Closing in on the Hubble constant.

In 1929, astronomer Edwin P. Hubble deduced that the universe is expanding. Ever since, astronomers have struggled to determine the exact rate of that expansion, now known as the Hubble constant. Given the separation between two bodies, the Hubble constant dictates that they will move away from each other at a rate proportional to their distance apart.

Researchers have now taken a key step toward pinning down the value of this much debated number, which plays a fundamental role in determining the age and size of the universe.

The new value, incorporating data from the Hubble Space Telescope, indicates that the universe is at least 15 billion years old - 3 billion to 5 billion years older than recent measurements of the constant suggested. While the numerical value of the constant remains a question, astronomers agree the finding represents important progress in determining how rapidly the universe has evolved since its explosive birth.

A team that includes Allan R. Sandage of the Observatories of the Carnegie Institution of Washington in Pasadena, Calif., and Abhijit Saha at the Space Telescope Science Institute in Baltimore described the new calculations this week at a workshop in Sardinia, Italy.

Measuring the Hubble constant requires two ingredients: the velocity with which one galaxy recedes from another and the distance between them. Astronomers can determine a galaxy's recession velocity relatively easily. Calculating distance has proved a challenge.

For years, astronomers have inferred the distance between certain galaxies by using a cosmic yardstick--the luminosity of a group of stars, called cepheid variables, located in those galaxies. By analyzing the rhythmic pulsation of cepheids, scientists can deduce their intrinsic brightness - how bright they would appear to an observer standing right in front of them. Knowing that the luminosity of any star appears to fall off in proportion to the square of its distance from Earth, researchers can in theory calculate the separation between any galaxy containing cepheids and our own.

Since telescopes can only resolve individual stars in relatively nearby galaxies, astronomers can't use cepheids to obtain direct measures of the distance of faraway bodies, which would offer a truer value for the expansion rate of the universe. But Sandage and his co-workers circumvented that limitation by using the Hubble telescope to measure the luminosity of 27 cepheids in a small galaxy known as IC 4182. That galaxy contains another, more luminous cosmic yardstick -- a special type of supernova called 1A -- that could help calculate the Hubble constant in galaxies many more millions of light-years away, Saha notes.

Astronomers believe that all type 1A supernovas attain the same peak intrinsic brightness, but no one knows the magnitude of that brightness. For this reason, reseachers have used the supernova yardstick only sparingly. But by using cepheids to calculate the distance to IC 4182, a galaxy in which researchers observed a 1A supernova in 1937, scientists can now determine the actual peak brightness of these stellar explosions, Saha says. Thus, the team has used one cosmic yardstick to calibrate another.

"This is an observation that has been needed for some 15 to 20 years," says George Jacoby of the Kitt Peak National Observatory in Tucson, Ariz. But he and others counsel caution about the new results. While the Sandage team calculates that IC 4182 lies about 16 million light-years from Earth, a team led by Kitt Peak's Michael J. Pierce, using a less reliable stellar standard, has come up with a far smaller distance - implying a smaller, younger universe. Pierce says his most recent calculations show that IC 4182 contains lots of dust, which could make the cepheids appear fainter and thus confound distance estimates for the galaxy. He adds that the space telescope's blurry optics might also have hampered efforts to measure the luminosity of the stars. Follow-up observations on the ground and with a repaired space telescope may resolve the discrepancies, he says.