A team of astronomers has found the best evidence yet for the very first generation of stars, ones made only from ingredients provided directly by the big bang. Made of essentially only hydrogen and helium, these so-called population III stars are predicted to be enormous in size and to live fast and die young. Until recently, many astronomers had thought they would never be able to see such stars, because they would have all burned and died in the universe’s early history—too far for us to see. But using new instruments on the world’s top telescopes, the team found a uniquely bright galaxy that seems to bear all the hallmarks of containing population III stars.
“The evidence is strong. They did a careful job,” says Avi Loeb, chair of Harvard University’s astronomy department.
Theorists predict that the clouds of gas in the early universe would have remained relatively warm from the big bang and so would resist condensing down to form stars. Mixing in a small amount of heavier elements helps gas clouds cool, because those elements are easier to ionize and so shed heat as radiation. But those heavy elements hadn’t yet formed in the early universe, so stars grew to enormous sizes—hundreds or even a thousand times as big as our sun—before their cores were dense enough to spark fusion. Once they did get started, they burned fast and hot, emitting lots of ultraviolet light and burning out in a few million years.
It was such burning that created the heavier elements that now populate the universe. Fusion in the cores of stars meld light atoms into heavier ones, all the carbon, oxygen, iron, and everything else needed to make dust clouds, planets, and life. These heavier elements are scattered around when a star ends its life and explodes. So all the gas that exists in the universe now has a smattering of heavier elements, which allow it to cool more easily. As a result, stars tend to be smaller, burn less brightly, and live longer than their ancient forebears.
Scientists thought population III stars probably resided in small dim primordial galaxies that astronomers would never see. But a team led by David Sobral of the University of Lisbon carried out a survey with the 8.2-meter Subaru Telescope at Mauna Kea, Hawaii, of galaxies shining brightly at ultraviolet wavelengths back as far as about 800 million years after the big bang, when the universe was about 6% its current age. They found an unexpected number of bright candidates. To rule out other possible objects that shine in the ultraviolet, they made follow-up observations of the two most promising sources using Europe’s Very Large Telescope in Chile, the Keck telescopes on Mauna Kea, and the Hubble Space Telescope.
The more interesting of the two, a galaxy dubbed CR7, proved to be the brightest galaxy yet found in the early universe—three times as bright as the previous record-holder. As well as its strong ultraviolet light from ionized hydrogen, it emitted a strong signal from helium—an expected signature from a galaxy of population III stars—but nothing else, suggesting that the stars lacked heavier elements. “There was no trace of other lines, only helium and hydrogen,” Sobral says. Looking at CR7 more closely with Hubble, the team made out three distinct regions with different emissions: one looking distinctly like population III stars and the others containing cooler, more normal stars, the team will report in an upcoming issue of The Astrophysical Journal. This, Sobral says, suggests a wave of star formation through the galaxy, with the first region to start shining later burning out and population III stars starting up elsewhere and so on. Such a wave at this stage of the universe’s evolution, “would be what you would expect,” Loeb says.
“This field, of first-generation stars and galaxies, was mostly theoretical until recently,” Loeb says. “It’s gratifying to see evidence that these are real things.” Mark Dijkstra of the University of Oslo cautions, however, that there are still some unexplained aspects of CR7, such as why such a massive population III galaxy would exist so long after the big bang. “Even if CR7 is not powered by population III stars at all, it will at least give us new insights into galaxy (and possibly black hole) formation in the early universe.”
But Sobral says the group’s survey has already turned up “even more spectacular” candidates. “This is just the beginning,” he says.