SPACE/COSMOS
IMPERIALISM IN SPACE
Space law crisis: Outer space security in an insecure world
If we can't get along on Earth, how will we ever get along in space, right? Experts told DW there are reasons to be optimistic.
Future security in space may be driven by private actors, such as tech billionaire Jared Isaacman, seen here on the first private spacewalk
Of all the threats in space, it's what the UN calls "a blurring of the line between civilian and military uses" that fires the imagination most. But there are other concerns: collisions between satellites, flight congestion, space debris hitting other spacecraft or falling to Earth, asteroids...
Viewed as a mass of problems, it may seem as though we're "staring at a perceived wild tiger," says Helen Tung, a space lawyer and lecturer based at the University of Huddersfield, UK. "It automatically engages our fear mechanisms," she told DW.
But you get what you want, Tung added: If you want war, you do everything to get war. If you want peace, you do what you can to get peace.
"I don't think we can say we want space development, that we want to engage more countries, we want peace and prosperity, and yet act contrary to that," said Tung. "If the intention is there, there must be a way to say, 'What can we do to move things forward?' And I think it's the private space sector which is going to push the legislation and states to change."
And change they must. A significant set of space treaties and legal frameworks for space exploration, science and communication are stuck in the past.
Some countries, such as Luxembourg, have tried to circumnavigate global treaties by passing their own legislation to allow commercial companies to mine for minerals in space and keep the rewards. But with more states entering space, from India and the UAE to Nigeria, Luxembourg's work around may not stand the test of time.
Space law designed for the Cold War
Time was when space was simple — a simple case of two states, the United States and the Soviet Union,(USSR) battling it out for supremacy — from the first dog and then the first person to orbit Earth (USSR) to landing and walking on the moon (US).
For a while, space was a game of just two players, with the rules laid out in a neat, four-page agreement, the 1967 Outer Space Treaty. This is still a guiding framework for human activity in space, but it is out of step with the times.
"The Outer Space Treaty is quite basic. It prohibits the stationing of weapons of mass destruction in space; it says space should be used for peaceful purposes. And that's kind of it. You could argue the bar is quite low," says Juliana Süß, an associate at the German Institute for International and Security Affairs in Berlin.
Now, said Süß in an interview with DW, "we're not only in a security dynamic and environment in which there's a whole host of different threats, a whole host of different actors, but we also have commercial players in the mix. One of them, SpaceX, with its Starlink constellation, owns [nearly 50% of active satellites in orbit]. So, the environment has changed. And we can't argue that the legal requirements have been in step with that because we're just lacking."
When the US launched renewed ambitions to get to the moon, it set up the Artemis Accords in 2020 — an agreement, which the US controls, with countries around the world.
But even these accords are lacking, believes Malcolm Macdonald, a professor of satellite engineering at the University of Strathclyde in Glasgow, Scotland, and president-elect of the UK's Royal Aeronautical Society.
"The Artemis Accords are really just an agreement among already aligned countries that hope to set norms of behavior, meanwhile other countries do this by seeking to create realities," wrote Macdonald in an email to DW.
He argues that the return to the moon, for example, is an attempt by China to create realities of behavior that suit them, and by US and others to do likewise.
"Until recently I had assumed that once the US realized China was looking like getting to the moon before the US returns there, the US would speed up what it's doing. But it seems the current US government is not willing to hear this," he said.
So, Artemis Accords or not, Macdonald thinks China will "do what it wants," and it will be "even easier [for them] if they get to the moon first."
Faced then with questions such as: Can diplomatic norms protect satellites or do treaties still work? Can voluntary diplomatic norms protect satellites, or how can we have established binding treaties? Or even the space arms race, can it be stopped? — it is perhaps understandable when Macdonald replies:
"One easy answer. Three times. No."
The outlook is depressing but it needn't be so.
Yes, Donald Trump declared space a military domain in his first term as US president and is now setting up a US Space Force.
And, yes, Russian President Vladimir Putin has considered foreign commercial satellites legitimate targets in war since October 2022, soon after the invasion of Ukraine.
Even on day-to-day, routine space maintenance, there is "potential for miscalculation, misunderstanding, miscommunication," thinks Süß, because Russia and China do not always share data, even on unintended satellite collisions.
