Star SDSS J0018-0939 is a small, second-generation star bearing the chemical imprint of one of the universe’s first stars. Seen as white here, the star would appear orange to human eyes.
After digging around in the light from an old, orange star, astronomers have found possible remnants of one of the universe’s first stars: a giant that may have been more than 140 times as massive as the sun.
The finding, reported Thursday in the journal Science, marks the first time astronomers have made observations suggesting that such massive stars populated the early universe, despite decades of theories proposing that some of those first stars must have been huge.
Billions of years ago, when that primordial star exploded in a spectacular supernova, it blasted its guts into space and seeded the cloud that eventually formed the orange star, which is now about 1,000 light-years from Earth.
Knowing the mass of these first stars—or the range of masses—is crucial for understanding how quickly the lights turned on in the very early universe and shifted it from a dark, gassy place to one filled with nascent galaxies and stars.
"How the cosmic Dark Ages ended—that really depends on the mass of the first stars," says University of Texas at Austin astrophysicist Volker Bromm, who calls the new findings "an important data point."
The universe’s first stars were born several hundred million years after the big bang, Bromm says, when hydrogen, helium, and dark matter were mixed into a thin soup that flooded space. At that time, irregular clumps of dark matter began to collapse, creating dense mixtures of gases that eventually ignited and became stars, lighting the universe and bringing an end to what astronomers call the cosmic Dark Ages.
Many of these first stars, called population III stars, were relatively small, several tens of times as massive as Earth’s sun. But astronomers think that some must have been huge-hundreds of solar masses. Formed from large clouds of slowly collapsing gas, these giant stars were energetic enough to reshape their surroundings, helping to form early galaxies and star clusters and burning away a long-standing cosmic fog.
But large stars live fast and die young. After a few million years, the primordial giants exploded in supernovae so spectacular they could destroy galaxies.
The chemical elements flung from these dying stars into the infant universe seeded the surrounding gas clouds, leaving patterns that astronomers can now read like fingerprints. Those elements helped gas clouds cool and condense more quickly and form a second generation of smaller stars, some of which still survive and can be mined for ancient stellar imprints.
Explaining an Oddball
A team led by astronomer Wako Aoki from the National Astronomical Observatory in Japan began its search for the fingerprints of ancient massive stars by looking for small, long-lived stars with low metal content. These might be stars that formed early in the universe and hadn’t been contaminated by the splatters from eons of supernovae.
"A low-metallicity star clearly records the products of a single supernova," Aoki says. "We can estimate the mass of the progenitor star from the chemical abundances of the products."
In other words, they were looking for the second-generation stars that might bear fingerprints from first-generation ancestors.
One of the stars in the survey, a small, orange star called J0018-0939, stood out. It had low levels of carbon, magnesium, and cobalt (all considered "metals" in astronomy) but a peculiarly high level of iron. Normally, that high iron reading would have been enough to exclude the star from the survey. But scientists were intrigued.
The team pointed the Subaru Telescope in Hawaii at the star and took a closer look at its chemical profile, making careful measurements of the abundance of chemical elements. It didn’t look like anything anyone had ever seen before.
"They took a risk and it paid off," Bromm says.
Then, astronomers compared the fingerprint to computer simulations of the chemicals that are created and chucked out by various types of supernovae. According to the team, the explosion model that best fits the chemical profile is what’s called a pair-instability supernova, a spectacular type of explosion that can be a hundred times more powerful than the supernovae seen today.
And the only way to get a pair-instability supernova and produce that much iron is by exploding a giant star at least 140 times as massive as the sun, Aoki says.
Not all the elements in the chemical profile fit the bill for this type of explosion, a result that astronomer Anna Frebel of MIT quibbles with. But Frebel notes that simulating what would happen in supernovae that have not been observed is incredibly difficult.
"I’m still a little bit on the fence about whether I believe these [primordial giant stars] existed," says Frebel, who has observed a handful of other second-generation stars.
Like J0018-0939, Frebel’s stars bear signatures of the universe’s first stars. But unlike this one, those signatures don’t reveal the presence of bulked-up behemoths. "But I would be absolutely thrilled if it turns out to be correct."
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