Hubble finds young white dwarfs on the fast track

These images show young and old white dwarf stars – the burned-out relics of normal stars – in the ancient globular star cluster NGC 6397.

The image at left, taken by a ground-based telescope, shows the dense swarm of hundreds of thousands of stars that make up the globular cluster. The white box outlines the location of the observations made by the NASA/ESA Hubble Space Telescope.

The image at top right, taken by Hubble's Advanced Camera for Surveys, reveals young white dwarfs less than 800 million years old and older white dwarfs between 1.4 and 3.5 billion years old. The photo shows 12 of the 84 white dwarfs in the Hubble study. The blue squares pinpoint the young white dwarfs; the red circles outline the older white dwarfs. The Hubble researchers distinguished the younger from the older white dwarfs based on their colour and brightness. The younger white dwarfs are hotter and therefore bluer and brighter than the older ones.

The astronomers were surprised to find young white dwarfs far away from the cluster's core. They had assumed that the youngsters would reside at the centre and migrate over time to the cluster's outskirts. The astronomers proposed that the cluster stars that burn out as white dwarfs are given a boost that propels them to the edge of the cluster.

Close-up images of the white dwarfs are shown at bottom right. The blue boxes represent the young white dwarfs; the red boxes indicate the older white dwarfs.

The ground-based image was taken June 5, 2005. The Hubble images were taken in March and April 2005.

Credit for ground-based image: D. Verschatse (Antilhue Observatory, Chile)

Credit for Hubble images: NASA, ESA, and H. Richer (University of British Columbia)

Other members of the team are S. Davis (University of British Columbia); I.R. King (University of Washington, Seattle); J. Anderson (Space Telescope Science Institute, Baltimore, Md.); J. Coffey (University of British Columbia, Vancouver); G.G. Fahlman (NRC Herzberg Institute of Astrophysics, National Research Council Canada, Saanich, British Columbia); J. Hurley (Swinburne University of Technology, Hawthorn, Australia); and J.S. Kalirai (University of California at Santa Cruz).