White dwarfs get a “kick” to hurl them to the edge of their cluster

December 5th, 2007 - 2:56 pm ICT by admin  

Washington, Dec 5 (ANI): NASA’s Hubble Space Telescope has provided evidence that white dwarfs, the burned out relics of stars, are propelled to the outer reaches of their star cluster at the time of their formation.
Evidence for this hypothesis emerged after Hubble’s Advanced Camera for Surveys uncovered the speedy 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. Because massive stars are thought 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.
According to the Space Telescope Science Institute in Baltimore, Maryland, the astronomers chose NGC 6397 because, at 8,500 light-years away, it is one of the closest globular star clusters to Earth.
The team studied 22 young white dwarfs less than 800 million years old and 62 older white dwarfs between 1.4 and 3.5 billion years old. The astronomers distinguished the younger from the older white dwarfs based on their color and brightness. The younger white dwarfs are hotter and therefore bluer and brighter than the older ones.
“The distribution of young white dwarfs is the exact opposite of what we expected,” said astronomer Harvey Richer of the University of British Columbia in Vancouver. “Our idea is that as aging stars evolve into white dwarfs, they are given a kick of 7,000 to 11,000 miles an hour (3 to 5 kilometers a second), which rockets them to the outer reaches of the cluster,” he added.
The new research suggests that white dwarfs propel themselves by ejecting mass, like rockets do. An explanation for this lies in the evolutionary process of a star from a Red Giant to a White Dwarf.
Before stars evolve into white dwarfs, they swell up and become red giants. Red giant stars lose about half their mass by shedding it into space. If more of this mass is ejected in one direction, it could propel the emerging white dwarf through space, just as exhaust from a rocket engine thrusts the rocket from the launch pad.
An earlier research by Michael Fellhauer of the University of California and colleagues in 2003 had calculated that if white dwarfs were given a small boost, they could be expelled from open clusters. This is because that it is easier for white dwarfs to escape the weak gravitational clutches of open clusters than to rocket out of globular clusters, which are as much as 100 times more massive than open clusters.
But after the latest theory regarding the white dwarfs, it was proposed that the formation of a white dwarf from a red giant gives it a “kick” to send it to the outer edge of its star cluster. (ANI)

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