Unexpected Find Opposite for White Dwarfs
White dwarfs or burned-out relics of stars are formed with a “kick”. The speedy white dwarfs were uncovered in the ancient globular star cluster NGC6397, a dense swarm of hundreds of thousands of stars by the sharp vision of the Hubble’s Advanced Camera for Surveys. The stars are found in the most massive stars in NGC6397 before they burnt out as white dwarfs.
It was thought that most of the newly minted white dwarfs dwelled near the center because massive stars are thought to gather at a globular cluster’s core. But the Hubble Telescope found young white dwarfs located at the edge of NGC6397, which is about 11.5 billion years old. “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 (three to five kilometers a second), which rockets them to the outer reaches of the cluster.”
By ejecting mass, like rockets do, the white dwarfs propel themselves, Richer suggested. The stars swell and become red giants before evolving into white dwarfs. The red giants lose about half their mass by shredding it into space. The mass can propel the white dwarf through space if it is ejected in one direction. Thirty years ago, the idea was suggested that the reason so few of the dwarfs were found in open star clusters was because they were born with a kick. In 2003, Michael Fellhauer of the University of California at Santa Cruz and colleagues said that if white dwarfs were given a small boost, they could be expelled from open clusters. Richer and his team, therefore, decided to test the acceleration theory in a globular cluster.
The astronomers chose NGC 6397 because, at 8,500 light-years away, it is one of the closest globular star clusters to earth. About 150 globular clusters exist in the Milky Way, each containing up to a million stars. The team studied twenty-two young white dwarfs less than 800 million years old and 62 older white dwarfs between 1.4 and 3.5 years old. Based on their color and brightness, the astronomers determined that the young white dwarfs are hotter and, therefore, blue and the older ones were brighter. While lighter stars pick up speed and move across the cluster to its outskirts while heavier stars slow down and sink to the cluster’s core.
The older white dwarfs were located throughout the cluster according to weight. However, the researchers were puzzled when they found the young white dwarfs at the edge of the cluster. “The first time we plotted up the distribution and found a difference, we though, “My goodness, what is happening?” said team member Saul Davis, a graduate student at the University of British Columbia in Vancouver. “For a long time, we thought we had made a mistake. But no matter what we did, it didn’t go away.”
Other ideas emerged including the possibility that they got a boost after coming in contact with heavier stars or they could have been a part of binary systems and got kicked out. These explanations were ruled out by computer simulations.
This entry was posted on Sunday, January 20th, 2008 at 4:54 am and is filed under Public Relations, Space Agency News. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
