Black hole stars could solve early universe puzzle

• Astronomers have discovered an object dubbed The Cliff. It could solve a riddle posed by some of the first observations of the distant universe with the Webb space telescope.
• Astronomers thought little red dots they saw in the early universe were young galaxies. But with such considerable mass, they would’ve been difficult to explain in current models of cosmic evolution.
• A new class of object called a black hole star could solve the problem. The Cliff led astronomers to propose this new class of object. The little red dots might not be galaxies at all but instead supermassive black holes embedded in a thick envelope of gas.

The Max Planck Institute for Astronomy published this original article on September 12, 2025. Edits by EarthSky.

Black hole stars could solve early universe puzzle

In the summer of 2022, less than a full month after the James Webb Space Telescope had begun to produce its first scientific images, astronomers noticed something unexpected: little red dots. Webb images showed a considerable number of extremely compact, bright red celestial objects quite clearly. It was apparently a whole new population of astronomical objects, which had eluded the Hubble space telescope. That latter part is unsurprising. In astronomy, when something is very red, it emits light predominantly at longer wavelengths. And the little red dots emit light at wavelengths beyond a 10 millionth of a meter, in the mid-infrared. Hubble is not designed to cover wavelengths this long, but Webb is.

Additional data showed these objects were far away indeed. Even the closest examples were so far away that their light had taken 12 billion years to reach us. So we see it as it was 12 billion years ago, a mere 1.8 billion years after the Big Bang.

Unexplainable young, massive galaxies?

When astronomers point to their data and say, “This is a star,” it’s trustworthy only because astronomers have robust physical models of what a star is. You also need a good understanding of how stars look, both in images and in its spectrum. In turn, if you see an object with the right kind of appearance and the right kind of spectrum, you can confidently state that it is a star.

The little red dots did not seem to fit into any of the usual slots. So astronomers set out to look beyond the standard objects. One of the first interpretations offered was a bombshell in and of itself: In this interpretation, little red dots were galaxies that were extremely rich in stars, their light reddened by huge amounts of surrounding dust. Within our own cosmic neighborhood, if you put our solar system in a cube 1 light-year a side, that cube would only contain a single star: our sun. In the star-rich galaxies postulated to explain little red dots, a cube that size would contain several hundred thousand stars.

In our home galaxy, the Milky Way, the only region that dense in stars is the central nucleus. But that contains only about 1/1000 of the stars needed in those little-red-dot models. The sheer number of stars involved, as high as hundreds of billions of solar masses’ worth less than a billion years after the Big Bang, raised major questions. Could we even explain how these galaxies produced so many stars, so quickly? Co-author Bingjie Wang (Penn State University) explains:

The night sky of such a galaxy would be dazzlingly bright. If this interpretation holds, it implies that stars formed through extraordinary processes which have never been observed before.

Galaxies vs. active galactic nuclei

So the interpretation remained controversial. The community split into two camps. One group favored the many-stars-plus-dust interpretation. Another group interpreted little red dots as active galactic nuclei, but also obscured by copious dust. Active galactic nuclei are what we see when a steady stream of matter falls onto a galaxy’s central black hole, forming an exceedingly hot so-called accretion disk around the central object.

But this second interpretation came with its own set of limitations. There are marked differences between the spectra of little red dots and those of the dust-reddened active galactic nuclei astronomers had previously observed. In addition, this interpretation would require extremely large masses for the supermassive black holes at the center of those objects. And surprisingly many of those, given the large number of little red dots found.

There was a consensus, too. In order to resolve the puzzle, astronomers would need more and different observational data. The original Webb observations had provided images. For testing physical interpretations, astronomers need spectra. And for the top telescopes, there is considerable competition for observing time. Once it became clear just how interesting little red dots were, numerous astronomers worldwide began to apply for time to observe them more closely.

One such application was the RUBIES program formulated by Anna de Graaff at the Max Planck Institute for Astronomy in Heidelberg and an international team of colleagues. The acronym stands for Red Unknowns: Bright Infrared Extragalactic Survey.

The distant treasures of RUBIES

The RUBIES application was successful. Between January and December 2024, astronomers used nearly 60 hours of Webb time to obtain spectra from a total of 4,500 distant galaxies. It was one of the largest spectroscopic data sets obtained with Webb to date. As Raphael Hviding (MPIA) said:

In that data set, we found 35 little red dots. Most of them had already been found using publicly available Webb images. But the ones that were new turned out to be the most extreme and fascinating objects.

