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Keen vision. The Palomar Testbed Interferometer's resolution is high enough to spot a basketball on the moon.

Measuring a Giant's Sighs

They're some of the oddest objects in the heavens: Cepheid variables, giant yellow stars that grow dimmer and brighter over periods of days or weeks. Astronomers have long presumed that the outer layer of such stars physically expands and contracts to cause their odd behavior. Now, as reported in this week's issue of Nature, they have observed it doing so for the first time.

"It's been something that we've always wanted to do," says graduate student Ben Lane of the California Institute of Technology in Pasadena, part of the five-person team that made the observations. The Cepheid variable they caught in the act was Zeta Geminorum, known to stargazers as the kneecap of one of the twins in the constellation Gemini.

Earlier astronomers inferred the size of the oscillations indirectly, through the well-known phenomenon of the Doppler shift. Seeing the size change directly, however, has been a daunting challenge in precision astronomy. Given Zeta Geminorum's diameter as seen from Earth, observing it grow and shrink would be equivalent to spotting a basketball on the moon. To accomplish that feat, the team used two telescopes linked into a 110-meter-wide interferometer, the Palomar Testbed Interferometer (PTI) in California, which has as much angular resolving power as a telescope with a mirror larger than a football field. Even so, it proved a technical tour de force; for one thing, the team had to modify the PTI to use shorter wavelength infrared light, thus improving its resolution.

By last Christmas, Lane already had clear evidence of the star's growth and shrinking, and by this spring he had the most accurate estimate ever of the angular size of the oscillations. Then, by dividing the angular size of the oscillations into their absolute size (as inferred from redshift measurements), Lane calculated the distance of Zeta Geminorum as 1100 light-years from Earth.

But the significance of the result extends far deeper into space. "In time, measurements like these will simplify and therefore strengthen astronomers' measurements of the distances of galaxies, and thus the size and age of the universe," says Jeremy Mould, an astronomer at the Australian National University in Canberra. That is because Cepheid variables are used to calibrate the distances to nearby galaxies, which in turn form a reference for estimating the distance to those farther away. This leads to an estimate for the Hubble constant--the ratio of the recession speed of the galaxies to their distance from Earth--which, finally, constrains the age and fate of the universe.

"Because we still have a 10% uncertainty, we're not making a dent in the Hubble constant today," says Shrinivas Kulkarni, who supervised Lane's research. "The excitement is that the technique does work."

Related sites
Animation of two Cepheid variables, showing that the brighter variables oscillate more slowly
The Palomar Testbed Interferometer
Shrinivas Kulkarni's home page at Caltech