NASA Scientists used high-energy radiation from blazars to measure the light from the first stars.

Astronomers have a pretty good idea of how the universe took shape following the Big Bang — with one glaring exception. About 400,000 years after the great detonation itself, as the superheated particles it created cooled and formed into atoms, the entire universe went black. A few hundred million years later, the darkness began to lift as the first stars congealed from clouds of cosmic gas. “The universe,” says Volker Bromm of the University of Texas at Austin, “underwent a crucial transition from a very simple state into a state of ever more complex structure.”

But exactly when and how that transition happened is still largely a mystery, because even the most powerful telescopes in existence can barely penetrate the murky period of cosmic evolution known, appropriately enough, as the Dark Ages.

That veil has been lifted slightly by a paper just published in Science. Using the Fermi Gamma-ray Space Telescope, an international team of more than 200 astrophysicists has probed deeper into the Dark Ages than ever before. The conclusion: the first stars must have appeared no more than 500 million years after the Big Bang, or just 100 million years after atoms first took shape. And those stars formed at a slower rate than anyone thought.

(PHOTOS: Happy Birthday Hubble. A Stunning New Picture for a Special Day)

To most of us, these results probably seem too vague to get excited about. But astronomers recognize that they’re a tour de force of observational finesse. It’s impossible to see the first stars, so the Fermi group took an indirect route instead: they measured the so-called extragalactic background light, or EBL — the light emitted by all the stars and galaxies in the universe, creating a sort of glow that comes from every direction.

Most of the 10 quadrillion or so stars in the visible universe, including the sun, were born after the first generation, so most of the EBL comes from them. But the Fermi scientists figured out a way to screen these out. They looked at objects known as blazars — giant black holes in the cores of distant galaxies that send out blasts of high-energy gamma rays, which can be spotted halfway across the universe.

(MORE: NASA Gets Two New Hubble Telescopes — for Free)

When photons of light from the EBL collide with gamma-ray photons from one of these blasts, it alters the blazar’s overall gamma-ray signature by suppressing the highest-energy gamma rays. The further back in time and out in distance you go, the greater the effect, since there is more intervening EBL to collide with the gamma rays. But at some point this effect stops, because you’ve gotten to an early enough stage that the only stars contributing to the EBL are the ones in that first generation.

By choosing 150 or so blazars that blazed as long as 10 billion years ago, the Fermi scientists were able to reach that stasis point. By comparing the abruptness of the drop-off with the best theoretical understanding of when the very earliest stars might have formed and how massive they should have been (hundreds of times as massive as the sun, it turns out), the authors conclude that there should have been about 1.4 stars per hundred billion cubic light-years, for an average distance of more than 4,000 light-years between one star and the next. In the Milky Way today, by contrast, the average separation is about 5 light-years.

(PHOTOS: Window on Infinity: Pictures from Space)

Coming up with these numbers was a painstaking job. The astrophysicists had to calculate what a blazar’s unaltered gamma-ray signal should look like so they could see how the actual signal differed. The difference caused by the EBL turned out to be awfully subtle. It took a long list of tricky calculations and simulations (thus explaining the need for so many co-authors) to arrive at their final numbers.

And they’re not done yet. “We measured starlight when the universe was only 4 billion years old,” said co-author Marco Ajello of Stanford’s Kavli Institute for Particle Astrophysics and Cosmology at a press conference, “but in the future we’ll be looking at even earlier times.” Those observations will also be indirect, but by 2018, he says, the James Webb Space Telescope will be in orbit around the sun, capable of imaging the first galaxies in the cosmos directly.

That won’t be quite as good as seeing individual stars, but those would be far too distant and tiny for any conceivable telescope to pick up. It will, however, be one more big step toward understanding what really happened during the Dark Ages of the universe.

(MORE: After Hubble: Will Budget Woes Kill NASA’s Next Great Telescope?)