The Universe began with a brilliant flash but soon descended into darkness – until finally, a few hundred million years after the Big Bang, the first stars flickered into life.
Astronomers believe they have now glimpsed some survivors from this pioneering generation of stars. These ancient ancestors of modern stars were monsters, hundreds of times more massive than our Sun and millions of times as luminous. Their short, intense lives ended in giant supernova explosions that enriched the cosmos with the first elements that were heavier than helium such as carbon, oxygen and nitrogen – the stuff of planets and ultimately of life.
Many astronomers thought we’d never glimpse these ancient ‘population III’ (or pop-III) stars. But tantalising evidence for these ancestral stars now comes from a newly discovered galaxy, 12.9 billion years old and by far the brightest from its epoch ever seen.
David Sobral at Leiden Observatory in the Netherlands and his team spied a vast, diffuse pocket of pure hydrogen and helium gas in the galaxy that was lit up from within. The lack of other elements shows the gas must be pristine Big Bang material. The team reasoned that the galaxy’s illumination could be explained by the pop-III stars within it. Their work was accepted for publication in The Astrophysical Journal last month. “It doesn’t really get any more exciting than this,” says Sobral.
In the mid-20th century, astronomers noticed the Universe contains two distinct populations of stars. Population I, the first stars they came across, were like our Sun, rich in metals (astronomers call any element heavier than helium a metal) and located around the fringes of galaxies. A second group, dubbed pop-II stars, were formed earlier than pop-I stars, were relatively poor in metals, and were situated near the cores of galaxies.
Only later, after the Big Bang theory was accepted, did astronomers wind back the cosmic timeline and reason that, since only hydrogen and helium were created in the Big Bang, there must have been an earlier population of stars containing no metals at all.
“Once you look where no-one’s looked, there's always the potential for interesting discoveries”
Astrophysicists soon realised these pop-III stars would be special. Stars form when a hot cloud of gas cools to a point when gravity takes over and the cloud collapses in on itself. A rapidly cooling cloud, such as one with heat-absorbing metals that speed up the process, spawns many small stars, but a slowly cooling cloud produces a few real monsters.
Because the clouds that spawned pop-III stars contained no heavier elements, these early stars could grow to an incredible size. A pop-III star would blaze with the brightness of a million suns. With a huge gravity powering their nuclear fusion, these stars would churn through their stock of hydrogen in only a few million years – a brief life by astronomical standards.
Would we ever glimpse these stars? To look back in time, we need to peer deeper into the universe. But whether we’d ever detect the faint and distant light of pop-III stars was a big question says Amanda Bauer, an astrophysicist at the Australian Astronomical Observatory.
Sobral and his team discovered the ancient galaxy by casting a wide net – a celestial survey about 10 times larger than the largest surveys previously attempted. This galaxy, CR7, stood out like a beacon – it was three times as bright as any other galaxy from its epoch. “Once you look where no-one’s looked, there's always the potential for interesting discoveries,” Sobral says.
The light reaching Earth from CR7 was emitted 800 million years after the Big Bang, long after the last pop-III stars were thought to have burned out. Sobral thinks we must be witnessing a pocket of late bloomers. He favours the idea that pop-III stars did not blaze on in synchrony across the entire galaxy, but rather lit up in waves. The fact some metal-containing pop-II stars are older than CR7’s cluster of pop-III stars supports this idea.
“It's pretty spectacular that they have evidence that's leaning towards that theory,” says Bauer. The only other explanation for Sobral’s observation could be that a black hole collapsed immediately from a gas cloud without first forming a star – an object itself only theorised and never seen. Radiation from the feeding black hole could be lighting up CR7. Such a discovery would be equally as exciting, Bauer adds, as it could help explain the origin of the supermassive black holes scientists now believe lurk at the centre of every galaxy. Getting a definitive answer will require a deeper look with Hubble, and by its successor, the James Webb Space Telescope, to be launched by NASA in 2018.