Number 200 (Story #1), October 26, 1994 by Phillip F. Schewe and Ben Stein ACCURATELY MEASURING THE HUBBLE CONSTANT (H) , the parameter describing the universe's rate of expansion, depends on having a reliable yardstick. For gauging distances to objects outside our galaxy, astronomers often use Cepheid variable stars, stars whose light emissions vary with a period proportional to their intrinsic luminosity; one can compare the intrinsic to the observed luminosity of the Cepheid to determine the distance to the galaxy in which it lies. The effort to extend this method over a larger distance scale would result in a more precise value for H because it would lessen the effect of local gravitational interactions. The trouble with this method, however, is that telescopes have trouble making out faint Cepheids in distant galaxies and following their pulsations unambiguously. Nevertheless, two sets of observations have now been made of Cepheids in galaxies in the faraway Virgo cluster, at a distance twice as far away as for Cepheids previously used for the purpose of finding H. One group, using the Canada-France-Hawaii telescope on Mauna Kea, monitored three Cepheids and worked out the distance to galaxy NGC4571. This results in a value for H of 87 km per second per megaparsec (Michael J. Pierce et al., Nature, 29 Sept.). A second team, using the (appropriately named) Hubble Space Telescope (HST) and looking at 20 Cepheids in galaxy M100, found a value of 80 for H (results announced at a NASA news conference today). Converting a measurement of H into an estimate for the lifetime of the universe is as not as easy as it once was since astronomers began to suspect that large amounts of dark matter lurk in and around galaxies. The presence of this matter can distort the cosmological "flow" of galaxies and complicate any determination of the expansion rate of the universe. Still, these new H values suggest a young universe; the HST results provide a lifetime estimate of 8 to 12 billion years.