Hubble Finds Evidence That White Dwarfs Once Rocketed Through Space

NASA has announced evidence of an unexpected beginning to white dwarfs. The Hubble Space Telescope found the white dwarfs in a globular cluster of hundreds of thousands of stars, but not where they expected.

In a previous life they had been the most massive stars in globular cluster NGC 6397. They were also the most ancient and among the first to burn out, and being the largest it was assumed their relics would be found near the center of the cluster. Instead, the white dwarfs are on the edge, in a much younger part of space.

Astronomer Harvey Richer, of the University of British Columbia in Vancouver, said the young white dwarfs are exactly opposite of what was expected. Based on the new evidence the astronomers have revised their ideas

"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."

Richer suggested that white dwarfs travelled there after losing half of their mass as red giants, a stage in which the star initially swells enormously. The stars then begin to eject mass and become smaller and colder. With enough mass ejected in the same direction the star will move through space, the same way rocket exhaust moves a rocket. The glowing material ejected by red giant stars becomes planetary nebulae, some of which have been seen to have a similar pattern of outward flow.

The theory that white dwarfs begin with propulsion was first proposed 30 years ago. At the time scientists wanted to explain why so few of them were in open, or less dense and massive, star clusters. In contrast, the white dwarfs in the globular cluster NGC 6397 were on the verge of escaping, but couldn't defeat the gravity of the larger group.

While testing this theory, Richer found that most white dwarfs were distributed according to weight, as stars usually are within a cluster. The heavier, more gravity effected stars in a group sink towards the middle, with the lighter stars being less affected. However, they found that even young white dwarfs (identified because they bluer, brighter, and hotter) were very near the edge of the cluster. After ruling out alternative explanations using computer models the team concluded that the stars must be getting a kickstart out of the globular cluster as they are reborn into white dwarfs.

HubbleSite.org, How White Dwarfs Get Their 'Kicks'