We finally know when the first stars in the universe switched on

Between 250 and 350 million years after the big bang, cosmic dawn broke. Measurements of six of the most distant galaxies we have ever seen have allowed researchers to make the most precise calculations ever of when the first stars formed.

“Before cosmic dawn the universe was dark and contained only hydrogen clouds, and now of course we are surrounded by all this beautiful cosmic structure and trillions of stars in the night sky,” says Richard Ellis at University College London. “The question is, when did all this begin?”

Ellis and his colleagues picked six of the most distant galaxies we have ever seen, all more than 25 billion light years away. Because light from those galaxies took a long time to travel to us, we see those galaxies as they were billions of years ago, making them a window into the early universe.

The researchers observed these six galaxies with four of the most powerful telescopes on Earth to measure their distances as precisely as possible and determine how old the stars in the galaxies are. Those distant stars are some of the very first stars that ever formed, so their ages tell us the date of cosmic dawn, which the researchers calculated to be around 13.5 billion years ago.

“If we’d have measured the age of one galaxy, sceptics would have said that maybe it’s a special galaxy, but we have six,” says Ellis. “It’s the first meaningful estimate of when cosmic dawn occurred because it’s based on a sizeable population of galaxies.”

None of our current telescopes are powerful enough to observe the first stars directly because they are simply too far away. But Ellis and his colleagues have calculated that given the timing that they found for cosmic dawn, the upcoming James Webb Space Telescope should be able to see them. It is scheduled to launch in November, and the team has already secured time to look for stars beginning to switch on. “We’re now very close to witnessing this dramatic moment directly,” says Ellis.

Journal reference: Monthly Notices of the Royal Astronomical Society, in press

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