Size matters for stars. Smaller stars may not have quite enough mass to sustain the fusion that keeps those just above the limit--known as red dwarfs--burning bright for billions of years. This deficiency condemns them to persist for much shorter periods as so-called brown dwarfs. And larger stars that have reached the end of their hydrogen fusion fuel burn out into white dwarfs--dense, hot but cooling remnants. New observations with the Hubble Space Telescope have captured images of both the very faintest stars of this ilk--those nearest to the lowest mass limit--as well as white dwarfs so old they are turning blue.

Astronomer Harvey Richer of the University of British Columbia and his colleagues trained Hubble on a nearby globular cluster--NGC 6397--a roughly spherical grouping of hundreds of thousands of stars in the southern hemisphere constellation Ara. By snapping nearly 400 overlapping images of a region of that cluster over the course of more than 110 hours--nearly five days--the astronomers hoped to capture the very faintest stars in the bunch. "The light from these faint stars is so dim that it is equivalent to that produced by a birthday candle on the moon as seen from Earth," Richer says.

Nevertheless, aided by computers the resulting images revealed such dim objects, interspersed among the heaviest and lightest stars in the universe. As predicted by theory, the smallest red dwarfs were just 8.3 percent the size of our sun, or roughly 80 times bigger than Jupiter. Once at this threshold or larger, the tiny stars can fuse hydrogen into helium for periods longer than the age of the universe, estimated at 13.7 billion yearsThe unlucky stars smaller than that peter out after roughly a billion years. "We have discovered the lowest mass stars capable of supporting stable nuclear reactions in this cluster," Richer says. "Any less massive ones faded early in the cluster's history and by now are too faint to be observed."

The search also detected the remnants of stars weighing in at seven solar masses that quickly raced through their hydrogen fuel and burst before collapsing into dense white dwarfs. These stars cool slowly over eons, making the most ancient cold and dim. Some of those captured in the Hubble survey showed their age by turning blue as they cooled enough to allow hydrogen molecules to form in their atmospheres, rendering them opaque to infrared light. Armed with such precise measurements of age, astronomers can now more confidently date the cluster, our galaxy and even our universe. "Pinning down their age narrows down the age range of the universe," Richer notes. "We also will use the white dwarfs to determine the age of the cluster to an accuracy of a few hundred million years." The research appears in today's Science.