The James Webb Space Telescope (JWST) was built to look deeper into the cosmos than any instrument before it. Its powerful infrared cameras allow astronomers to glimpse light that has been traveling for billions of years, carrying with it stories from the dawn of the universe. By capturing this faint, stretched light, JWST lets us see galaxies as they were more than 12 billion years ago—when the universe was only a fraction of its current age.
From the moment JWST began sending back images in the summer of 2022, the scientific community was stunned. Among its breathtaking views of cosmic nurseries and ancient galaxies, something unexpected appeared: tiny, intensely red points scattered across the images. They weren’t just artifacts of the camera or distant stars. These compact, glowing “small red dots” represented an entirely new kind of object that Hubble, with its limited infrared vision, could never have revealed.
But as astronomers began to study these dots in more detail, the mystery deepened. They didn’t look like anything in the existing catalog of cosmic structures. Were they young galaxies? Enormous black holes? Or something stranger altogether?
The Puzzling Nature of the Small Red Dots
At first glance, astronomers thought the red dots might be galaxies in their infancy—distant clusters of stars forming rapidly in the early universe. That would fit with their extreme distances: the closest ones are about 12 billion light-years away, meaning their light began its journey when the cosmos was only 1.8 billion years old.
But when researchers ran the numbers, something didn’t add up. The supposed galaxies appeared far too massive for such an early epoch. To match the brightness JWST observed, they would need to contain hundreds of billions of stars packed into volumes far denser than anything we see in the Milky Way today. It would be as though the night sky of such a galaxy was ablaze, every square degree shimmering with starlight.
If these dots really were galaxies, they would force us to rewrite our understanding of cosmic evolution. How could so many stars form so quickly, so soon after the Big Bang? And if not galaxies, then what were they?
Galaxies or Active Black Holes?
Another explanation soon emerged: perhaps these dots weren’t packed galaxies at all, but active galactic nuclei—supermassive black holes consuming matter at ferocious rates, surrounded by disks of incandescent gas. Such objects can outshine entire galaxies and are often obscured by thick clouds of dust, which would explain their deep red glow.
Yet this theory also had problems. The spectra of the red dots, when analyzed, didn’t match those of known active black holes shrouded in dust. The amount of mass required to power them seemed improbably high, and the sheer number of dots JWST detected suggested something else might be at play.
Astronomers were left in a tantalizing limbo, torn between models that didn’t quite fit the evidence.
Enter the RUBIES Program
Recognizing the importance of solving the puzzle, an international team of astronomers led by Anna de Graaff at the Max Planck Institute for Astronomy launched an ambitious project: the Red Unknowns: Bright Infrared Extragalactic Survey—RUBIES. Using nearly 60 hours of JWST’s precious observation time, they collected detailed spectra from more than 4,500 galaxies, including 35 of these enigmatic red dots.
One object in particular stood out. Found in July 2024, it was among the most extreme examples yet—its light taking nearly 12 billion years to reach Earth. The team nicknamed it The Cliff, after the steep “drop” in its spectral curve. The name wasn’t just poetic: its spectrum showed a striking Balmer break, a feature that usually indicates older galaxies with little star formation. But here, the break was so sharp and unusual that it defied every conventional model.
No standard explanation—whether galaxy or black hole—fit the data. The Cliff was something new.
The Birth of a Radical Idea: Black Hole Stars
Faced with the failure of existing models, de Graaff and her colleagues proposed a bold alternative. What if these dots were neither ordinary galaxies nor typical active black holes, but a hybrid of sorts—what they called black hole stars (abbreviated BH*)?
The concept is extraordinary. Imagine a supermassive black hole, millions of times the mass of the Sun, sitting at the center of a galaxy. Around it swirls an envelope of hydrogen gas so thick, so turbulent, that it engulfs the black hole in a cocoon. The black hole’s accretion disk heats this gas shell, making it glow in ways reminiscent of a star’s atmosphere—though unlike stars, there is no nuclear fusion at its core.
This “black hole star” would shine brightly in the infrared, its light shaped by the turbulence of its gaseous shell. Crucially, such a model naturally reproduces the strange cliff-like Balmer break seen in the spectrum of The Cliff, something no galaxy or black hole model could explain.
What Makes The Cliff Special
If the BH* interpretation holds, The Cliff is an extreme example in which the black hole star dominates the object’s brightness. Other small red dots may represent less extreme cases, where the light we see is a mix of the black hole star and surrounding starlight.
The Cliff’s spectrum doesn’t just hint at this possibility—it challenges us to think differently about how galaxies and black holes evolved in the early universe. Perhaps these strange hybrids were common in the past, their turbulent shells fueling rapid black hole growth.
This is especially exciting because JWST has already shown us that massive black holes existed surprisingly early, just a few hundred million years after the Big Bang. Black hole stars could provide the missing mechanism that allowed them to grow so quickly, bridging a gap in our understanding of cosmic history.
A New Path in Galaxy Evolution
The implications are profound. If black hole stars are real, they could represent a stage of cosmic evolution previously hidden from us. They may explain how galaxies in the early universe developed central black holes so rapidly, setting the stage for the galactic structures we see today.
The theory also pushes the boundaries of astrophysics. Black hole stars had been considered before, but only in purely theoretical work involving intermediate-mass black holes. To see them potentially emerge in JWST data—and at such extreme scales—suggests they could be more than a curiosity. They could be a key piece in the puzzle of galaxy formation.
Questions That Remain
As with any new discovery, caution is essential. The black hole star model is still in its infancy—a proof of concept, not yet a settled fact. Many questions remain. How do such enormous gas envelopes form and stay stable around black holes? How are they replenished as the black hole consumes them? Do they leave behind signatures we can detect beyond spectra, such as distinctive X-ray or radio emissions?
These are mysteries for future observations. De Graaff’s team already has JWST follow-up programs approved, targeting The Cliff and other promising red dots. Over the next few years, as more spectra come in, astronomers will be able to test whether black hole stars truly exist—or whether another explanation awaits discovery.
A Universe Full of Surprises
The story of the small red dots is a reminder of why we build telescopes like JWST in the first place. Each time we look deeper into the universe, we uncover phenomena that challenge our understanding and push science forward.
When astronomers first noticed those tiny crimson points in JWST’s images, they could not have imagined that they might reveal a new class of cosmic objects—part black hole, part star-like cocoon—reshaping our picture of the early cosmos.
Whether black hole stars prove to be the answer or not, the mystery of The Cliff shows us something even more profound: the universe still has secrets waiting to be uncovered, and each discovery brings us closer to understanding the vast story we are part of.
More information: Anna de Graaff et al, A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5, Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202554681
