Despite its lack of iron, Keller’s Star has measurable amounts of carbon, oxygen and other metals. “That’s the really remarkable thing,” says Beers. “We see a pattern that for the time being can only be explained by a population of first-generation stars.”
Understanding the First Generation
Originally, astronomers simply assumed that when the first stars went supernova, the explosions uniformly spewed their innards far and wide. But even before finding Keller’s Star, they’d already begun to wonder if this picture was oversimplified.
“We’ve seen this distinctive element pattern in other old stars as well,” Beers says — very little iron, but more of the other metals than you’d expect. The implication: Some first-generation or progenitor stars exploded evenly, as expected, but others must have somehow held onto iron during their death throes while allowing lighter metals to spread out into space.
Key among these lighter elements is nitrogen. Some iron-poor stars (including Keller’s) have a fair amount of nitrogen, while others have essentially none. “My suspicion,” Beers says, “is that this variation in nitrogen tells us that we’re seeing evidence for at least two classes of progenitor stars.” Stellar archaeologists had found the stellar equivalent of Neanderthals, a separate but similar species that coexisted with our suspected forebears.
According to theorists like Volker Bromm of the University of Texas at Austin, the iron-poor, nitrogen-rich second-gen stars come from one specific class of progenitor stars with 10 to 100 times the mass of the sun. Simulations show that these stars would die in dramatic explosions that leave behind black holes, which would trap the heaviest elements in place. “Lighter stuff like carbon and oxygen and nitrogen will get out,” says Bromm.
The other class of first-generation star, whose imprint is seen in iron-poor, nitrogen-poor stars, generally would have been even bigger, between 50 and 100 solar masses. (The apparent overlap between the two classes reflects uncertainties in the numbers, but modelers know for sure that this second category would have been more massive.)
“When stars this massive form,” says Beers, “they tend to spin very rapidly.” In contrast to the first group, the metals in these larger stars get churned up to the upper levels, so they’re thoroughly mixed in when the star explodes. That means the black holes left behind swallow a representative mix of elements, not just the heavier ones — and that some iron is allowed to escape. Second-generation stars made from this debris would still have relatively little iron, like any other ancient star, but they’d have correspondingly little nitrogen as well.
So the first stars came in at least two distinct flavors, and astronomers suspected an even more rare third kind, downright enormous between 140 and 260 solar masses. These gigantic stars would have had surface temperatures of millions of degrees, making them not red-hot or blue-hot, but hot enough to produce gamma rays, the most energetic form of light. The laws of physics dictate that gamma radiation can decay into pairs of elementary particles: electrons and positrons. The star’s gamma rays exerted outward pressure, keeping the massive star from collapsing, but once they’d turned into particles, that outward pressure would be gone, resulting in a catastrophic collapse. This would trigger gigantically powerful supernovas, known to astronomers as “pair-instability” supernovas, which would have added their own, slightly different mix of elements to interstellar gas clouds, and to stars that formed from them.