JWST peers through cosmic dust to reveal newborn star clusters in NGC 628

The James Webb Space Telescope continues to rewrite what we know about star formation. In a new study posted to arXiv and led by Helena Faustino Vieira of Stockholm University, an international team used JWST’s NIRSpec spectrograph to pierce through the dense dust clouds of NGC 628 — the galaxy also known as Messier 74 — and directly characterize a population of star clusters so young they are still invisible to optical telescopes. The results offer a rare, detailed window into one of the most elusive phases of stellar evolution.

A galaxy caught in the act of building itself

NGC 628 is a textbook grand-design spiral galaxy, some 30 million light-years away, with two well-defined spiral arms and an active star-formation rate of approximately 1.7 solar masses per year. At an estimated age of 10 to 13 billion years, it has had ample time to accumulate a rich population of young star clusters — dense, gravitationally bound groups of newly formed stars that represent the basic structural units from which galaxies are assembled.

But star clusters don’t announce their arrival in the optical band. In their earliest phase — the emerging phase — they remain deeply buried inside the same dusty molecular clouds from which they formed. Intense radiation and stellar winds from the most massive stars in the cluster heat and ionize the surrounding gas, but the dust absorbs much of the optical light, rendering these objects effectively invisible to conventional surveys. This is precisely the gap that JWST was built to close.

What NIRSpec revealed inside the dust

The team used JWST’s Near Infrared Spectrograph (NIRSpec) as part of the FEAST program — Feedback in Emerging extrAgalactic Star clusTers — to obtain spectra of 14 emerging young star clusters (eYSCs) in NGC 628, along with their photodissociation regions and the surrounding diffuse interstellar medium.

The spectral data are rich. Among the most significant detections: hydrogen and helium recombination lines tracing ionized gas regions powered directly by the clusters; molecular hydrogen transitions; and bright 3.3 µm emission from polycyclic aromatic hydrocarbons (PAHs) — complex carbon molecules that fluoresce in infrared light when irradiated by ultraviolet photons from young, hot stars. Together, these signatures paint a coherent picture of clusters embedded in, and actively reshaping, their birth environment.

The ionizing photon fluxes measured from the clusters are consistent with stellar populations dominated by O8.5V–O8V type stars — among the hottest and most massive stars that exist. Age estimates derived from the spectral fits confirm that these are genuinely young objects, with a median age of around 3 million years. Clusters showing spectral signatures of more evolved stars, such as red supergiants, have age estimates exceeding 9 million years.

The fading fingerprint of star cluster birth

One of the more subtle findings of the study concerns what happens as the clusters age. The data show that as the star clusters grow older and begin to physically emerge from their natal cloud — driven outward by their own radiation pressure and stellar winds — both the molecular hydrogen emission and the PAH emission systematically decrease.

This is not a coincidence. It reflects a tight physical coupling between the cluster and its photodissociation region: as the cluster disperses the surrounding gas and dust, it also destroys the very material that was producing those infrared signatures. The PDR doesn’t just surround the cluster — it evolves in lockstep with it, and its morphology changes as the cluster emerges. Tracking this evolution observationally is key to understanding how stellar feedback shapes the interstellar medium and ultimately regulates star formation on galactic scales.

JWST as a tool for galactic archaeology

This study is part of a broader FEAST observational program designed to systematically map emerging star cluster populations in nearby galaxies. By building statistically meaningful samples of eYSCs with detailed spectral characterization — something ground-based and even optical space telescopes could never achieve for dust-embedded objects — JWST is enabling a new empirical foundation for models of star formation and galaxy evolution.

NGC 628, with its proximity, face-on orientation, and vigorous star formation, makes an ideal laboratory. The 14 clusters examined here are a first sample; deeper observations will extend the census and allow researchers to map the full life cycle from embedded protostellar cloud to exposed, optically visible star cluster across a representative galaxy.

Source: Helena Faustino Vieira et al., FEAST: a NIRSpec/MOS survey of emerging young star clusters in NGC 628, arXiv (2026). DOI: 10.48550/arxiv.2603.09866 © SKYCR.ORG — Reproduction with attribution.