The Fading Firmament

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Science  29 Aug 2003:
Vol. 301, Issue 5637, pp. 1159
DOI: 10.1126/science.301.5637.1159b

You may not have noticed it, especially if you live in an urban area, but the skies are getting darker. Using spectra from the Sloan Digital Sky Survey and an agile data compression algorithm called MOPED (Multiple Optimized Parameter Estimation and Data compression), Panter et al. have determined that the rate of star formation has been decreasing for the past 6 billion years. About 30% of the stars formed more than 8 billion years ago, and a similar percentage of stars are observed in elliptical galaxies, which is consistent with the theory that most elliptical galaxies formed early and contain the oldest stars. In addition, the computed average metallicity (abundance of elements heavier than He) of the gas in which the stars formed started increasing about 8 billion years ago, peaked at about the solar value, and then decreased to about half the solar value about 2 billion years ago. The rise and fall of metallicity favors star formation models in which infall of relatively unprocessed gases onto galaxies was prevalent in the universe from 4 to 0.1 billion years ago. — LR

Mon. Not. R. Astron. Soc. 343, 1145 (2003).

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