First stars not all so lonely: simulations suggest cosmic partnerships.

It's always nice to have a companion. And in the lonely, dark expanse of the early universe, even some of the first stars had soul mates, new simulations reveal.

Previous work had indicated that the first stars were extremely massive--at least 100 times as heavy as the sun--but were loners (SN: 6/8/2002, p. 362).

Now, more detailed modeling, including a careful consideration of how atomic and molecular hydrogen interact at low densities, reveals that at least 5 percent and perhaps as many as half of these heavyweights were gravitationally bound to similar-mass companions, says Tom Abel of Stanford University. He and his colleagues, Matthew Turk of Stanford and Brian O'Shea of Michigan State University in East Lansing, report their findings online July 9 in Science.

Pairs of massive stars are intriguing, notes Abel, because each star will probably collapse into a black hole. The eventual coalescence of the adjacent black holes would be a key source of gravitational waves, ripples in spacetime predicted by general relativity but never directly detected.

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A second star's presence could also add to its partner's spin, and the resulting more rapid rotation would enhance the production of gamma-ray bursts, flashes of high-energy light that have long-lasting afterglows and provide a window on the early universe.

Only one in five of the team's simulations, which model star formation about 200 million years after the Big Bang, produced pairs. And the team can only provide a rough estimate of the percentage of partnered stars.

"The simulations make good sense," says theorist Volker Bromm of the University of Texas at Austin. Bromm says that his own team's simulations track the evolution of pairs further, long enough to see the stars mature, and suggest that the fledgling stars remain close partners. His team plans to post a paper online describing the results.