According to the German expert, UN working groups on the prevention of an outer space arms race and transparency and confidence building in space activities have revealed that a lot of states have similar feelings and have similar motivations.
"So, talking about norms and behaviors is the way to go because when it comes to the treaties, people are going to pick apart things such as, 'How do you define a space weapon?' We've been stuck on that debate for decades, especially with dual-use technologies and robotics, and I don't think we can solve it right now."
Meanwhile, for the private sector it may mean introducing financial penalties for bad actors in space by involving new forms of insurance — it works in maritime law, said Tung: "That's going to be a game changer when the private sector steps up and says, 'If you do shoot this satellite, insurers will get involved, there'll be an economic cost to this."
Ultimately, or at least for now, "everything that we see happening in space," said Süß, "is a reflection of how we act on Earth."
Edited by: Uwe Hessler
Zulfikar Abbany Senior editor fascinated by space, AI and the mind, and how science touches people
How interstellar objects similar to 3I/ATLAS could jump-start planet formation around infant stars
Europlanet
image:
Interstellar object 3I/ATLAS imaged by the Hubble Space Telescope. Could similar objects be the seeds of new planets around young stars?
view moreCredit: NASA/ESA/David Jewitt (UCLA). Image Processing: Joseph DePasquale (STScI).
Interstellar objects like 3I/ATLAS that have been captured in planet-forming discs around young stars could become the seeds of giant planets, bypassing a hurdle that theoretical models have previously been unable to explain.
Interstellar objects are asteroid- and comet-like bodies that have been ejected from their home system and now wander through interstellar space, occasionally encountering other star systems. Since 2017 astronomers have detected three interstellar objects passing through our Solar System: 1I/’Oumuamua, 2I/Borisov and most recently 3I/ATLAS, discovered in summer 2025.
However, interstellar objects may be more influential than they at first appear to be, says Professor Susanne Pfalzner of Forschungszentrum Jülich in Germany, who presents her new findings on the subject at this week’s EPSC-DPS2025 Joint Meeting in Helsinki.
“Interstellar objects may be able to jump start planet formation, in particular around higher-mass stars,” said Pfalzner.
Planets form in dusty discs around young stars through a process of accretion, which according to theory involves smaller particles come together to form slightly larger objects, and so on until planet-sized bodies have assembled. However, theorists struggle to explain how anything larger than a metre forms through accretion in the hurly-burly of a planet-forming disc around a young star – in computer simulations, boulders either bounce off each other or shatter when they collide rather than sticking together.
Interstellar objects can potentially bypass this problem. Pfalzner’s models show how the dusty planet-forming disc around each young star could gravitationally capture millions of interstellar objects the size of 1I/’Oumuamua, which was estimated to be around 100 metres long.
“Interstellar space would deliver ready-made seeds for the formation of the next generation of planets,” said Pfalzner.
If interstellar objects can act as the seeds of planets, it also solves another mystery. Gas giant planets like Jupiter are rare around the smallest, coolest stars, which astronomers refer to as ‘M dwarfs’. They are more commonly found around more massive stars similar to the Sun. The problem, though, is that planet-forming discs around Sun-like stars have a lifetime of about two million years before dissipating and it’s very challenging to form to form gas giant planets on such a short timescale. However, if captured interstellar objects are present as seeds onto which more material can accrete, it speeds the process of planet formation up and giant planets can form in the lifetime of the disc.
“Higher-mass stars are more efficient in capturing interstellar objects in their discs,” said Pfalzner. “Therefore, interstellar object-seeded planet formation should be more efficient around these stars, providing a fast way to form giant planets. And, their fast formation is exactly what we have observed.”
Pfalzner says that her next steps are to model the success rate of these captured interstellar objects – investigating how many of the millions of captured interstellar objects are able to form planetary bodies, and whether they are captured evenly across a planet-forming disc, or whether they are concentrated in certain areas that could become hotspots for planet-birth
Planets without plate tectonics and too little carbon dioxide could mean that technological alien life is rare
image:
An artist’s impression of the rocky, habitable-zone exoplanet Kepler-168b.
view moreCredit: NASA Ames/NASA/JPL–Caltech/Tim Pyle (Caltech).
