White dwarf stars propelled like rockets

Hubble Space Telescope images of white dwarfs in NGC 6397. Young white dwarfs were found far away from the center of the globular cluster, where they were expected to be found. (NASA Photo)

BEIJING, Dec. 5 (Xinhuanet) -- Hubble images suggest that white dwarfs, the burned out relics of stars, are born with a natal "kick" that propels them to travel at a speed of thousands of miles per hour, media reported Wednesday.

Using Hubble's Advanced Camera for Surveys, astronomers from University of British Columbia look at the distribution of white dwarfs in the ancient globular star cluster NGC 6397, a dense swarm of hundreds of thousands of stars.

Before the stars burned out as white dwarfs, they were among the most massive stars in NGC 6397. Since massive stars tend to gather at a globular cluster's core, astronomers assumed that most newly minted white dwarfs dwelled near the center.

Hubble, however, discovered young white dwarfs residing at the edge of NGC 6397, which is about 11.5 billion years old.

"The distribution of young white dwarfs is the exact opposite of what we expected," said Profesor Harvey Richer, whose study will appear in the Monthly Notices of the Royal Astronomical Society Letters in January 2008.

"Our idea is that when these white dwarfs were born, they were given a small kick of 7,000 to 11,000 miles an hour (three to five kilometers a second), which rocketed them to the outer reaches of the cluster."

Using computer simulations, Richer and his team showed that white dwarfs propel themselves by ejecting mass, just as rockets do. If more of this mass is ejected in one direction, it could propel the emerging white dwarf through space. If these jets are not perfectly balanced, the stronger jet could accelerate the white dwarf in the opposite direction.

White dwarfs represent the third major stage of a star's evolution. Like the Sun, each star begins its life with a long stable state where nuclear reactions take place in the core supplying the energy. After the core fuel is depleted, it swells up and turns into a huge red giant. Later, the red giant ejects its outer atmosphere and its core becomes a white dwarf that slowly cools over time and radiates its stored thermal heat into space.

(Agencies)