Aug. 17, 2006

Erica Hupp 
Headquarters, Washington 
202-358-1237 

Donna Weaver 
Space Telescope Science Institute, Baltimore 
410-338-4493 

RELEASE: 06-292

HUBBLE SEES FAINTEST STARS IN A GLOBULAR CLUSTER

NASA's Hubble Space Telescope has uncovered what astronomers are 
reporting as the dimmest stars ever seen in any globular star 
cluster. Globular clusters are spherical concentrations of hundreds 
of thousands of stars. 

These clusters formed early in the 13.7-billion-year-old universe. The 
cluster NGC 6397 is one of the closest globular star clusters to 
Earth. Seeing the whole range of stars in this area will yield 
insights into the age, origin and evolution of the cluster. 

Although astronomers have conducted similar observations since Hubble 
was launched, a team led by Harvey Richer of the University of 
British Columbia, Vancouver, is reporting they have at last 
unequivocally reached the faintest stars. Richer's team announced 
their findings Thursday at the 2006 International Astronomical Union 
General Assembly in Prague, Czech Republic and in the August 18 
edition of Science.

"We have run out of hydrogen-burning stars in this cluster. 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," Richer said.

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. 

The light from these faint stars is as dim as the light produced by a 
birthday candle on the moon seen from Earth. NGC 6397 is 8,500 
light-years away from Earth. Analyzing the burned-out remnants of 
stars that died long ago, Hubble showed the dimmest white dwarfs have 
such low temperatures 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. 

These white dwarfs are the relics of stars up to eight times as 
massive as the sun, which 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.

Astronomers have used white dwarfs in globular clusters as a measure 
of the universe's age. The universe must be at least as old as the 
oldest stars. White dwarfs cool down at a predictable rate. The older 
the dwarf, the cooler it is, making it a perfect "clock" that has 
been ticking for almost as long as the universe has existed. Richer 
and his team are using the same age-dating technique to calculate the 
cluster's age. NGC 6397 is estimated to be nearly 12 billion years 
old.

A globular cluster's dimmest stars have eluded astronomers because 
their light is too feeble. Richer's team used Hubble's Advanced 
Camera to probe deep within the cluster for nearly five days to 
capture the faint stars. 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 
Galaxy, and Hubble was able to pinpoint which stars were moving with 
the cluster. The Hubble team 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. 

For images and additional information about NGC 6397, visit: 

http://www.nasa.gov/hubble

	
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