INFO 13-1996: HST observations determine the age of some of the oldest stars in our Galaxy
28 May 1996New observations with the Hubble Space Telescope have determined the age of the stars in the globular Cluster NGC 6752 with unprecedented accuracy. The age of the stars in globular clusters is significant because these stars are believed to have formed during the era of the formation of our galaxy, an event which probably occurred only 1 to 2 billion years after the birth of the Universe itself. An accurate age estimate for these cluster stars is thus regarded as an important means of gauging the age of the Universe.
Globular clusters like NGC 6752 are compact, spherically shaped collections of several hundred thousand stars which orbit our galaxy. Virtually all of the stars in these clusters began their lives at the time of the formation of our galaxy.
Astronomers therefore believe that globular clusters contain the oldest sample of stars that we can easily study and that an accurate determination of the ages of such stars would also define the age of our own galaxy since its formation. The age of our galaxy, in turn, sets a firm lower limit on the age of the Universe.
Obtaining accurate ages of stars in globular clusters has previously been hindered by the inability to determine accurate distances to these clusters. Using the Wide Field and Planetary Camera on the Hubble Space Telescope, a team of astronomers in Italy, the US and Canada obtained long exposure images of NGC 6752 in April 1995. These sharp images allowed them to detect and measure the brightness and color of a number of faint white dwarf stars in the cluster.
With dwarfs are the condensed cores of stars that have reached the final stage of their lives as stars. They represent the stellar remains of the more massive stars in NGC 6752 which have completed their evolution. By comparing the apparent brightness and color of these stars with a sample of white dwarfs in the neighborhood of the Sun which have known distances, the team determined the distance to NGC 6752 to be 13,300 light years with an uncertainty of less than 5%. This distance provides an accurate measure of the luminosity of the brightest stars in NGC 67523 that are still burning hydrogen at their center. In turn, the knowledge of this luminosity allows to estimate the time elapsed since the formation of the stars in the cluster, hence the cluster age. This leads rather directly to an age of 15 billion years with a 10% uncertainty.
Using current estimates of the time interval between the Big Bang and the era of galaxy formation gives 16-17 billion years for the age of the Universe. The age of NGC 6752 based on the Space Telescope data agrees well with previous estimates; what is new, however, is that the uncertainty in the age has been reduced to the point that the age of the cluster is in apparent conflict with recent estimates of the age of the Universe derived from its rate of expansion. Some of these estimates, also based on Space Telescope observations, give a much younger age of 9 - 14 billion years. It is not yet clear how this conflict over the age of the Universe may involve changes in the fundamental assumptions used to relate the expansion of the Universe to its age, unless inadequacies in the calculations used to estimate the ages of stars will emerge.
The Hubble Space Telescope observations of NGC 6752, which are to be published in the 1 July edition of the Astrophysical Journal, were conducted by an international team of astronomers lead by Dr. Alvio Renzini of the Universita di Bologna in Italy, presently at the European Southern Observatory (ESO) in Munich, Germany. Other European members include Drs. Angela Bragaglia and Francesco Ferraro of the Osservatorio Astronomico di Bologna, Italy, Dr. Roberto Gilmozzi also at ESO, and Dr. Sergio Ortolani of the Universita di Padova in Italy. US and Canadian members include, Drs. Jay Holberg and James Liebert of the University of Arizona, Dr. Ralph Bohlin of the Space Telescope Institute in Baltimore Maryland and Dr. François Wesernael of the Université de Montréal in Québec, Canada. The team is currently analyzing Space telescope observations of additional globular clusters to further improve estimates of cluster ages, and possibly determine the duration of the formation process of the galactic spheroid.