Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

February 24, 1998
See Explanation.  Clicking on the picture will download 
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The Lyman Alpha Forest
Credit & Copyright: J. Shalf, Y. Zhang (UIUC) et al., GCCC

Explanation: We live in a forest. Strewn throughout the universe are "trees" of hydrogen gas that absorb light from distant objects. These gas clouds leave numerous absorption lines in a distant quasar's spectra, together called the Lyman-alpha forest. Distant quasars appear to be absorbed by many more Lyman-alpha clouds than nearby quasars, indicating a Lyman-alpha thicket early in our universe. The above image depicts one possible computer realization of how Lyman-alpha clouds were distributed at a redshift of 3. Each side of the box measures 30 million light-years across. Much remains unknown about the Lyman-alpha forest, including the real geometry and extent of the clouds, and why there are so many fewer clouds today.

Tomorrow's picture: The Solar Neighborhood

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