The closest technological species to us in the Milky Way galaxy could be 33,000 light years away and their civilisation would have to be at least 280,000 years, and possibly millions of years, old if they are to exist at the same time that we do, according to new research presented at the EPSC–DPS2025 Joint Meeting in Helsinki this week.
These numbers are reflective of the strong odds against finding Earth-like worlds with plate tectonics and a nitrogen-oxygen dominated atmosphere with just the right amount of oxygen and carbon dioxide.
By considering these factors, the possibility for the success of SETI (Search for Extraterrestrial Intelligence) seems bleak, according to Dr Manuel Scherf and Professor Helmut Lammer of the Space Research Institute at the Austrian Academy of Sciences in Graz.
“Extraterrestrial intelligences – ETIs – in our galaxy are probably pretty rare,” says Scherf.
The more carbon dioxide a planet has in its atmosphere, the longer it can sustain a biosphere and photosynthesis for, and prevent the atmosphere from escaping into space, but it’s a careful balance: too much carbon dioxide and it can lead to a runaway greenhouse effect, or render the atmosphere too toxic for life. Plate tectonics regulate the amount of carbon dioxide in the atmosphere as part of the carbon-silicate cycle, and so a habitable planet requires plate tectonics. Gradually, though, the carbon dioxide that is drawn out of the atmosphere gets locked away in rocks rather than recycled.
“At some point enough carbon dioxide will be drawn from the atmosphere so that photosynthesis will stop working,” says Scherf. “For the Earth, that’s expected to happen in about 200 million to roughly one billion years.”
Earth’s atmosphere is dominated by nitrogen (78 per cent) and oxygen (21 per cent), but it also contains trace gases including carbon dioxide (0.042 per cent). Scherf and Lammer consider what would happen on a planet with ten per cent carbon dioxide (such a planet could avoid a runaway greenhouse if it is further from its sun, or its sun is younger and less luminous) and find that its biosphere could be maintained for 4.2 billion years. Alternatively, an atmosphere with one-per-cent carbon dioxide would maintain a biosphere for a maximum of 3.1 billion years.
These worlds would also need no less than 18 per cent oxygen. Not only is more oxygen needed by larger, complex animals, but previous studies have shown that if oxygen levels fall below this then there is not enough free oxygen to enable open-air combustion. Without fire the smelting of metal would be unfeasible and a technological civilisation would be impossible.
Scherf and Lammer then contrasted these biosphere lifespans with the amount of time it takes for technological life to evolve, which on Earth was 4.5 billion years, and the possible lifetime of a technological species. This is important because the longer their species survives, the greater the chance that they will exist at the same time that we do.
Combining all these factors is what led Scherf and Lammer to the conclusion that technological species living on a planet with 10 per cent carbon dioxide would have to survive for at least 280,000 years for there to even be one other civilisation in the galaxy at the same time we are.
“For ten civilisations to exist at the same time as ours, the average lifetime must be above 10 million years,” says Scherf. “The numbers of ETIs are pretty low and depend strongly upon the lifetime of a civilisation.”
This means that if we do detect an ETI, it is almost certainly going to be much older than humanity.
It’s these numbers that also lead to the estimate that the next closest technological civilisation is about 33,000 light years away. Our Sun is about 27,000 light years from the galactic centre, which means that the next closest technological civilisation to our own could be on the other side of the Milky Way.
These numbers are not absolutes – Scherf points out that there are other factors that should be included, such as the origin of life, the origin of photosynthesis, the origin of multi-cellular life and the frequency with which intelligent life develops technology, but they cannot be quantified at present. If each of these factors has a high probability, then ETIs might not be as rare. If each of these factors has a low probability, then a more pessimistic outlook is required.
Nevertheless, Scherf strongly believes that SETI should continue the search.
“Although ETIs might be rare there is only one way to really find out and that is by searching for it,” says Scherf. “If these searches find nothing, it makes our theory more likely, and if SETI does find something, then it will be one of the biggest scientific breakthroughs ever achieved as we would know that we are not alone in the Universe.”
An artist’s impression of our Milky Way Galaxy, showing the location of the Sun. Our Solar System is about 27,000 light years from the centre of the galaxy. The nearest technological species could be 33,000 light years away.
