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How the first stars
came in pairs

Posted: JULY 10, 2009

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We know that the majority of the stars that exist in the Galaxy today exist in multiple systems of two, three, four or even five stars. New and detailed computer simulations that describe how the very first stars in the Universe formed show that binary star systems were also common occurrences just a few hundred million years after the big bang too.

Image: X-ray (NASA/CXC/CfA/E.O'Sullivan); Optical (Canada-France-Hawaii-Telescope/Coelum)

The simulations have been run by Matthew Turk and Tom Abel of the Kavli Institute for Particle Astrophysics and Cosmology, and Brian O’Shea of Michigan State University, and the results published in the 9 July edition of the journal Science Express. The trio ran five representations depicting the growth of the first stars (termed Population III stars), which incorporated data on the distribution of both ordinary matter and dark matter in the Universe shortly after the big bang, based on observations of the cosmic microwave background radiation. Each simulation covered a virtual space of 650 quadrillion (650,000,000,000,000,000) cubic kilometres and ran for three weeks on 64 processors. The results were a tad surprising.

“We used to think that these stars formed by themselves, but now we see from our computer simulations that sometimes they have siblings,” says Turk. In four of the five simulations, gravity pulled matter in to form one giant star, but in one of the simulations two stars formed, with masses of ten times and 6.3 times the mass of our Sun. The simulations ended while the stars were still growing; the first Population III stars are speculated to have been true behemoths reaching one hundred times the mass of the Sun or more.

Consequently, the simulation points to the existence of two stars in a binary system that could both explode as supernovae, or as even more violent gamma-ray bursts, potentially creating two black holes, or a black hole and a neutron star. No one is exactly certain when the Population III stars formed – they hark back to an era during the first few hundred million years called the dark ages when the unique was flooded with neutral hydrogen gas and was opaque to light – so therefore no one has ever seen such a star. However, the discovery of a binary system in the simulations raises hope that they could be detected by other means.

“These stars could evolve into two black holes, which could have created gravitational waves that we could detect,” says Tom Abel. “Or one of the stars could evolve into a black hole that could create gamma-ray bursts that we could detect with the Swift mission and the Fermi gamma-ray space telescope.”

Indeed, astronomers have only recently identified several gamma-ray bursts originating just 800–900 million years after the big bang (read our news stories Dark GRBs illuminate early star formation and Swift sees most distant gamma-ray burst for more information).

These are still too young to be true Population III stars (which are made purely from hydrogen and helium – it is these first stars that created all the other elements, and they lasted only a few million years) but it may only be a matter of time before we detect gamma-ray bursts from even earlier – the death throes of the very first stars in the Universe.