New Cluster Data Puts Universe at 13 Billion Years

One of the outstanding issues in astrophysics is determining the age of the universe. Some measurements of the Hubble constant suggest an age as low as 8 billion years, while studies of the very old stars in globular clusters indicate an age of 13 billion years or more. At a Thursday morning session at the 1996 Joint APS/AAPT Meeting, Don Vandenberg of the University of Victoria reported on measurements of globular star clusters that support previous estimates of their age to be at least 13 billion years. Based on measurements of the distances to galaxies in the Virgo cluster and elsewhere, the new data has important implications for the ongoing debate over the large-distance scale of the universe.

An important adjunct to the debate is the notion that the universe cannot be older than its older stars, which appear to be those in globular clusters, spherical clumps of hundreds of thousands or millions of stars found near and around our galaxy. VandenBerg uses the Canada-France-Hawaii telescope to view the ancient, metal-poor stars in globular clusters, which largely lack the elements heavier than helium that many younger stars inherit from earlier supernova explosions. By plotting the stars' luminosities versus their colors, and by employing the standard model for stellar evolution, the age of the stars can be calculated.

Vandenberg said the oldest reliably dated stars, in globular cluster M92, were most likely 15 billion years old. Uncertainties in the determination of the distances to the clusters - effecting calculations of the stars' luminosities - might permit an age of 12 or even 13 billion years. But he asserted that the ages could not be much younger than that. New observations of his in globular cluster M13 did not alter this assessment.

The session also featured a talk by Wendy Freedman of the Carnegie Institute, who presented new Hubble Space Telescope results based on measurements of galaxies in the Fornax cluster, at a distance of 60 million light years. Freeman reported at a NASA press conference in early May that she and her colleagues were finding that values for the Hubble constant (H), a measure of the expansion of the universe, hovered in the range 68 to 78 km/sec/Mpc. (In 1994, they reported a preliminary value of 80.) A separate group led by Allan Sandage, also of Carnegie, recently reported a Hubble constant of 57.

Freedman's team is midway through a three-year program of measuring the distance to 20 distant galaxies by observing Cepheid variable stars, whose intrinsic brightness is related to the rate at which their luminosity varies. These observations in turn can be used to calibrate other means for determining distances to objects at even larger scales where local gravitational interactions have a lesser impact on a calculation of H. The secondary yardstick methods include the determination of the peak brightness of type-Ia supernovas and the use of the Tully-Fisher relation, according to which a galaxy's luminosity is related to its rotation rate. The latest entry in Freedman's inventory is galaxy NGC1365 in the Fornax cluster.