Credit
NASA/JPL–Caltech/R. Hurt (SSC–Caltech).
This graph shows the maximum number of ETIs presently existing in the Milky Way. The solid orange line describes the scenario of planets with nitrogen–oxygen atmospheres with 10 per cent carbon dioxide. In this case the average lifetime of a civilization must be at least 280,000 years for a second civilization to exist in the Milky Way. Changing the amount of atmospheric carbon dioxide produces different results.
Credit
Manuel Scherf and Helmut Lammer.
Manuel Scherf and Helmut Lammer.
Mysterious ‘red dots’ in early universe may be ‘black hole star’ atmospheres
The objects that astronomers at Penn State dubbed ‘universe breakers’ could be an exotic black hole atmosphere, representing a missing link in the rapid growth of supermassive black holes
Penn State
image:
Artist’s impression of a black hole star (not to scale). Mysterious tiny pinpoints of light discovered at the dawn of the universe may be giant spheres of hot gas that are so dense they look like the atmospheres of typical nuclear fusion-powered stars; however, instead of fusion, they are powered by supermassive black holes in their center that rapidly pull in matter, converting it into energy and giving off light.
view moreCredit: T. Müller/A. de Graaff/Max Planck Institute for Astronomy
UNIVERSITY PARK, Pa. — Tiny red objects spotted by NASA's James Webb Space Telescope (JWST) are offering scientists new insights into the origins of galaxies in the universe — and may represent an entirely new class of celestial object: a black hole swallowing massive amounts of matter and spitting out light.
Using the first datasets released by the telescope in 2022, an international team of scientists including Penn State researchers discovered mysterious “little red dots.” The researchers suggested the objects may be galaxies that were as mature as our current Milky Way, which is roughly 13.6 billion years old, just 500 to 700 million years after the Big Bang.
Informally dubbed “universe breakers” by the team, the objects were originally thought to be galaxies far older than anyone expected in the infant universe — calling into question what scientists previously understood about galaxy formation.
Now, in a paper published today (Sept. 12) in the journal Astronomy & Astrophysics, the international team of astronomers and physicists, including those at Penn State, suggest that the dots may not be galaxies but an entirely new type of object: a black hole star.
They said their analysis indicates that the tiny pinpoints of light may be giant spheres of hot gas that are so dense they look like the atmospheres of typical nuclear fusion-powered stars; however, instead of fusion, they are powered by supermassive black holes in their center that rapidly pull in matter, converting it into energy and giving off light.
“Basically, we looked at enough red dots until we saw one that had so much atmosphere that it couldn't be explained as typical stars we’d expect from a galaxy,” said Joel Leja, the Dr. Keiko Miwa Ross Mid-Career Associate Professor of Astrophysics at Penn State and co-author on the paper. “It’s an elegant answer really, because we thought it was a tiny galaxy full of many separate cold stars, but it’s actually, effectively, one gigantic, very cold star.”
Cold stars emit little light due to their low temperatures compared to normal stars, Leja explained. Most stars in the universe are low-mass, colder stars, but they are typically harder to see as they are washed out by rarer, more luminous massive stars. Astronomers identify cold stars by their glow, which is primarily in the red optical or near-infrared spectrum, wavelengths of light that are no longer visible. While the gas around supermassive black holes is typically very hot, millions of degrees Celsius, the light from these “red dot” black holes was instead dominated by very cold gas, the researchers said, similar to the atmospheres of low-mass, cold stars, based on the wavelengths of light they were giving off.
The most powerful telescope in space, JWST was designed to see the genesis of the cosmos with infrared-sensing instruments capable of detecting light that was emitted by the most ancient stars and galaxies. Essentially, the telescope allows scientists to see back in time roughly 13.5 billion years, near the beginning of the universe as we know it, Leja explained.
From the moment the telescope turned on, researchers around the world began to spot “little red dots,” objects that appeared far more massive than galaxy models predicted. At first, Leja said, he and his colleagues thought the objects were mature galaxies, which tend to get redder as the stars within them age. But the objects were too bright to be explained — the stars would need to be packed in the galaxies with impossible density.
