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Saturday, February 16, 2008

Were the First Stars Powered by Dark Matter?

When scientists look at what makes a star and what makes it burn, they turn to fusion. However, according to new research, this may not have been the case for the stars classified as “Population 3."

“The first stars were different in a lot of ways,” Katherine Freese, a theoretical physicist at the University of Michigan, told According to Freese, dark matter annihilation was the source of energy for the earliest stars, rather than fusion, when the universe was only 100 to 200 million years young.

“Annihilation means that matter goes into something else,” Freese explains. She says that everything has a partner opposite – matter and anti-matter, electrons and positrons. When these opposites meet, their identity is lost and the energy goes elsewhere. “Dark matter particles are their own anti. When they meet, one-third of the energy goes into neutrinos, which escape, one-third goes into photons and the last third goes into electrons and positrons.”

“In order for a star to form, in order for its matter to collapse into a dense object, it has to be able to cool off,” Freese continues. “We noticed that in the first stars something was competing with the cooling. The stars couldn’t collapse down small enough to get fusion going. But they were still giving off energy. They were in a phase we hadn’t discovered before.”

But how did they move from a dark matter phase and in to the more standard fusion stage that we are more familiar with? Freese explains; “The annihilation products getting stuck is what allows the dark matter heating to stay inside the star, and is what prevents the star from collapsing into a fusion driven one.” When all the dark matter has been annihilated the star then collapses enough so that fusion can take over.

What happens next is another “circle of life” thing that really makes you think. Hydrogen and helium atoms are forced together by the fusion process within the star and form new elements such as carbon, nitrogen, oxygen and various metals. Once it becomes dense enough with these new elements it collapses in on itself and goes supernova. Then, all the new elements made up within the old star spread out in to the universe to help in the creation of new stars.

“This new phase is only true in the first stars,” Freese insists. “The stars we see today are called population one stars. Earlier stars were population two stars. The first stars are referred to as population three stars. Our work is to modify how we believe population three stars developed. At first, they weren’t fusion driven.”

If the three are indeed right about this new theory, it will change what we know about how stars are formed. “It adds a new phase of stellar evolution,” Freese says.

Sadly though, according to Freese, any further study of this theory is going to have to wait until at least 2013, when NASA is scheduled to launch the James Webb Space Telescope. “We call them dark stars,” Freese explains, “but they would still shine, looking a little different. They would be cooler than a fusion driven star. We hope the next phase telescope will be able to tell between the standard stars now, and what we think happened in the first stars.”

Their study appears in Physical Review Letters with the title “Dark Matter and the First Stars: A New Phase of Stellar Evolution.”

Thanks to PhysOrg for the permission to work from their article, found at the link below.

Posted by Josh Hill.

Original here

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