This form of hydrogen, called deuterium, is comprised of a proton and a neutron and was created a few minutes after the Big Bang. Deuterium is also present in the local universe and serves as a tracer for star creation and galaxy building throughout the eons.

Stars consume deuterium, and no mechanism creates it in significant amounts, so the amount of deuterium present today was expected to be much lower than the amount initially produced. Deuterium emits a telltale spectral fingerprint in the FUSE ultraviolet energy range.

The FUSE deuterium survey, six years in the making, solves a 35-year mystery concerning the uneven distribution of deuterium in the Milky Way galaxy. Yet the solution poses new questions about how stars and galaxies are made. A team led by Jeffrey Linsky of JILA, a joint institute of the University of Colorado, Boulder, and the National Institute of Standards and Technology, Gaithersburg, Md., discusses this result in the August 20 issue of The Astrophysical Journal.

"Since the 1970s we have been unable to explain why deuterium levels vary all over the place," Linsky said. "The answer we have found is as unsettling as it is exciting."

In the 1970s, NASA's Copernicus satellite found the galactic deuterium distribution to be patchy. Early FUSE observations confirmed this, a perplexing result because deuterium should be evenly mixed and as readily available as other elements for the creation of new stars.

In 2003, Bruce Draine of Princeton University, a co-author on the new result, developed computer models that showed how deuterium, compared to hydrogen, might preferentially bind to interstellar dust grains, changing from an easily detectable gaseous form to an unobservable solid form. The new FUSE data strongly support this theory.

In regions that remain undisturbed for long periods, deuterium atoms systematically leave the gas phase and replace normal hydrogen atoms in dust grains. FUSE cannot detect this non-gaseous form, which explains the low detection level of 15 parts per million hydrogen atoms within a hundred light years of the sun and values as low as 5 parts per million elsewhere.

When a region is disturbed by a supernova or hot stars, dust grains are vaporized, releasing deuterium atoms back into a gas phase. FUSE detects high deuterium levels in such regions.

Primordial deuterium concentrations are about 27 parts per million. FUSE finds concentrations as high as 23 parts per million in our galaxy. Scientists had assumed, based on theory that at least a third of the deuterium would have been destroyed over time, leaving less than the amount seen in the Milky Way. But only about 15 percent of the deuterium has been destroyed, FUSE has found, suggesting that less than the amount expected has cycled through stars.

"The peak galactic detection levels are likely close to the real total deuterium abundance in the Milky Way, with the rest of it in hiding, not destroyed," said Warren Moos of Johns Hopkins University, Baltimore, FUSE principal investigator and co-author.

If the peak levels are 23 parts per million, 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 our galaxy over its lifetime than had been thought.

In either case, our models of the chemical evolution of the Milky Way galaxy will have to be revised significantly to explain this new result, the team said. FUSE is a sensitive probe of how much gas has entered into galaxies and has been processed in stars.

"FUSE has solved the mystery about why the deuterium is where it is, but now scientists need to try to explain why there is so much of it," said Brian Fields of the University of Illinois, Urbana, Ill.

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