First stars 'may have been dark'

By Roland Pease
BBC Radio Science Unit

The "dark" stars may have been large and diffuse

The first stars to appear in the Universe may have been powered by dark matter, according to US scientists.

Normal stars are powered by nuclear fusion reactions, where hydrogen atoms meld to form heavier helium.

But when the Universe was still young, there would have been abundant dark matter, made of particles called Wimps: Weakly Interacting Massive Particles.

These would have fused together and obliterated each other long before nuclear fusion had the chance to start.

As a result, the first stars would have looked quite different from the ones we see today, and they may have changed the course of the Universe's evolution - or at least held it up.

The theory, published in the journal Physical Review Letters, depends on particles that astronomers can't see, but are certain exist, and physicists have never detected. But the indirect evidence for their existence is overwhelming.

"Dark matter particles make up more than three-quarters of the mass of the Universe," says theoretical physicist Katherine Freese from the University of Michigan.

"In fact, billions of them are passing through each of us every second."

In the early Universe, there would have been even more.

Changing course

The nature of the first stars has long puzzled astronomers. Immediately after the Big Bang, the Universe expanded and cooled, so that for millions of years it was filled with dark, featureless hydrogen and helium - and perhaps Wimps.

Astronomers can see that there were normal stars 700 million years after the Big Bang - the Hubble Telescope looking to the edges of the Universe, which is like looking back billions of years in time, can see whole galaxies of them.

JWST could see back to the first stars

But how did the Universe change course?

The leading theory is that gravity pulled balls of dark matter and hydrogen together.

"These 'haloes', as we call them, are about a million times as massive as the Sun, and the first stars formed inside their centres," Professor Freese told the BBC.

It had been thought the hydrogen brought together by these dark matter haloes would collapse to make the first small stars, and would start to make inside themselves the first new elements - carbon, oxygen, silicon and other materials needed by planets and life.

But the new paper says reactions between the Wimps, colliding and annihilating each other, would have generated enough heat to keep the protostars inflated - like hot air balloons. And as more Wimps rained down on them the heating would have kept going.

These giant, diffuse stars could have filled the orbit of the Earth.

The details of what the stars would have looked like have yet to be worked out. But in five years' time, Nasa will be launching its James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope, and that might be able see right back to these "dark" stars.

There is also the intriguing possibility, says Professor Freese, that in some corner of our local Universe, there may be a few survivors lurking unnoticed.