Could some of the first “stars” in the universe have been powered not by fusion, but by black holes? The James Webb Space Telescope has been discovering a population of faint compact objects dubbed “little red dots” (LRDs) that don’t fit neatly into either the galaxy or stellar category. One extreme specimen among those, now named “The Cliff,” has become the focus of a radical new hypothesis: that these are “black hole stars,” massive stellar-like envelopes of gas lit from inside by supermassive black holes.
The JWST started mapping the far universe in infrared light, with an unprecedented sensitivity that spotted LRDs over more than 11 billion light-years away. But when they were classified as very ancient, unusually mature galaxies, their compactness and spectral signatures soon raised doubts. SEDs (a detailed comparison of brightness across wavelengths, through which their physical properties are inferred) demonstrated that the objects were much too cool and too dense to represent typical early galaxies or active galactic nuclei. “Determining whether the [spectral energy distributions] of LRDs are dominated by stars or [active galactic nuclei] is a major challenge,” the research team said.
The Cliff whose light set off just 1.8 billion years after the Big Bang hosts an unusually strong Balmer break, meaning a sudden jump in brightness around particular wavelengths of light that is due to hydrogen absorption. In normal astrophysical contexts, that’s a sign of a population of A-type stars in a genuinely evolved galaxy. But at this early time, there simply isn’t time for stellar populations to have aged into such a configuration. The break here is the most pronounced yet for any LRD, and forces astronomers to go back to the drawing board, and come up with entirely new models, said Anna de Graaff of the Max Planck Institute for Astronomy.
The emerging model by the team envisions a supermassive black hole possibly millions of solar masses enveloped in a dense, turbulent hydrogen envelope, from which matter spirals inward via an accretion disk. The gravitational energy released by accretion heats the surrounding gas until it glows like a cold, red supergiant. Unlike fusion-powered stars, black hole stars shine because infalling material is converted into radiation with efficiencies up to 10%, a process that can outpace stellar luminosity despite lower surface temperatures. Moreover, the dense gas readily absorbs high-energy X-rays.
This concept dovetails with broader questions about early black hole formation. JWST has already identified quasars hosting billion-solar-mass black holes less than a billion years after the Big Bang, which challenges standard growth models. Simulations of clumpy gas accretion suggest short, intense feeding episodes from dense clouds accelerate black hole growth, allowing small seeds to reach supermassive scales in a very short period of time. This might occur within low-spin dark matter halos-rare structures comprising only about 1% of the population-which concentrate gas toward the center, thus providing ideal conditions for either rapid starbursts or runaway black hole feeding.
The spectral profiles of The Cliff and its kin indeed hint at broad emission lines from fast-moving gas, in accordance with accretion dynamics, though without the unobscured glare of quasars. This is a suggestion that LRDs might represent a phase of transition whereby black holes grow inside stellar-scale cocoons before coming out as fully fledged AGN. If so, they may provide a missing link between a first generation of black hole seeds-whether from collapsing Population III stars, direct gas collapse, or even primordial origins-and the supermassive black holes anchoring modern galaxies.
JWST’s infrared capabilities are crucial here. By probing rest-frame optical and near-infrared wavelengths at high redshift, it can detect cold starlight and subtle spectral breaks invisible to ground-based optical telescopes. The Cliff’s spectrum, captured over nearly 60 hours of observation, encompasses a wide wavelength range, enabling stringent constraints on models. Simulations of black hole stars reproduce both the Balmer break strength and the overall SED far better than galaxy-based interpretations.
Many questions remain: How common are these objects? Are they a brief growth stage or a stable configuration? Upcoming JWST programs will target brighter LRDs in order to resolve their structure and search for more examples. Black hole stars, if confirmed, would rewrite the timeline of cosmic structure formation to show that in the early universe, some of the brightest “stars” were not stars at all, but black holes wearing stellar disguises.