Most interesting of all was the spectrum for an object the astronomers found in July 2024. The astronomers dubbed the object in question The Cliff. It seemed to be an extreme version of the population of little red dots. And, by that fact, a promising test case for interpretations of just what little red dots were. The Cliff is so distant from us that its light took 11.9 billion years to reach us.

The Cliff gets its name from the most prominent feature of its spectrum: a steep rise in what would be the ultraviolet region, at wavelengths just a little shorter than that of violet visible light. “Would” because our universe is expanding. A direct consequence is that, for an object as distant as The Cliff, that wavelength is stretched to almost five times its original value. So it lands squarely in the near-infrared.

A prominent rise of this kind, at these wavelengths, is known as a Balmer break. Balmer breaks can also be in the spectra of ordinary galaxies, usually in galaxies that form little to no new stars. But in those cases, the rise is much less steep than The Cliff.

A curious similarity to single stars

Thus, The Cliff looked like it did not fit any of the interpretations proposed for little red dots. But De Graaff and colleagues wanted to make sure. They constructed diverse variations of all the models that tried to cast little red dots either as massive star-forming galaxies or as dust-shrouded active galactic nuclei. And they attempted to reproduce the spectrum of The Cliff with each one and failed every single time.

De Graaff said:

The extreme properties of The Cliff forced us to go back to the drawing board, and come up with entirely new models.

By that time, the idea that Balmer-break features in a spectrum might be due to something other than stars had entered the discussion. De Graaff and her colleagues had started to wonder about something similar themselves. Balmer breaks can be found both in the spectra of single, very hot, young stars and in the spectra of galaxies containing a sufficient number of such very hot, young stars. Weirdly, The Cliff looked more like the spectrum of a single star than that of a whole galaxy.

Enter black hole stars

So de Graaff and her colleagues developed a model they’re calling a black hole star. It’s an active galactic nucleus – a supermassive black hole with an accretion disk – but surrounded and reddened by virtue of being embedded in a thick envelope of hydrogen gas.

The black hole star is not a star in the strict sense. There’s no nuclear fusion reactor in its center. In addition, the gas in the envelope is swirling much more violently than in any ordinary stellar atmosphere. But the basic physics is similar: The active galactic nucleus heats the surrounding gas envelope, just like the nuclear-fusion-driven center of a star heats the star’s outer layers, so the external appearance has marked similarities.

The models of de Graaff and colleagues at this point are proofs-of-concept. It’s pioneering work but not by any measure a perfect fit. Still, these black hole star models describe the data much better than any other type of model. In particular, a turbulent, dense, spherical gas envelope around an active galactic nucleus nicely explains the shape of the cliff in the spectrum. From that perspective, The Cliff would be an extreme example where the central black hole star dominates the object’s brightness. For the other little red dots, their light would be a more even mixture of the central black hole star with the light from stars and gas in the surrounding parts of the galaxy.

A new mechanism for rapid early galaxy formation?

If a black hole star is the solution, it might have another potential advantage. Previously, astronomers studied systems of this kind in a purely theoretical setting, with much lighter intermediate-mass black holes. There, the setup with central black hole and surrounding gas envelope was a way for the mass of very early galaxies’ central black holes to grow quickly.

Given that Webb has found solid evidence for high-mass black holes in the early universe, a configuration that could explain ultra-fast mass growth of black holes would be a welcome addition to current galaxy evolution models. We still don’t know whether the supermassive black hole stars can do the same. But it would be an intriguing expansion of their role if they did!

As promising as this sounds, cautions are in order. It’s a brand-new result. Reporting on it conforms with accepted practice of covering scientific results once they are published in, or at least accepted by, a peer-reviewed journal. But in order to know whether this becomes a trusted part of astronomy’s view of the universe, we will need to wait at least a few more years.

Questions remain with black hole stars

The present result does represent a major step forward. It’s the first model that can explain the unusual shape of The Cliff. Like any significant step forward, it leads to new, open research questions. How could such a black hole star have formed? How can the unusual gas envelope be sustained over a longer time? (Since the black hole gobbles up surrounding gas, there needs to be a mechanism for “refueling” the envelope.) How do the other features of the spectrum of The Cliff come about?

Answering those questions requires contributions from astrophysical modeling. But it is also set to benefit from further in-depth observation. In fact, de Graaff and her team already have the approval of Webb followup observations scheduled for next year.

These future observations will shed light on whether black hole stars are indeed the explanation for how today’s galaxies came to be what they are. At this point in time, that outcome is an intriguing possibility, but far from certain.

Bottom line: Astronomers have proposed black hole stars to explain the mysterious little red dots that Webb has seen in the early universe.

Source: 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

Via MPIA

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