Weekly Science Update

Wednesday, October 24, 2007
The Second Stars
The first stars in the universe are thought to have begun forming about one billion years after the big bang, as the neutral hydrogen atoms in the expanding universe gathered together into filaments and clumps under the influence of gravity. These first stars were probably monsters, each a hundred times more massive than our sun, the result of their forming in a comparatively warm, dense environment. They lived only briefly, dying in supernova explosions that seeded the early universe with the first of the heavy elements essential to life - elements that were not produced in the beginning.

CfA astronomer Lars Hernquist and two of his colleagues wondered what came next. Their detailed computer simulations of the first generation of stars showed that these monsters were able to ionize much of the neutral hydrogen gas in their vicinity. This gas, as it cooled off, apparently recollapsed into the next generation of stars in another one hundred million years. In a new paper published this month, the astronomers present the results of computations that describe these second generation stars.

It turns out that the gas forming the second stars is much cooler than the gas producing the first generation. As a result, the second stars can begin their nuclear reactions when they are only about forty times as massive as the sun -- much less massive than their predecessors. Their smaller mass is significant because it means that as a class they can more efficiently produce heavy elements. The team also notes that such medium-sized stars are thought to explode as black-hole forming supernovae, perhaps in gamma-ray emitting bursts that might be detected with today's instruments. The new results are an important step in deciphering how stars and galaxies gradually came to light up our universe, and how the elements of life were produced.


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