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'Groundbreaking' Discovery: First Direct Observation of Dark Matter
By Robert Roy Britt
Senior Science Writer
posted: 02:00 pm ET
22 March 2001
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Challenging ideas about star formation
The study found that
some of the white dwarfs had a very low temperature, indicating they were
very old, said Hansen, the Princeton researcher. That is consistent with
the belief that the halo contains the galaxy's first generations of stars,
he said, adding that the work may also help determine a more precise age
for our galaxy.
But because the newly found white dwarfs
represent stars that in their youth were between 0.8 and eight times as
massive as our Sun, the discovery "demonstrates that the earliest stellar
populations had a greater fraction [of stars] more massive than our Sun,"
Hansen said.
This flies in the face of existing
models of star formation, which predict that many more low-mass stars should
form, rather than high-mass ones, in a given galaxy.
"If it is right and there are far more
white dwarfs than expected, as also suggested by the MACHO project, then
it does raise some important questions, such as how come there are relatively
few low-mass stars," said Roger Blandford, professor of theoretical astrophysics
at the California Institute of Technology.
One possible explanation is that star
formation once followed different rules, an idea that is not new but which
has never had much direct evidence to lean on.
"It seems that there are far too many
of these white dwarfs to presume that the stars that the white dwarfs come
from were formed in the way that stars are formed today," Oppenheimer said.
"No one has ever come up with real evidence for the notion that star formation
could proceed somewhat differently at different places or times in the
universe. I think that this is one of the things we've shown."
Crowding out other candidates
Researchers have been nearly certain
since the MACHO results that white dwarfs, black holes or other hard-to-find
objects were roaming the halo of our Milky Way, a region thought to harbor
the ancient remnants of the galaxy's formation dating back 10 billion to
13 billion years.
David Bennett, a University of Notre
Dame professor who worked on the MACHO project, said the new research "appears
to solve the mystery of the MACHO Project's microlensing results" in favor
of the white dwarfs.
It had been considered possible, Bennett
said, that the MACHO observations were the result of red dwarf stars (which
would not be considered dark matter at all) in a nearby galaxy called the
Large
Magellanic Cloud.
Maverick black
holes remain a dark matter candidate, Bennett said, but their contribution
to the overall equation will likely be limited.
Next page: MACHOs 1, WIMPs 0
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