Simulations trace how the first stars left fingerprints still visible today

Astronomers have long pursued two seemingly separate windows onto the cosmic dawn. The first is direct: the James Webb Space Telescope now captures the light of galaxies that existed when the Universe was only a few hundred million years old. The second is indirect but remarkably durable: ancient, metal-poor stars surviving in and around the Milky Way preserve, in their atmospheres, the chemical imprint of the very first stellar generations. A new international collaboration called MEGATRON, led from the University of Bath with partners at the University of Chicago and the Institut d’Astrophysique de Paris, has built some of the most detailed simulations yet of the early Universe to demonstrate that these two windows can be unified within a single physical framework. The project’s first four papers, published in the Open Journal of Astrophysics, argue that only by simulating starlight, gas and chemistry together can astronomers reliably translate what JWST sees into what the fossil record of nearby old stars records.

The technical ambition of MEGATRON lies in its simultaneous treatment of processes that are usually modelled separately. The simulations track the hydrodynamic motion of gas, the propagation of radiation emitted by young stars, the non-equilibrium chemistry of the cooling and fragmenting gas, and the synthesis and dispersal of heavy elements by supernova explosions. All of these are evolved self-consistently at exceptionally high resolution, following a single young galaxy that will eventually grow into a system comparable in mass to the Milky Way. That choice matters: rather than simulating hundreds of crude galaxies, the team resolved the internal structure of one progenitor deeply enough to capture the small-scale filaments and dense clumps where the first stars actually formed, structures that simpler models smear out entirely.

The simulations begin from pristine gas containing no heavy elements at all, mirroring the composition of the cosmos shortly after the Big Bang, when hydrogen and helium were essentially the only ingredients available. From those initial conditions, the code follows the birth of the first stars, the ultraviolet radiation they pour into their surroundings, the supernovae that end their brief lives, and the dispersal of newly forged carbon, oxygen and iron into the gas that will form subsequent generations of stars. Each stage feeds back into the next: radiation heats and ionises gas and suppresses fragmentation, supernovae drive metal-rich winds out of nascent galaxies, and the changing chemical composition alters how efficiently the gas can cool and form new stars. It is this coupled chain of events that determines the chemical fingerprints later recorded in long-lived low-mass stars.

One of the collaboration’s headline results concerns the smallest dwarf galaxies, the systems least able to hold onto their gas and metals. The published work examines how the first stars can create an iron metallicity plateau in these tiny systems, a regime where the iron abundance of successive stellar generations stops rising and instead flattens out. Understanding such plateaus is crucial for stellar archaeology, because astronomers infer the properties of the first stars from statistical patterns in the abundance ratios of ancient stars. If simplified models mispredict how metals are mixed, expelled and diluted in dwarf galaxies, the inferred properties of the first stellar generations could be systematically wrong. By resolving the interplay of radiation, gas flows and chemical enrichment at high resolution, MEGATRON provides a physically grounded calibration for interpreting those abundance patterns.

The broader conclusion of the first wave of results is a cautionary one for the field: simplified subgrid models may substantially underestimate the influence of stellar radiation and complex chemical processes on the gas surrounding early galaxies. Radiation from massive stars does not merely illuminate the gas; it heats it, ionises it, drives photoevaporative flows and regulates where subsequent stars can form. Chemical enrichment, meanwhile, does not proceed as a smooth, well-mixed process but through inhomogeneous, clumpy channels that leave distinct signatures in different environments. When these effects are modelled at high resolution, structures emerge in the circumgalactic gas that coarser simulations simply do not capture, and predictions for what JWST should observe change accordingly. Improving those predictions is one of the project’s central practical goals.

Dr Martin Rey of the Department of Physics at the University of Bath, a lead contributor to the collaboration, framed the significance of the work in terms of connection. “The James Webb Space Telescope gives us a direct glimpse of the infant cosmos, while stellar archaeology allows us to study the relics of those earliest times in our own Galactic neighbourhood,” he said. “MEGATRON provides a physical bridge between the two.” The phrase captures the project’s core claim: the same physical processes that JWST observes in action in distant young galaxies are the ones that wrote the chemical record now readable in ancient stars much closer to home.

That bridge also speaks to one of astronomy’s most fundamental questions: where the elements that make up today’s Universe came from. Every atom of carbon in DNA, every atom of oxygen breathed on Earth and every particle of iron in the planet’s core was forged inside a star and scattered into space by stellar death. “The elements that make our world and life possible – carbon, oxygen, iron and many others – were forged by stars,” Dr Rey explained. “To understand where those elements came from, we need to understand how the first stars formed and enriched their surroundings. MEGATRON allows us to test these ideas directly by comparing detailed simulations with observations from JWST and the chemical fingerprints preserved in ancient stars.” In this sense, the first stars are not merely a remote curiosity of the early Universe; they are the opening chapter of the chemical history that made rocky planets and life possible.

The timing of the project could hardly be better. JWST is transforming the observational picture of the earliest galaxies, revealing objects that are brighter, more numerous and more chemically evolved than many models had anticipated. At the same time, large-scale stellar surveys are cataloguing hundreds of thousands of ancient stars in the Milky Way and its dwarf satellites, measuring their abundance patterns with ever greater precision. Each dataset is powerful on its own, but the combination is uniquely constraining. “MEGATRON provides a common physical framework for interpreting two of astronomy’s most exciting new datasets: JWST’s view of the earliest galaxies and the stellar fossil record,” said Dr Rey. “Together, these complementary observations allow us to test competing models of the first stars in ways that weren’t previously possible.” A model of the first stars must now survive two independent tests at once: it must reproduce the galaxies JWST sees, and it must reproduce the abundance patterns written into the oldest stars.

The computational demands of this programme are enormous. The project has been awarded 40 million processor hours on the United Kingdom’s national supercomputers, a resource allocation the team compares to running five million laptops in parallel for a full year. Those hours are already being spent developing the next generation of MEGATRON simulations, which will push to higher resolution and incorporate more complete physical models, from improved treatments of radiative transfer to richer chemical networks. The payoff, the researchers argue, will be increasingly direct comparisons between simulated and observed galaxies, and between simulated and measured chemical abundance patterns, tightening the link between theory and data at every stage of cosmic history.

MEGATRON began in 2023 and is scheduled to run until 2030, with the four papers just published representing only the first substantial body of results and further releases expected to follow. If the collaboration’s central thesis holds, the coming years should see the two great archives of the cosmic dawn – the photons collected by JWST from the infant Universe and the chemical fossils preserved in the oldest stars of the Milky Way – converge into a single, mutually consistent account of how the first stars formed, died and seeded the cosmos with the elements from which everything, including us, is ultimately built.

Subject of Research: Cosmological simulations of the first stars and galaxies and their chemical enrichment of the early Universe

Article Title: New simulations connect the first stars to cosmic fingerprints still visible today

Article References: New simulations connect the first stars to cosmic fingerprints still visible today. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: first stars, early galaxies, MEGATRON simulations, James Webb Space Telescope, stellar archaeology, chemical enrichment, cosmic dawn, dwarf galaxies, supernovae, metallicity, University of Bath, computational astrophysics