Determining the expansion rate of the universe, known as the Hubble constant, is a tall order. Astronomers have approached this problem in part by studying celestial objects whose observed brightness relates in a predictable manner to their distance from us, such as the so-called Cepheid variable stars. Large and luminous, the Cepheid variables are a class of pulsating stars that regularly change in size and brightness. By measuring the period of the shifts and using the period-luminosity relationship, researchers can infer the distance of Cepheids from their luminosity. And because these stars are so bright, they can serve as distance indicators in galaxies beyond our own. For theorists to derive the Hubble constant from such distances, they must first calibrate the period-luminosity relationship--a task made difficult by the low number of neighboring Cepheids. As a result, astronomers have had to rely on indirect methods of calibration.

But no more. According to a report in this weeks issue of the journal Nature, astronomers have for the first time measured the changing diameter of a Cepheid star directly. Using the Palomar Testbed Interferometer on Palomar Mountain in California, researchers studied the Cepheids in the Large Magellanic Cloud--a satellite galaxy of the Milky Way. Although their results come with an uncertainty of 15 percent, the distance derived from the teams observations accords well with previous indications. Further work should fine-tune the results."We anticipate that over the next few years, distances to several dozen Cepheids will be determined with an accuracy of a few percent," the researchers wrote in their report, "providing a direct calibration of the Cepheid period-luminosity relation."