Heavy hydrogen clumps in space
Monday, 21 August 2006
A six-year study of a primordial element created just after the birth of the universe may force scientists to revise theories about how stars and galaxies formed.
Using a telescope sensitive to far ultraviolet light, astronomers have discovered that there is much more deuterium, a heavy form of hydrogen created minutes after the Big Bang, than expected.
"Our models of the chemical evolution of the Milky Way will have to be revised significantly," says astrophysicist Professor Jeffrey Linsky with the University of Colorado at Boulder.
All the deuterium that ever existed was believed to have been forged within a few minutes of the Big Bang 13.7 billion years ago.
Deuterium is a heavy version of regular hydrogen, the lightest and most abundant element. But its nucleus harbours a neutron in addition to the normal proton, so it is twice hydrogen's mass.
Because stars consume deuterium and there is no known process to create more, the isotope serves as a tracer for star formation and galaxy creation throughout time.
Scientists thought they knew how much deuterium there was, figuring about one-third of the original deuterium was remaining.
Not so, according to scientists using data from the Far Ultraviolet Spectroscopic Explorer, or FUSE.
Launched in 1999, the FUSE telescope breaks light into component wavelengths so scientists can determine which chemicals and other phenomena the beams might have passed through before striking the observatory's instruments.
The team provides the first hard evidence that deuterium can attach to interstellar dust grains, as well as exist as a free-floating gas.
The finding explains why deuterium is not spread evenly throughout the galaxy, but instead appears in clumps.
Solving one puzzle, however, laid the groundwork for a far thornier question. In adding up the amount of deuterium, whether free-flying or bound to dust, there was far more than expected.
"Since the 1970s, we have been unable to explain why deuterium levels vary all over the place," Linsky says. "The answer we found is as unsettling as it is exciting."
FUSE found that the present-day deuterium abundance is less than 15% below the primordial values, rather than about 33%.
"This implies that either significantly less material has been converted to helium and heavier elements in stars or that much more primordial gas has rained down onto the galaxy over its lifetime than had been thought," says Linsky.
Associate Professor Brian Fields, with the University of Illinois in Urbana says:
"FUSE has solved the mystery about why the deuterium is where it is. Now scientists need to try to explain why there is so much of it."
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