“The night sky of such a galaxy would be dazzlingly bright,” said Bingjie Wang, now a NASA Hubble Fellow at Princeton University who worked on the paper as a postdoctoral researcher at Penn State. “If this interpretation holds, it implies that stars formed through extraordinary processes that have never been observed before.”
To better understand the mystery, the researchers needed spectra, a type of data that could provide information about how much light the objects emitted at different wavelengths. Between January and December 2024, the astronomers used nearly 60 hours of Webb time to obtain spectra from a total of 4,500 distant galaxies. It is one of the largest spectroscopic datasets yet obtained with the telescope.
In July 2024, the team spotted an object with a spectrum that indicated a huge amount of mass, making it the most extreme case of such an early and large object. The astronomers nicknamed the object in question “The Cliff,” flagging it as the most promising test case to investigate just what those “little red dots” were.
“The extreme properties of The Cliff forced us to go back to the drawing board, and come up with entirely new models,” said Anna de Graaff, a researcher for the Max Planck Institute for Astronomy and corresponding author on the paper, in a Max Planck Institute press release.
The object was so distant that its light took roughly 11.9 billion years to reach Earth. The spectra analysis of that light indicated it was actually a supermassive black hole, pulling in its surroundings at such a rate that it cocooned itself in a fiery ball of hydrogen gas. The light that Leja and his colleagues spotted was coming not from thick clusters of stars, but from one giant object.
Black holes are at the center of most galaxies, Leja explained. In some cases, those black holes are millions or even billions of times more massive than our solar system’s sun, pulling in nearby matter with such strength that it converts to energy and shines.
“No one's ever really known why or where these gigantic black holes at the center of galaxies come from,” said Leja, who is also affiliated with Penn State's Institute for Computational and Data Sciences. “These black hole stars might be the first phase of formation for the black holes that we see in galaxies today — supermassive black holes in their little infancy stage.”
He added that JWST has already found signs of high-mass black holes in the early universe. These new black hole star objects, which are essentially turbocharged mass-builders, could help explain the early evolution of the universe — and may be a welcome addition to current models. The team is planning future work to test this hypothesis by examining the density of gas and strength of these early black hole stars, Leja said.
Of course, the mysterious “little red dots” are great distance away in both time and space — and their small size makes it especially challenging to get a clear picture.
“This is the best idea we have and really the first one that fits nearly all of the data, so now we need to flesh it out more,” Leja said. “It's okay to be wrong. The universe is much weirder than we can imagine and all we can do is follow its clues. There are still big surprises out there for us.”
A full list of authors is available in the paper. The Penn State aspects of this work were funded by NASA.
Journal
Astronomy and Astrophysics
Method of Research
Imaging analysis
Subject of Research
Not applicable
Article Title
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
Article Publication Date
12-Sep-2025
“Black Hole Stars” could solve JWST riddle of overly massive early galaxies
image:
Artist’s impression of a black hole star (not to scale). The cut-out reveals the central black hole with its surrounding accretion disk. What makes this a black hole star is the surrounding envelope of turbulent gas. This configuration can explain what astronomers observe in the object they are calling “The Cliff.”
view moreCredit: MPIA/HdA/T. Müller/A. de Graaff
In the summer of 2022, less than a full month after the James Webb Space Telescope (JWST) had begun to produce its first scientific images, astronomers noticed something unexpected: little red dots. In pictures taken at JWST’s unprecedented sensitivity, these extremely compact, very red celestial objects showed very clearly on the sky and there appeared to be a considerable number of them. JWST had apparently discovered a whole new population of astronomical objects, which had eluded the Hubble Space Telescope. That latter part is unsurprising. “Very red” is astronomy lingo for objects that emit light predominantly at longer wavelengths. The little red dots emit light predominantly at wavelengths beyond a 10 millionth of a meter, in the mid-infrared. Hubble cannot observe at wavelengths this long. JWST, on the other hand, is designed to cover this range.
Additional data showed that 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. Astronomers always look into the past, and we see an object whose light takes 12 billion years to reach us as it was those 12 billion years ago, a mere 1.8 billion years after the Big Bang.
Unexplainable young, massive galaxies?
