Astronomers Find New Evidence for Elusive First-Generation Stars

Astronomers with the European Southern Observatory have identified a series of stars hailing from the very earliest epoch of star formation—members of the elusive Population III classification. Likely, the clusters, which are composed of materials leftover from the Big Bang itself, aka the most original star stuff, are the last of their kind.

The universe began in a flash of heat energy, a uniform expanding miasma of everything-to-be betraying little of the modern universe: stars, galaxies, light, people. There was a beginning, the Big Bang, and then there was the end, or an end. The universe cooled, and the physical forces as we know them precipitated out of this cooling through the process of symmetry breaking. For a while, it was a place of featureless blackness. The new universe was blank.

There were new forces in the cosmic dark ages, however, and the first atoms: hydrogen and helium. The stage just before the dark ages is known as recombination, which is when all of the free-floating electrons and protons saturating space as an energetic, charged plasma got together to form neutral atoms. Recombination prevailed over the ionized universe and neutrality took over. The universe became transparent as all of this free-floating charged junk settled down into neutral arrangements. It would have been transparent, but still completely dark. It was that way for a very long time.

What happened next is speculative in the sense that there's a lack of evidence explaining it. This would be the period in which actual structures like stars and eventually galaxies formed. We know of course that first-generation stars should have existed in the universe, but they've been in short supply, observationally speaking. Now, astronomers at the European Southern Observatory have found new observational evidence, described in this week's Astrophysical Journal, for this first generation in the form of two extremely distant, extremely dim galaxies: CR7 and MASOSA.

The end of the cosmic dark ages was a stage called reionization. As one might imagine, this is sort of just the opposite of the process that formed the dark ages in the first place. Objects materialized in the cooling, condensing universe—their precise nature being in large part a mystery—and the result of all of this settling down and object formation was a release of energy as photons. These photons, as they blasted the then-neutral atoms of the dark universe, stripped away electrons, once again leaving ions. As before the the dark ages, the universe was again a charged plasma.

And so the ball was at the top of the hill that is modern cosmic evolution. The objects at the heart of reionization have been theorized to be Population III stars, or first-generation stars. This classification of star is characterized by having a chemical composition consisting entirely of Big Bang leftovers: hydrogen, helium, tiny bits of lithium. There had been no stars before them to produce all of the other elements we know today, so the absence of such elements implies that these stars were the very first stars, the bootstrappers of the prototypical cosmos.

The new ESO observations suggest that the galaxies CR7 and MASOSA, also known as Lyman-alpha emitters, are of this epoch and, thus, are among the most distant (oldest) objects ever to have been detected. It's here that all of the metals that are found in Population II and I stars, such as our own, would have come from. It gets better.

"We may be witnessing, for the first time, direct evidence for the occurrence of waves of PopIII-like star formation which could happen from an original star cluster outwards," the ESO group, led by University of Lisbon astrophysicist David Sobral, explains. So, as the first PopIII stars started blinking out, new regions of space (still poor in metals) were coalescing into the next wave of PopIII stars.

The stars observed by the group are thought to make up the very last wave of PopIII stars, those recent enough to reside even among PopII and PopI stars. This suggests that PopIII stars may be much easier to find and study than previously thought. Needless to say, that's an exciting prospect.

"The discovery challenged our expectations from the start," Sobral offered in an ESO statement, "as we didn't expect to find such a bright galaxy. Then, by unveiling the nature of CR7 piece by piece, we understood that not only had we found by far the most luminous distant galaxy, but also started to realise that it had every single characteristic expected of Population III stars. Those stars were the ones that formed the first heavy atoms that ultimately allowed us to be here. It doesn't really get any more exciting than this."

An open-access version of the current paper is available at the arXiv pre-print server.