April 9, 1998
Based on Hubble's detection of a rare, young white dwarf star, astronomers conclude that its progenitor was a whopping 7.6 times the mass of our Sun. Previously, astronomers had estimated that stars anywhere from 6 to 10 solar masses would not just quietly fade away as white dwarfs, but abruptly self-destruct in torrential explosions.
This new lower limit will help astronomers refine theories of how galaxies developed in the early universe, determine the rate at which supernovae enrich interstellar space with heavy elements for building new generations of stars and planets, and estimate the number of neutron stars in space (neutron stars are the crushed stellar cores resulting from supernovae).
Rebecca Elson and Steinn Sigurdsson of Cambridge University, and co-investigators, discovered the ultra-hot white dwarf during a search in archival Hubble Wide Field Planetary Camera 2 pictures of the young star cluster NGC 1818, located 164,000 light-years away in the Large Magellanic Cloud -- a satellite galaxy of our Milky Way.
The trick was to identify a newly formed white dwarf that was still exceptionally hot and bright immediately after the burnout and collapse of its progenitor star. Such a dwarf would be so "young" - relative to older fainter dwarfs in the cluster -- it would allow a direct link back to the most massive stars now present in the cluster. That's because the most massive stars are the shortest lived, and so are first to burnout as white dwarfs.
Because the star cluster NGC 1818 is ten times larger than those found closer to us within our own galaxy, chances were far better for catching the young dwarf before it swiftly dimmed on its way to the "graveyard" of faint dwarfs. Also, the cluster is only about 40 million years old, and so still contains massive stars.
Hubble is ideally suited for hunting for white dwarfs so far away because its exquisite resolution can pinpoint them among the cluster's crowded stellar population, and can easily detect the blue light from the sizzling 50,000 degree Fahrenheit surface temperature of the young dwarf.
Once the candidate star was identified, a spectrum of the star obtained at the Anglo-Australian Observatory. "Our spectrum indicates that it is neither a foreground nor background object, and detailed modeling is underway to understand its exact evolutionary state," says Elson.
The results will be reported in the Astrophysical Journal Letters.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
Images to accompany this release are available electronically via the World Wide Web at:
PHOTO NO.: STScI-PRC98-16:
The circled star is a young white dwarf star, which has only very recently formed following the burnout of a red giant. Based on this observation astronomers conclude that the red giant progenitor star was 7.6 times the mass of our Sun. Previously, astronomers have estimated that stars anywhere from 6 to 10 solar masses would not just quietly fade away as white dwarfs but abruptly self-destruct in torrential explosions.
Hubble can easily resolve the star in the crowded cluster, and detect its intense blue-white glow from a sizzling surface temperature of 50,000 degrees Fahrenheit.
Date taken: December 1995
Wavelength: natural color reconstruction from three filters (I,B,U)
Field of view: 100 light-years, 2.2 arc minutes
Name: NGC 1818
Distance: 164,000 light-years
Age: 40 million years
Class: Rich star cluster
Apparent magnitude: 9.7
Apparent diameter: 7 arc minutes
Credit: Rebecca Elson and Richard Sword, Cambridge UK, and NASA (Original WFPC2 image courtesy J. Westphal, Caltech)