Science: Hubble’s Views of Aging Stars Reveal Secrets of the Universe

Astronomers have identified two fundamental transitions in the physics of stars related to age that may help further refine the Milky Way’s age. The results appear in the 18 August 2006 issue of the journal Science.

Using the Hubble Space Telescope, Harvey B. Richer and colleagues peered into the faintest stars in the globular star cluster NGC 6397 with Hubble’s advanced camera for five days to capture high-resolution images of the faint stars.

“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,” says Richer of the University of British Columbia in Vancouver and lead author of the Science article.

“These are the deepest images yet taken of a globular star cluster,” said Science Associate Editor Joanne Baker. “They present a double whammy that will appear in future textbooks.

“By tracing the tiniest and faintest members of the population of stars in the cluster, the study reports the threshold mass at which a star becomes large enough to burn hydrogen by fusion and, second, the onset of molecular hydrogen being formed in the cooling atmospheres of dying white dwarfs.”

Both effects confirm earlier theoretical predictions.

The research revealed the mass that determines which stars will burn hydrogen through fusion and live many billions of years and which ones will never grow large enough to be self-sustaining. Stars that are not self-sustaining only live about 1 billion years at most.

Richer and colleagues also detected a characteristic change in the color of white dwarfs in the cluster that is related to the onset of molecular hydrogen being formed in the cooling atmospheres as the white dwarfs die.

With this information in hand, astronomers can learn more about the physics of low-mass stars and white dwarfs and perhaps improve the estimate of the ages of these stars and the universe.

“These stars, which died long ago, were among the first to have formed in the universe,” said Richer. “Pinning down their age narrows down the age range of the universe.

“We also will use the white dwarfs to determine the age of the cluster to an accuracy of a few hundred million years. This will eventually confront models of star formation in the early universe.”

Richer will present the team’s findings at a AAAS/Science press conference during the General Assembly of the International Astronomical Union in Prague, Czech Republic, on Thursday 17 August 2006.

Natasha Pinol

17 August 2006