This is where things get dicey. In order to interpret astronomical observations, you need a model of the object in question. When astronomers point to their data and say, “This is a star,” the statement comes with a lot of baggage. It is trustworthy only because astronomers have robust physical models of what a star is – in short, a giant plasma ball held together by its own gravity, producing energy by nuclear fusion in its center. You also need a good understanding of how stars look both in images and in the rainbow-like decomposition of light known as a 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 neighbourhood, if you put our solar system in a cube one 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 one thousandth 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 about astronomers’ basic understanding of galaxy evolution: 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
The interpretation itself remained controversial. The community split into two camps: One group that favored the many-stars-plus-dust interpretation, and another that 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 that had been found.
There was a consensus, too: that in order to resolve the puzzle, astronomers would need more and different observational data. The original JWST observations had provided images. For testing physical interpretations, astronomers need spectra: detailed information about how much light an object emits at different wavelengths. 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, where the acronym stands for “Red Unknowns: Bright Infrared Extragalactic Survey.”
The distant treasures of RUBIES
The RUBIES application was successful, and between January and December 2024, the astronomers used nearly 60 hours of JWST time to obtain spectra from a total of 4500 distant galaxies, one of the largest spectroscopic data sets obtained with JWST to date. As Raphael Hviding (MPIA) says, “In that data set, we found 35 little red dots. Most of them had already been found using publicly available JWST images. But the ones that were new turned out to be the most extreme and fascinating object.” 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,” and it seemed to be an extreme version of the population of little red dots – and by that very 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 (redshift z=3.55).
“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, landing squarely in the near-infrared (“cosmological redshift”). A prominent rise of this kind, at these wavelengths, is known as a “Balmer break.” Balmer breaks can be found in the spectra of ordinary galaxies, where they are usually seen in galaxies that form little to no new stars at the time. But in those cases, the rise is much less steep than The Cliff.
A curious similarity to single stars
With this unmissable, unusual feature, The Cliff looked like it did not fit any of the interpretations that had been proposed for little red dots. But De Graaff and her 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, attempted to reproduce the spectrum of The Cliff with each one, and failed every single time.
Anna de Graaff says: „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 (in the shape of a September 2024 article by two researchers based in China and the UK). De Graaff and her colleagues had started to wonder about something very 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 hole galaxy.
Enter black hole stars
On this basis, de Graaff and her colleagues developed a model some of them have taken to calling a “black hole star,” written as BH*: An active galactic nucleus, that is, a supermassive black hole with an accretion disk, but surrounded and reddened not by dust, but by virtue of being embedded in a thick envelope of hydrogen gas. The BH* is not a star in the strict sense, since there is no nuclear fusion reactor in its center. In addition, the gas in the envelope is swirling much more violently (there is much stronger turbulence) 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 formulated by de Graaff and colleagues at this point are proofs-of-concept – 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, the shape of the name-giving cliff in the spectrum is nicely explained by assuming a turbulent, dense, spherical gas envelope around an AGN. 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 indeed the solution, it might have another potential advantage. Systems of this kind had previously been studied in a purely theoretical setting, with much lighter intermediate-mass black holes. There, the setup with central black hole and surrounding gas envelope was seen as a way for the mass of a very early galaxies’ central black holes growing particularly quickly. Given that JWST 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. Whether the supermassive black hole stars can do the same is still undetermined, but it would be an intriguing expansion of their role if they did!
As promising as this sounds, caveats are in order. The new result is brand-new. 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.
Open questions
The present result does represent a major step forward: the first model that can explain the unusual shape of The Cliff, the extreme object’s Balmer break. 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 JWST follow-up observations for little red dots of particular interest, such as The Cliff, 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.
Background information
The results described here have been accepted for publication as A. 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” in the journal Astronomy & Astrophysics. The paper led by Raphael Hviding that presents the full sample of Little Red Dots in the RUBIES data set has been accepted for publication in the same journal,
The MPIA researchers involved are Anna de Graaff, Hans-Walter Rix and Raphael E. Hviding, in collaboration with Gabe Brammer (Cosmic Dawn Center), Jenny Greene (Princeton University), Ivo Labbe (Swinburne University), Rohan Naidu (MIT), Bingjie Wang (Penn State University and Princeton University), and others.
Journal
Astronomy and Astrophysics
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
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
Article Publication Date
10-Sep-2025
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