Tentative Evidence of the First Generation of Stars

Big Bang cosmology tells us that the early universe was made up of only hydrogen, helium, and a smattering of lithium. From this, we can reason that the first generation of stars would have been quite different from the stars around us today. We’ve never seen this first generation— the first stars are too far away to image directly. But now, we have tentative evidence that they really did exist, and in the pure form astronomers had predicted.

The first stars were “pure” in the sense that they must have been free of metals, the term astronomers use to mean any elements heavier than hydrogen and helium. For historical reasons, we call these stars Population III (Pop III). Because they were missing metals, these stars were huge, with hundreds of Suns’ worth of mass. Astronomers have hypothesized that some of them ended their lives in a peculiar kind of explosion called a pair-instability-supernova (PISN).

Unlike most supernovae we observe, a PISN does not leave behind a black hole or neutron star— just gas leftover from a shredded star. Subsequent generations of stars — Pop II, then Pop I — were formed from the heavier elements left behind. Neither Pop III stars, nor the explosions they ended in, have ever been directly observed. But a recent article in The Astrophysical Journal claims to have found evidence of this peculiar type of supernova in the gas around ULAS J1342, one of the most distant quasars known.

Yuzuru Yoshii (The University of Tokyo), who led the project, has been researching no-metal and low-metal stars since the 1990s. His team analyzed spectra of the gas around the quasar, collected with the Gemini North telescope on Mauna Kea in Hawai‘i, converting the intensity of light emitted by specific heavy elements into abundances of those elements. The results were intriguing, showing much less magnesium relative to iron than expected. This was promising because it is theorized that a PISN could produce the particular ratios they detected: about 10 times as much iron as magnesium, compared to the signature of a Sun-like star.

In the past decade there have been a few light curve observations that might have been associated with PISN. But those observations have been inconclusive, claims team member Timothy Beers (University of Notre Dame).

From far away, Beers says, these supernovae might look similar to regular supernovae because astronomers are limited in what they can observe from such distances. “You can’t even resolve the galaxy, all you really see is a brightening, and that’s a tricky observation at best,” he notes. “So instead of trying to catch a star at the moment of explosion, what we have done is find a chemical pattern, that as far as we know, is only formed by this kind of an explosion.”

But the team has been careful to call their finding a potential signature of a Population III star.

This is the first detection using this innovative technique, so it’s pushing the envelope, says Volker Bromm (University of Texas, Austin), who was not involved with the study. “Even so, it's  pretty convincing.”

More observations of similarly distant quasars could verify this observation. So could observations of individual far-away stars, made possible by gravitational lensing. The star Earendel, discovered this way in the distant Sunrise Arc galaxy, is also a Pop III candidate.

With the new technique, Beers adds, astronomers should also be able to study stars in the outskirts of the Milky Way that have similarly high-iron, low-magnesium content — descendants of ancient PISN explosions.

Next-generation instruments such as the University of Tokyo Atacama Observatory (TAO), led by Yoshii and set atop the summit at an altitude of 5640 meters of Cerro Chajnantor in Chile, could aid these efforts. Set to be completed next year, it will be the world's highest observatory having 6.5-meter infrared telescope, with unprecedented deep-sky survey capabilities. Pop III hunters have set their hopes on this and other instruments, like the James Webb Space Telescope. With discoveries like this one, the future certainly looks bright.

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