Astronomers have discovered an ancient star a few thousand light-years from Earth whose composition is opening a window on the life and times of its immediate ancestors: the universe's first stars.
A team led by Australian National University astronomer Stefan Keller found the star as part of a dedicated search for these second-generation stars. The hope is that by studying the composition of second-generation stars, researchers will be able to learn more about their predecessors – the first population of stars to emerge after the big bang, the enormous release of energy that gave rise to the universe.
These first stars are thought to have appeared some 1 million to 10 million years after the big bang. They would have formed as part of small galaxy wannabes. These protogalaxies ultimately would merge to form larger galaxies whose ionizing radiation would lift a fog of molecular hydrogen that had prevented light from traveling across the cosmos.
Known as Population III stars, the first-generation stars were nothing but burning balls of hydrogen with small amounts of helium. They lacked any trace of heavier elements, such as carbon, oxygen, or calcium. Because of their mass, they burned hot, bright, and only for about 10 million years before they exhausted their fuel and exploded as supernovae.
These stars are long gone. In their stead, they have left astrophysicists with nagging questions about the stars' purported role in an infant universe. Did they re-ionize the cosmos? How many Population III stars does it take to make a Population II star? And what is their life history?
The newly discovered star, with the ungainly label SMSS J031300.36-670839.3, appears to be a second-generation star. And it's hinting at possible answers to some of those questions, suggests Anna Frebel, an astronomer at the Massachusetts Institute of Technology in Cambridge, Mass., and a member of the team reporting the discovery in the current issue of the journal Nature.
The team discovered the star using a telescope specially designed for the survey and for hunting old stars at the Siding Spring Observatory, about 300 miles northwest of Sydney, Australia.
When the team followed up by collecting the star's chemical fingerprints, or spectra, the results were stunning, Dr. Frebel recalls.
She called her colleagues at Siding Spring and said, "Hey, you've got to look at this!" she recalls. The elements heavier than hydrogen or helium that she expected to see in a second-generation star were hardly present. This is unexpected, she says, because the supernovae that appear in today's universe form heavier elements, up to iron, as the progenitor stars burn through the last of their fuel. The star collapses, then explodes, shedding these elements into the cosmos, where they are free to become incorporated into a new generation of stars.
The star that the team discovered had relatively small amounts of these elements, and it had iron in amounts so small that the team wasn't sure the star had any.
The low abundances for elements heavier than hydrogen and helium gave away the star's age in general terms – putting it among the second-generation stars. The team estimates that the supernova seeding it was triggered by the collapse of a star 60 times more massive than the sun.
The low abundances also provided clues about the intensity of the death throes of Population III stars.
In essence, the low abundances suggest an explosion less energetic than those of later populations of massive, more metal-rich stars. The lack of iron suggests that the explosion culminated in a black hole.
As a star burns through its fuel and forms successively heavier elements, these arrange in layers that are blown away from the core in the explosion. Since iron is the final element formed, it's the last element to escape – unless the star's collapse instantly forms a black hole. In that case, the iron wouldn't have traveled far enough to escape the black hole's gravitational grip.
If this scenario for supernovae less energetic than previously expected holds up, it would suggest that Population III stars may have played a weaker role in bringing the universe out of the dark ages than previously thought.
And it suggests that these first very massive stars as a group may have persisted in the early universe longer than previously thought, the team posits. Lower-energy explosions would have been less disruptive to the enormous clouds of gas that serve as the nurseries and raw material for new stars.