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Science 18 August 2006:
Vol. 313. no. 5789, p. 888
DOI: 10.1126/science.313.5789.888a
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This Week in Science
Soon after the universe formed, it was filled with hydrogen atoms, yet today almost all the diffuse hydrogen between galaxies is ionized. Barkana (p. 931) reviews how and when the first stars and black holes lit up and ionized primordial hydrogen gas throughout the universe. Some understanding has come from computer simulations of the change that show the ionization is patchy and happens first in the densest regions of space. However, a full picture must await a new generation of radio telescopes that will map out this key epoch. Stars must exceed a certain size if they are to burn hydrogen through fusion, and Richer et al. (p. 936; see the news story by Bhattacharjee) have identified this fundamental mass limit in a deep census of globular cluster stars in our Milky Way taken with the Hubble Space Telescope. They also see a characteristic change in the color of white dwarfs in the cluster caused by the onset of molecular hydrogen formation in their atmospheres. Both effects had been predicted by theorists, and this experimental confirmation helps improve our understanding of the physics of low-mass stars and white dwarfs.
CREDIT: BARKANA ET AL. |
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Science. ISSN 0036-8075 (print), 1095-9203 (online)
