Peering to the edge of the visible cosmos with some of the most powerful telescopes on the planet, astronomers have detected light from the very first generation of stars to emerge after the big bang.
These behemoths weighed 100 or more times the mass of the sun but died young. After only a million years or so—compared with the sun’s projected ten-billion-year lifetime—they exploded, spewing out elements that were incorporated into a second generation of stars and then a third that included our sun.
“It’s quite fantastic,” says astronomer and lead author David Sobral of the University of Lisbon in Portugal and Leiden Observatory in the Netherlands. “We’re finally seeing what theorists have been predicting for many years.”
Sobral and his colleagues didn’t identify individual stars. Instead, they captured the collective light of stars in the galaxy CR7, named for Portuguese soccer star Cristiano Ronaldo. They found CR7 in a survey that looked for unusually bright galaxies in the early universe using the Subaru and Keck telescopes in Hawaii. CR7, the brightest galaxy found yet from that time, is so far away that its light began the journey to Earth 13 billion years ago, just 800,000 years after the big bang. Details about CR7 and its ancient stars appear in a paper that will be published in the Astrophysical Journal.
In the moments after the big bang, the universe had only three elements: hydrogen, helium, and the faintest traces of lithium. The first stars—known somewhat confusingly as “Population III” stars—would have formed out of these alone. The nuclear fusion within those stars then produced heavier elements, including carbon, oxygen, and nitrogen, which were released into space when the stars exploded. Those elements were incorporated into second- and third-generation stars and, eventually, planets and even people.
When Sobral and his colleagues looked closely at CR7, they found that light from one part of the galaxy was being emitted by heated hydrogen gas. The light clearly came from stars, but that wasn’t all. “The other thing we realized,” Sobral says, “was that there was excess of light we couldn’t explain.” So they went to the Very Large Telescope in Chile for another look.
The excess, they found, was coming from helium gas. Though helium is a basic component of stars, it is difficult to detect unless the stars are incredibly hot—which means they’re extremely large. That alone isn’t enough to prove the stars belong to the first generation, but the astronomers also failed to detect any other, heavier elements.
That was a dead giveaway. If some of the stars in CR7 are both unusually hot and don’t contain at least some of these heavy elements, they must be from the first generation, Sobral and his colleagues argue.
“The evidence is convincing,” says Harvard astrophysicist Avi Loeb, one of the theorists who predicted what first-generation stars should look like. “It provides the strongest observational evidence to date for the stars made out of pristine hydrogen and helium, left over from the big bang.”
CR7 also has clumps of stars that are not from the first generation. This is consistent with theoretical predictions, Loeb notes. Modern galaxies, including the Milky Way, are thought to have assembled themselves from much smaller proto-galaxies that began forming a few hundred million years after the big bang. CR7 is probably a snapshot of the early stages of that process, in which some parts have recently formed their very first stars while others have already moved on to the second generation.
In the coming years, powerful new telescopes, such as the James Webb Space Telescope and giant telescopes now under construction in Chile and Hawaii, should make observations like these easier to carry out and lead to even more impressive discoveries.
“I’m happy that our predictions are being validated,” says Loeb, “but I would be even happier if something completely unexpected will be found, because then we will learn something new.”