UBC astronomers discover how white dwarf stars get their 'kicks'

December 04, 2007

University of British Columbia astronomer Harvey Richer and UBC graduate student Saul Davis have discovered that white dwarf stars are born with a natal kick, explaining why these smoldering embers of Sun-like stars are found on the edge rather than at the centre of globular star clusters.

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.

Using NASA's Hubble telescope, Richer and his team looked at the position of white dwarfs in NGC 6397, one of the globular star clusters closest to Earth. Globular clusters are dense swarms of hundreds of thousands of stars. About 150 of these clusters exist in the Milky Way, each containing between 100,000 and one million stars.

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

Richer explains that globular clusters sort out stars according to their mass, governed by a gravitational billiard-ball game among stars. 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. The team found that the older white dwarfs were behaving as expected; they were scattered throughout the cluster according to weight.

"Newly-minted white dwarfs should be near the center, but they are not," says Richer. "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 when white dwarfs were born, their own mass acts like "rocket fuel" propelling them forward.

"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 it from the launch pad," says Richer.

The researchers studied 22 young white dwarfs up to about 800 million years old and 62 older white dwarfs between 1.4 and 3.5 billion years old. They distinguished the younger from the older white dwarfs based on their color and brightness. The younger ones are hotter, and therefore bluer and brighter than the older ones.
Study co-authors are: I. King, University of Washington; J.Anderson, Space Telescope Science Institute; J. Coffey, UBC, G. Fahlman, National Research Council of Canada's Herzberg Institute of Astrophysics; J Hurley. Swinburne, University of Technology; and J. Kalirai, University of California, Santa Cruz.

A high-resolution photo is available by request from Prof. Richer or Lorraine Chan. View thumbnail at: www.ubc.ca/whitedwarfs

An embargoed media release is also available at the Space Telescope Science Institute. Contact Donna Weaver at 410.338.4493/4514. dweaver@stsci.edu

University of British Columbia

Related White Dwarf Articles from Brightsurf:

Two planets around a red dwarf
The 'SAINT-EX' Observatory, led by scientists from the National Centre of Competence in Research NCCR PlanetS of the University of Bern and the University of Geneva, has detected two exoplanets orbiting the star TOI-1266.

A white dwarf's surprise planetary companion
For the first time, an intact, giant exoplanet has been discovered orbiting close to a white dwarf star.

KU astronomer helps confirm first-ever planet found orbiting white dwarf
A University of Kansas astronomer played a key role on the team that today announced the first-ever discovery of a planet orbiting a white dwarf.

HKU's Laboratory for Space Research member co-discovers first planet found around white dwarf star
An international team of astronomers led by the University of Wisconsin-Madison, including NASA co-authors, and Thomas G.

Experiments replicate high densities in 'white dwarf' stars
In a project conducted at the National Ignition Facility at Lawrence Livermore National Laboratory, a research team including University of Rochester engineering professor Gilbert (Rip) Collins, simulated the crushing pressure created as white dwarf stars cease to produce their own fuel, leaving only an extremely dense core.

Measuring the wind speed on a brown dwarf
Strong winds blow high in the atmosphere of the brown dwarf 2MASS J1047+21, according to a new study, which presents a simple method to deduce the windspeed in other brown dwarf atmospheres, too.

Astronomers measure wind speed on a brown dwarf
Using VLA and Spitzer observations, astronomers are able to determine wind speeds on a brown dwarf for the first time.

Observed: An occultation of a brown dwarf by another
An international team of astronomers in the project SPECULOOS, dedicated to the search for habitable planets, with scientists participating from the Instituto de Astrofísica de Canarias (IAC) has discovered an eclipse (termed an occultation) in a peculiar brown dwarf formed by two stars orbiting around each other.

Two stars merged to form massive white dwarf
A massive white dwarf star with a bizarre carbon-rich atmosphere could be two white dwarfs merged together according to an international team led by University of Warwick astronomers, and only narrowly avoided destruction.

Pulsar-white dwarf binary system confirms general relativistic frame-dragging
A century after it was first theorized, researchers have detected the effects of Lense-Thirring precession -- an effect of relativistic frame-dragging -- in the motion of a distant binary star system, a new study reports.

Read More: White Dwarf News and White Dwarf Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.