Extra proof of large first-generation stars

A few days ago I wrote about the search for Population III stars. These stars were the first stars of the universe. Huge animals hundreds of times more massive than the Sun and composed only of hydrogen and helium. These massive stars would have been very short-lived, exploding as luminous supernovae in less than a million years. But Population III stars were so massive that their supernovae were quite different from those we see today. So our best bet for finding evidence of them is by looking for their supernova remnants. And a recent study published in Nature may have found some.

For a star to die as a supernova, it must have at least nine times the mass of the Sun. Smaller stars can swell into red giants before evolving into white dwarfs or neutron stars, but they don’t explode quickly. Core-collapse supernovae occur when the core suddenly becomes depressurized, causing atomic nuclei to rapidly fall inward. When they collide, the nuclei produce new heavy elements and a tremendous amount of neutrinos and gamma rays, which leak out and tear the star apart.

Supernovae can occur in stars up to about 50 solar masses. Above this mass limit, the core is so dense and collapses so suddenly that it forms a black hole straight away. That means there are no supernovae, or at least not very bright ones. But if a star is really massive, over 150 solar masses or so, then extremely bright supernovae, known as hypernovae, can occur. This happens due to a physical process known as pair instability.

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Pair formation by high-energy gamma radiation. Photo credit: U Mallik, University of Iowa

The more massive the star, the hotter and denser its core. And when it collapses, the neutrinos and gamma rays are all the more intense. In really massive stars, a gamma-ray photon is so intense that it can create an electron-positron pair when it hits a nucleus. These charged particles interact incredibly strongly with the surrounding nuclei, creating pressure too strong for gravity to withstand. They also cause another set of heavier elements to form, and this is where this study comes in.

The team studied a distant star called J1010+2358, which may be the earliest star we’ve ever observed. It is not a Population III star, but it is low in metals. From spectral observations of the star, they found that it had extremely low levels of sodium and cobalt. Less than one percent of the frequency found in the sun. The team found larger amounts of magnesium and nickel.

This is interesting because of the atomic numbers of these elements. Sodium (11) and cobalt (27) have an odd number of electrons, while magnesium (12) and nickel (28) have an even number. This split between even and odd frequencies is exactly what one would expect in the remnant of a pair-instability supernova. Based on the observations, the team estimates that J1010+2358 formed from the remnant of a 260 solar-mass progenitor that was likely a first-generation Population III star. From these and other observations of ionization in distant galaxies, it seems clear that we have evidence of massive first-generation stars in the early Universe.

Reference: Xing, Qian-Fan et al. “A metal-poor star with frequencies of a pair-instability supernova.” Nature (2023): 1-4.

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