The Hubble Space Telescope has provided strong evidence that white dwarfs, the burned-out relics of stars, are given a "kick" when they form. The idea that young white dwarfs are born with a kick was suggested decades ago to explain why there were so few of them in open star clusters.
Hubble's sharp vision uncovered 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 to become 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.
To explain this, some researchers suggested that white dwarfs propel themselves by ejecting mass, like rockets do. 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.
Globular clusters continue to sort out member stars according to their mass. Heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to its outskirts.
For more information, and to see where astronomers found white dwarfs in this Hubble image, visit:
hubblesite.org/contents/news-releases/2007/news-2007-42.html
Credit: NASA, ESA, and H. Richer (University of British Columbia)
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