Understanding the nature of the first stars, whose formation marked a pivotal epoch in the Universe's history, is at the frontier of astronomy. An analysis of stellar data indicates that they were fast-rotating objects. See Letter p.454
Like many of the dinosaurs, the first stars in the Universe were massive beings that died out long ago. On page 454 of this issue, Chiappini et al.1 report measurements of the abundances of heavy elements in stars from one of the Galaxy's oldest star clusters, denoted NGC 6522, that suggest that the earliest generations of stars were not only massive but also rotating rapidly.
Astronomers have made major strides in the study of the first stellar generations using a technique that draws its inspiration from old bones. Massive stars (those at eight times the Sun's mass or more) live fast and die young (after only 30 million years or less), so the first stars that formed within a billion years of the Big Bang are no longer around to illuminate even our largest telescopes. We can read the astronomical 'fossil record' because, when stars die as explosions called supernovae or become red giants, they cast the heavy elements (those heavier than helium) that they have synthesized in their nuclear-burning cores into interstellar space. The heavy elements, which astronomers call metals, then get mixed into fresh gas and are eventually recycled into new stars. Any star formed with roughly the Sun's mass (2 × 1030 kg) or less lives for at least the whole 13.7-billion-year history of the Universe, so small stars bearing the metals produced by the first stars can be studied and the properties of the earlier generations deduced from the relative quantities of each observed element.
By reanalysing data that their group had obtained with the European Southern Observatory's Very Large Telescope, Chiappini et al.1 have measured greatly elevated abundances of the heavy elements strontium (Sr) and yttrium (Y) in eight old stars in NGC 6522, a tightly bound cluster of stars associated with the Galaxy's bulge that is at least 12 billion years old2 (Fig. 1). The observed enhancements in these two rare elements when compared with iron suggest that the 'slow neutron capture' process for creating elements heavier than iron is up to 10,000 times more effective in the stars that preceded and enriched NGC 6522 than in non-rotating stars of the same mass. Like a spinning vessel filled with water, rotating stars experience mixing between inner and outer nuclear-burning gas layers that would not otherwise overlap. A cascade of nuclear reactions in these overlapping layers generates radioactive neon, which in turn emits a high flux of neutrons that are captured by iron and other heavy nuclei to create rare elements such as Sr and Y. After the death of the original, rapidly rotating stars, these elements eventually found their way into new star-forming clouds and then into the stars in NGC 6522.
To explain the large enhancements seen in Sr and Y, the authors invoke a rotating-star model with a surface velocity of 500 kilometres per second — a dizzying spin compared with the Sun's stately turns at 2 km s−1, or the typical value of 100 km s−1 seen in massive stars in the Milky Way. Indeed, 300–500 km s−1 is rare for Milky Way stars, but could be typical of the first stars. This rapid spin is likely to have affected all stages of stellar formation and evolution in the first generations, from their initial mass to their chemical composition and products. Rapidly rotating first stars may in fact be more likely to die as energetic γ-ray bursts than their slowly spinning cousins, which may make them more easily detectable in the early Universe.
Chiappini and colleagues' results add some observational urgency to a question that has taxed theorists of the first stars in the past few years. Over the past decade, there developed a tentative theoretical consensus that the first stars formed with typical masses of tens to hundreds of solar masses in the first few hundred million years of cosmic history. The chemical fossil record generally supported this picture. But the exact distribution of masses could not be calculated precisely, because it depended on poorly understood details of the radiation transport and gas chemistry that occur as a forming star's core makes the transition from a dense cloud to stellar densities and begins its nuclear burning. Another wild card in the formation models of the first stars is the total amount of rotation (or angular momentum) in the original cloud from which the star forms: fast rotation could influence the final mass of the star3 or even promote the cloud's fragmentation, leading to multiple star systems4,5.
Until now, these models have had no observational constraint on how fast the final star should be rotating, and they could not predict its rotation a priori because they stopped well short of the densities at which stellar evolution and nuclear burning begin. Chiappini et al. have provided important guidance to this effort: whatever happens during the complicated final stages of star formation at these early epochs, it must be able to lead to rapidly spinning stars. Indeed, one recent theoretical study6 suggested that primordial stars can form with both 125 solar masses and at least 800 km s−1 of rotation.
Despite these encouraging developments, many questions remain about these eight stars, the cluster in which they reside, and their true relationship to the first stars. Although NGC 6522 has an age that places it within the first billion years of cosmic history, the uncertainty in this age and the cluster's relatively high metal content (one-tenth of the Sun's value) relative to stars at that epoch not associated with clusters (one-hundredth to one-thousandth of the Sun's value) leaves open the possibility that these eight fossils bear the rotation-driven products of stars from several generations after the first.
The rotating-star models used by Chiappini and colleagues to interpret the abundances in NGC 6522 need further development and independent confirmation by other researchers. Clusters such as NGC 6522 are generally not included in numerical simulations of the first stars and galaxies such as those described above; integrating them is a challenge for theory. Many details remain to be worked out concerning how these rapidly rotating stars form, evolve throughout their lives and die. Finally, additional abundance signatures of rotating metal-poor stars beyond Sr and Y should be developed and searched for in these stars and others in the fossil record to test the rotating models against multiple lines of evidence. Notwithstanding these open issues, these old stellar fossils will no doubt impart a fresh spin to our thinking about the earliest stars in the Universe.