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Astronomers see faintest stars in a globular cluster

August 21, 2006

Astronomers report in the Aug. 18 issue of the journal Science seeing the faintest stars ever seen in any globular star cluster. The light from these dim stars is only as bright as the light produced by a birthday candle on the moon, as seen from Earth. The astronomers used NASA's Hubble Space Telescope.

"These are the faintest stars that we are likely to see and the faintest stars we think are actually there," said report co-author Brad Hansen, a UCLA associate professor of physics and astronomy and a member of UCLA's Institute of Geophysics and Planetary Physics. "There aren't any significant populations of stars that remain to be seen in this cluster."

Globular clusters are spherical concentrations of hundreds of thousands of stars that formed early in the universe. Hansen calculates that this star cluster, known as NGC 6397, is approximately 12 billion years old; over the next several months, Hansen will calculate the margin of error on that figure. Edward L. Wright, a UCLA professor of physics and astronomy, is among a team of astronomers who have estimated that the Big Bang occurred 13.7 billion years ago.

The star cluster is one of the oldest objects in the Milky Way Galaxy, Hansen said. NGC 6397 is some 8,500 light-years from Earth, which astronomers consider quite close. A light-year is approximately 6 trillion miles. Seeing the whole range of stars in NGC 6397 will yield insights into the origin and evolution of the cluster, as well as its age, and eventually will provide clues to galaxy formation and star formation in the early universe.

"We have run out of hydrogen-burning stars in this cluster," said Harvey Richer of the University of British Columbia Department of Physics and Astronomy, lead author of the Science paper. "There are no fainter such stars waiting to be discovered. We have discovered the lowest-mass stars capable of supporting stable nuclear reactions in this cluster. Any less massive ones faded early in the cluster's history and by now are too faint to be observed."

"This was a very long exposure," Hansen said, "so we could see fainter objects than had ever been seen before. It's crowded in a globular cluster; there are stars of all kinds and all brightnesses."

Hubble's Advanced Camera for Surveys completed a census of two distinct stellar populations in NGC 6397. Hubble surveyed the faintest red dwarf stars, which fuse hydrogen in their cores, like our sun, and the dimmest white dwarfs, which are the burned-out relics of normal stars.

Analyzing the burned-out remnants of stars that died long ago, Hubble showed that the dimmest white dwarfs have such low temperatures that they are undergoing a chemical change in their atmospheres that makes them appear bluer rather than redder as they cool. This phenomenon had been predicted but never observed before. These white dwarfs are the relics of the cluster stars, up to eight times as massive as the sun, that have exhausted the fuel capable of supporting nuclear reactions in their cores. Stars that were initially even more massive died as supernovae very early in the cluster's life, leaving behind neutron stars, black holes or no debris at all.

The lowest-mass cluster stars seen by Hubble - red dwarfs - are 8 percent as massive as the sun and match theoretical predictions. Less massive stars are called brown dwarfs, objects so low in mass that they never reach the temperature necessary to sustain nuclear fusion, the power source in normal stars. If ancient, cool brown dwarfs are present in the surveyed area, they are so faint that they are beyond Hubble's viewing limit.

Astronomers have used white dwarfs in globular clusters as a measure of the universe's age. White dwarfs cool down at a predictable rate; the older the white dwarf, the cooler it is, allowing white dwarfs to serve as a "clock" that has been ticking almost since the dawn of the universe.

Globular clusters' dimmest stars have eluded astronomers because their light is so faint. Richer's team used Hubble's Advanced Camera to probe deep within the cluster for nearly five days to capture the faint stars; Richer conducted some of this research during a year at UCLA.

The camera's resolution is so sharp that it is capable of isolating cluster stars in this crowded cluster field, enabling cluster members to be distinguished from foreground and background stars. The cluster stars move together as the cluster orbits the Milky Way, and Hubble was able to pinpoint which stars were moving with the cluster. The astronomers used this technique together with archival Hubble images taken as much as a decade earlier to make sure they had a pure sample of cluster stars.

University of California-Los Angeles