ANN ARBOR—The first stars to form in the early universe may have been “dark stars” fueled by an altogether different engine than the stars visible in the night sky now, according to a team of physicists that includes professor Katherine Freese of the University of Michigan.

Ordinary stars like the sun burn bright because they are fueled by nuclear fusion in their core that converts hydrogen to helium. But these theoretical dark stars would have run on dark matter particles colliding and annihilating each other.

Dark matter is a substance astronomers have not directly observed, but they deduce it exists because they detect its gravitational effects on visible matter. The prevailing theory is that the visible parts of the universe make up just 15 percent of its total matter.

Freese and her colleagues analyzed the young universe through the lens of the dark matter theory.

“We asked: Well, what about this enormous reservoir of dark matter? What does it do and what does it mean for star formation,” said Freese, a professor in the Department of Physics who studies particle astrophysics. She is an author of a paper on this research to be published in the January edition of Physical Review Letters.

The findings dramatically alter the current theoretical framework for the formation of the first stars, the paper says.

“This is a new phase of stellar evolution, like when they first found that fusion was important in the sun,” Freese said. “It’s a completely new kind of star. It’s what they looked like billions of years ago.”

The universe is estimated to be 15 billion years old, with the first stars having formed when the cosmos was just 150 million years old.

The first stars are thought to have formed inside clouds of dark matter, when hydrogen and helium gases cooled to a temperature at which nuclear fusion could begin. Conventional theory says dark matter didn’t affect this process except to provide the gravity to bring the gases together.

Freese and her colleagues believe otherwise. They say the dark matter concentrations were high enough for the particles in the dark matter clouds to collide with each other, destroying themselves and, more importantly, keeping the burgeoning star too hot to collapse to a high enough density for fusion to begin.

The dark matter particles on which this new framework is based are called “weakly interacting massive particles,” or WIMPs. They are a leading candidate for the composition of dark matter.

Freese and her colleagues will now shift toward detecting these dark stars, or their footprints, which would include the products from the dark matter particles annihilating each other. Gamma-rays, particles called neutrinos or antimatter could be tell-tale signs.

“The next thing we’re going to do is figure out what these things look like, what kind of light they give off, what is their temperature. We have to make predictions for what we’ll see when we detect them,” Freese said. “They will shine, but they will look different than stars that operate by fusion.”

These stars might have eventually become regular stars, which would have burned out by now, or they could be “very different beasts.” Freese believes the James Webb Space Telescope scheduled to be launched in 2013 will be able to detect them.

Early stars were made only of hydrogen and helium, the simplest elements. When they finished their lifetimes and became supernovas, they left behind heavier elements such as carbon, nitrogen and oxygen that became part of the next generation of stars and eventually, the earth.

The paper is called, “Dark matter and the first stars: A new phase of stellar evolution.” Freese’s collaborators are: Paolo Gondolo, associate professor at the University of Utah and Douglas Spolyar, a doctoral student at the University of California, Santa Cruz.

Freese is also associate director of the Michigan Center for Theoretical Physics.

Related Links:

• Michigan Center for Theoretical Physics