Astronomers have a pretty good idea about what the first stars in the universe must have looked like. Theorists say they should have been gigantic, weighing in at anywhere from 20 times the mass of the Sun all the way up to 100 Suns’ worth of material or more. These giants would have burned far hotter than our own star, and far faster as well. The Sun, for example will live for about 10 billion years (it’s about half that old now), but the first stars should have torn through their fuel supply in just a few million years before blowing themselves apart in gigantic explosions.
Unfortunately, it all happened more than 13 billion years ago, and while powerful new instruments like NASA’s partially built James Webb Space Telescope might one day be able to to pick out the light of these mammoth stars, still streaming faintly across the universe after all that time, there’s no way at present to image them directly.
But a team of observers is now reporting in Science that they’ve picked up the telltale signature of the most massive of those first stars. “They’ve been predicted for years, but never seen before,” says Timothy Beers, of Notre Dame, one of the report’s co-authors. To be precise, they still haven’t seen the stars themselves; instead, the astronomers detected their chemical signatures, imprinted on a second generation of stars born just a bit later. Because they’re trying to understand a long-lost era of cosmic history indirectly, Beers and his colleagues call their field “stellar archaeology,”
Those second-generation stars were for more modest in terms of size and temperature and much slower-burning, which has allowed some of them to survive right up to the present. That includes SDSS J0018-0939, the star described in the new Science paper. It’s somewhat less massive than the Sun, and it’s relatively deficient in elements heavier than hydrogen and helium.
That’s a clue that it was formed early in the life of the universe.
Right after the Big Bang, those heavier elements, including everything from oxygen to carbon to silicon to iron, didn’t even exist; they were created in the nuclear furnaces at the cores of stars (which means that the calcium in your bones and the carbohydrates in your breakfast cereal were manufactured inside a star, long ago). For historical reasons, astronomers call any elements heavier than helium “metals” (carbon and nitrogen count as metals in astronomical jargon.)
Those “metals” were spread far and wide when the original stars exploded and incorporated into new stars, and since stars have been forming and exploding for billions of years now, those that formed relatively recently, such as the Sun, are relatively metal-rich. “Our Sun,” says Beers, “is a is a bucket into which the entire history of chemical evolution was poured.”
Stars that formed early on, by contrast, when there was still mostly just hydrogen and helium to be had, are metal-poor. SDSS J0018-0939 is one of them—but given its metal-poor status, it has a surprisingly large amount of iron. And given what theorists know about star formation and evolution, the only place it could have come from so early in the lifetime of the universe was the core of a gigantic star.
The evidence that such stars really did exist is still circumstantial, but that’s a lot better than being purely theoretical. It also adds to a growing understanding of what the universe must have looked like when the stars first turned on. Earlier efforts at stellar archaeology had yielded circumstantial evidence of much smaller (but still huge) first-generation stars, which were unusually rich in carbon rather than iron.
But to understand how the modern universe began to take shape, and how the galaxies came form out of the diffuse gases that dominated the earliest years of the cosmos, astronomers need to know the range of sizes those first stars came in—because how they lived and how they died set the stage for what would come afterward.
“It’s a complicated story,” says Beers, “but it’s incredibly interesting. You’re talking objects that exploded 13 billion years ago. I find it remarkable,” he admits, “that the question can be addressed at all.”