Faintest Flickers of Star Cluster Found

Aug. 18, 2006 — Imagine looking up from Earth and straining to see a one-year-old’s birthday cake candle — on the moon. That dimmest of flickers is comparable to what NASA’s Hubble Space Telescope has spied in a cluster of stars 8,500 light years away.

The spherical collection of hundreds of thousands stars is what is known as a globular cluster and this newly-found bunch formed early on in the 13.7-billion-year-old universe.

Harvey Richer of the University of British Columbia, Vancouver, led the observations and claims the faint find, dubbed NGC 6397, is among the dimmest star discoveries in a globular cluster to date.

"We have run out of hydrogen-burning stars in this cluster," he said in a statement. "There are no fainter such stars waiting to be discovered."

Richer and his team announced the discovery at the 2006 International Astronomical Union General Assembly in Prague, Czech Republic and in the current issue of Science.

The team used Hubble’s Advanced Camera for Surveys to analyze two distinct star types in the cluster. One set are known as red dwarfs, which are very low-mass stars that burn hydrogen in their cores slowly and have extremely long lifetimes.

The other group is made up of white dwarfs, which are the burnt relics of stars nearing the end of their lives.

These stars are the ones that appear as faint as that far away candle on the moon and the recent Hubble observations showed for the first time that they undergo chemical changes in their atmosphere as they cool. These changes cause white dwarfs to take on a bluish, rather than reddish hue as they age.

Thanks to their old age, astronomers use white dwarfs as "clocks" to measure the universe’s age. The cooler the white dwarf, the older it is and the older the age of the universe. Richer and his team used the white dwarfs in NGC 6397 to gauge the globular cluster’s age to be nearly 12 billion years old.

Besides this dim cluster, astronomers have recently detected other pale glimmers in the solar system.

Rene Mendez of the Universidad de Chile in Santiago announced earlier this month that his team had found a brown dwarf star 100 million times dimmer than our sun.

The brown dwarf star was close by astronomical standards — "only" 16.2 light years away — which is why Mendez’ team could make out its dim glimmer. But brown dwarfs technically don’t rate as stars since they don’t undergo hydrogen fusion.

As Richer points out, the new Hubble discovery qualifies as the faintest finds among actual stars seen in a globular cluster.

"We have discovered the lowest-mass stars capable of supporting stable nuclear reactions in this cluster," he said in his statement. "Any less massive ones faded early in the cluster's history and by now are too faint to be observed."