Light from the very first stars in the universe has been measured – and there is less of it than previously thought. The discovery should help us better understand how the hot haze of hydrogen that existed shortly after the big bang transformed into the complex web of stars and galaxies we see today.

Although the first stars are too distant to be glimpsed directly, their light contributes to a diffuse fog of photons known as the extragalactic background light, which permeates the universe. The intensity of this light through cosmic time holds a record of all the stars that have ever existed.

But directly observing the background light has proved challenging, too, because of the local glow of our own solar system and galaxy. Instead, distant galaxies called blazars that shine like headlights through the fog have been the best targets for observation.

Gamma-ray dimming

Blazars have central supermassive black holes that are feeding so violently they emit especially energetic gamma rays. They can travel billions of light years to our telescopes, but along the way, they occasionally collide with photons from the extragalactic background light and convert into an electron-positron pair.


A beacon in the photon fog (Image: NASA/JPL-Caltech)

So, the further away a blazar is, the more its high-energy gamma-ray output should appear dimmed by the extragalactic fog it has to pass through en route to us, says Anita Reimer of the University of Innsbruck in Austria. Because the team had separate data on how bright the blazars should be and how far away they are, they could work out how much the blazar’s gamma rays had been dimmed by the photon fog, and thus work out the intensity of the fog.

Subtract out the known intensity of younger stars and galaxies, and the part of the fog that’s left must be light from the first generation of stars.

Reimer and colleagues studied 150 blazars from more than 1000 spotted by the Fermi Gamma-ray Space Telescope. The team chose targets between 0.4 billion and 9.5 billion light years distant.

Sure enough, the farther away the blazars were, the dimmer they appeared in high-energy gamma rays. It is the first time this gamma ray die-off has been observed, says Volker Bromm of the University of Texas at Austin, who was not involved in the new work.

“Speculations were in the literature for many years, but this is one of the first hard numbers, which makes it special,” he says.

Lifting the curtain

Astronomers think the first stars were at least 100 times more massive than the sun. By comparing the new gamma-ray results with theoretical models of how much light such stars should contribute to the extragalactic background, Reimer and colleagues determined that formation of the first generation of stars peaked no earlier than 500 million years after the big bang. They also found that these first stars formed at a slower rate than some models predicted.

“We already know now with the Fermi result that the contribution from the very first stars cannot be too extreme,” says Bromm. “Probably there were just fewer stars.”

More data from Fermi in the coming years should help probe the background light even further. New telescopes expected to come online in the next decade, like the James Webb Space Telescope and the ground-based European Extremely Large Telescope, may be able to see the first generation of stars directly.

“The curtain is about to be lifted on this formative stage in the universe,” says Bromm. “Fermi gives us a first glimpse of what to expect.”

Journal reference: Science Express, DOI: 10.1126/science.1227160