http://www.space.com/scienceastronomy/astronomy/red_giants_010703-1.html

Lasers Help Show Stars are Larger Than Thought

By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
03 July 2001

Using a novel observational technique that one scientist likened to alchemy, an 85-year-old Nobel laureate has mixed lasers with starlight and, with a sprinkling of luck, discovered that some old stars are larger than expected.

If it holds up, the finding could force astronomers to rethink theories about how stars evolve and behave.

Charles Townes, a professor emeritus at the University of California at Berkeley, led a team of researchers that peered through the deceptive gas veils of three well-known aged stars and found each of them to be larger than previous measurements.

One of the objects, an old gas-enshrouded star called Mira, now seems to be 30 percent larger than what the textbooks say, a figure Townes calls "troublesome" for current theories that try to explain the structure and dynamics of many of our galaxy's older stars.

The potential theory-busting data was collected only after Townes drew from his experience in quantum electronics, for which he received a Nobel Prize in 1964, to develop a way to effectively cancel out all but the most telling wavelengths of radiation coming from the stars.

"Charlie Townes and his colleagues are pioneering a very difficult technique, but one that can confront directly our fundamental concepts about stars and their evolution," said Andrea Dupree, a researcher at the Harvard-Smithsonian Center for Astrophysics.

Dupree and Ronald Gilliland were credited in 1996 with making the first direct image of star's surface -- other than the Sun. That star, Betelgeuse, was also studied by Townes and found to be 10 percent larger than previously measured, as was another star called Antares (also known as Alpha Scorpii).

All three stars in Townes' study are currently bright and easily visible to the naked eye.

Cloudy picture

Betelgeuse (often pronounced "beetle juice"), Antares and Mira are red giants, a class of ancient objects that are running out of fuel and preparing to kiss their stellar lives goodbye. Our own Sun will one day become a red giant, likely swallowing Earth as it swells and contracts over thousands and millions of years.

Some red giants, like Mira, also oscillate in brightness on short time scales of months or years for reasons scientists don't fully understand. Mira (officially called Omicron Ceti) is in fact the namesake for a special class of red giants whose brightness oscillates on relatively short cycles.

Mira's intensity waxes and wanes every 330 days. At its dimmest, it is invisible to the naked eye and just 1 percent as bright in the sky as when it reaches peak brightness.

"Mira has an awesome amount of material around it," Townes explained in a telephone interview, "and when it oscillates in intensity it blows off material, every year, as it gets brighter and weaker, brighter and weaker."

He says the star's diameter ought to change in step with this oscillation.

"Theory says it's probably at its biggest at about the time when it's at its brightest, but we don't know that for sure," he said, adding that research will continue later this year in an attempt to see how the size and shape of the star change over time.

Meanwhile, the leading theory that explains this short-term oscillation requires that Mira be no larger than previous estimates.

"If the [Townes] results hold up, and if current theory is at a loss, then a real problem for astrophysics faces us," Dupree told SPACE.com.

Lack of information

If theorists have to rethink how red giants work, they can be excused for not having had much to go on. Red giants are surrounded by so much gas and dust that scientists have difficulty observing their true size, shape and behavior.

So knowledge of these stars, which are often more than 10 billion years old, has relied in part on estimates of the type and amount of debris surrounding them, along with assumptions based on what we know about our own Sun, which is roughly 4.6 billion years old and not yet anything like a red giant.

"But what if other stars are not like the Sun?" asks Peter Tuthill, an astronomer at the University of Sydney. "The only way to really find out is of course to go out and make measurements, and the stars studied by Townes are very unlike our Sun, so the orthodox theory is on very shaky ground indeed."

Peeking inside

With the new laser technique, the Townes team set out a decade ago to map the quantity and distribution of gas and dust around the three stars. They observed the stars in infrared light using Berkeley's Infrared Spatial Interferometer (ISI). The interferometer amounts to a pair of telescopes mounted on commercial tractor-trailers and hooked together to increase their resolving power by gathering starlight in tandem from two locations.

But as the researchers began to study the data and the method, they made the fortuitous realization that the important aspect to it all was the fact that they could peer right through the cloudy cocoon to measure the stars themselves.

"These new mid-infrared measurements are important, as this wavelength avoids some of the problems and complexities which make it difficult to interpret earlier data," Tuthill says. "Hopefully, with the new ISI measurements, a more complete model of the atmospheres of red giants might finally be constructed."

The results, if confirmed, could apply to many or most red giants, Townes said. But to a lesser extent the results might have implications for other stars.

Very young stars are surrounded by natal debris clouds and have also evaded measurement; they might be studied with the new technique. Even many "normal" or middle-aged stars might now be found to be slightly larger than thought, Townes said, adding that the difference would be minimal among these stars, however.

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."