Interferometer Gauges Space

Daniel S. Burgess

In the seemingly infinite universe, it is difficult to tell just how far that galaxy far, far away really is. Advances in stellar interferometry, however, promise astronomers a means to more accurately determine cosmic distances.

Researchers working at the testbed interferometer at the Palomar Observatory have confirmed the feasibility of the Baade-Wesselink approach to measuring the distance to Cepheid variables, stars that pulsate in diameter over a regular period and that display an equally regular change in their apparent luminosity.

Astronomers typically gauge the distance to Cepheid stars -- and the region of space in which they reside -- by analyzing this change in apparent luminosity against their intrinsic luminosity, which can be inferred from the pulsation period.

Two-step procedure

Baade-Wesselink, however, does not rely on estimated changes in brightness. Rather, it is based on the true measurement of changes in a Cepheid's diameter using stellar interferometry.

Therefore, the technique may better establish period-luminosity standards based on the variable stars within our galaxy and enable more accurate calculations of the distance to the extragalactic Cepheids beyond its reach.

"Baade-Wesselink is conceptually easy but technically challenging," said Benjamin F. Lane, a graduate student at California Institute of Technology and a member of the research team, which reported its work in the Sept. 28, 2000, issue of Nature.

The technique involves a two-step process. First, astronomers calculate the radial velocity of a Cepheid by measuring the Doppler shift of its spectral lines. Then, integrating the radial velocity yields the change in the physical size of the star.

"Here is where we come in," Lane said. The technique requires a high-resolution stellar interferometer to determine the apparent change in the angular diameter of the Cepheid, which may be thousands of light-years away. The ratio of the physical and angular sizes yields the distance.

For two weeks, the researchers observed changes in the Cepheid variable Zeta Geminorum. This, in itself, was no easy feat because the star has an average angular diameter of only about 1.5 milliarc seconds.

The observations revealed that Zeta Geminorum increases in diameter by approximately 10 percent over its 10.15-day pulsation period and that it is approximately 1095 light-years away, with an uncertainty of 13 percent.

New age of interferometry

Interferometers, of course, are not new instruments. Albert A. Michelson himself used a 20-foot device in 1920 to measure the diameter of the red giant Betelgeuse.

Certain technological advances were required, however, to realize the precision of the 300-foot Palomar testbed interferometer and to design the 1000-foot monsters currently in the works.

"The biggest technical enabler is high-speed computers," Lane said. "There are also a few other things that had to be developed to make it work: laser distance gauges, piezoelectric actuators and various kinds of light detectors."

Lane noted that Michelson measured his fringe patterns looking through an eyepiece; the new breed of researchers reaps the benefits of accuracy in a world of adaptive optics, digital cameras and image processing.

The longer baselines of the US Navy's prototype optical interferometer and the Center for High Angular Resolution Astronomy at Georgia State University in Atlanta will enable researchers to apply the technique to more Cepheids.