 percent larger than expected is in line with what might be expected from limb darkening, Townes said, but it does not account for the 30 percent increase in the size estimate for Mira.
Gasoline on a fiery debate
The new finding "throws more gasoline on a fiery debate concerning the pulsation of Mira variable stars," Tuthill said. He said researchers disagree over whether the stars oscillate in a fundamental mode, like a pure note struck by a tuning fork, or in a more complex overtone mode, as when multiple frequencies combine to create the sound of a note produced by a particular instrument.
"Most theorists would like to see fundamental mode pulsation, and have been somewhat dismissive of measurements which imply that the stars are too large to be fundamental pulsators and must therefore be pulsating in an overtone."
But Tuthill said the new measurements, implying larger sizes, might force astronomers "to sit up and take notice of what nature is telling us, rather than relying on their computer simulations for guidance."
The work was recently published in the Astrophysical Journal and was discussed at a June meeting of the American Astronomical Society by Jonathan Weiner, a graduate student working under Townes. David D. Snyder Hale of the Mount Wilson Observatory and John H. Lacy of the University of Texas also contributed to the study.
The ISI has been sponsored by the National Science Foundation and the Office of Naval Research. A third telescope will go online later this year and should allow Townes and his colleagues to determine if Mira and stars like it are round, asymmetrical, or if they have patches of brightness sticking out in certain locations.
"Everybody assumes they're pretty much round," Townes said. "My guess is it's probably true, but we feel it ought to be measured carefully."
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