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Helium poses interstellar puzzle.

Scientists continue to analyze data from Astro, the X-ray and ultraviolet observatory that orbited Earth during a nine-day mission last December. One of the spectrographs on board, the Hopkins Ultraviolet Telescope (HUT), detected the absorption of extreme ultraviolet light by neutral helium and hydrogen gas in our galaxy. Researchers used those measurements to infer the ratio of neutral helium to neutral hydrogen in the region between Earth and the white-dwarf star named G191 B2B (SN: 1/5/91, p.10). Last week, the same team announced that its calculated ratio -- about 8.5 to 100 -- matches the cosmic abundance predicted by the Big Bang theory.

At first glance, that finding appears straightforward. But Arthur F. Davidsen, HUT's principal investigator, says the data may prove puzzling; his team had expected a slightly higher ratio. Because it takes more energy to strip an electron from neutral helium than from neutral hydrogen, it's likely that less helium gas would undergo this ionization, says Davidsen, an astronomer at Johns Hopkins University in Baltimore. The lower ionization rate should create a relative surplus of neutral helium compared with neutral hydrogen, he explains, resulting in a higher ratio of the two gases than the ratio believed created at the birth of the universe.

Davidsen suggests several explanations for the apparent discrepancy. Astro's preliminary results indicate that the region between us and the white dwarf may be too chilly to ionize much gas. However, he adds, several earlier studies suggest that although Earth may lie inside a relatively cold, neutral gas cloud, the region closer to the white dwarf contains warmer, less dense gas where significant ionization should occur.

Alternatively, he says, local conditions might somehow conspire to ionize helium and hydrogen gas at identical rates, resulting in a ratio that would remain unchanged from the Big Bang. Says Davidsen, "I don't know the answer to this one yet."
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Title Annotation:Astro space observatory data
Author:Cowen, Ron
Publication:Science News
Date:May 4, 1991
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