Small red dots glow at the edge of the universe. Could these be black holes disguised as stars?

Expectations were huge when the James Webb Telescope began operations in the summer of 2022. Many astronomers hoped the space telescope would change our view of the early universe.

They weren't disappointed. Even the first images from the James Webb Telescope revealed extremely compact and very red objects. These Little Red Dots were thought to be precursors to today's galaxies. Some of these objects, however, were very massive. Astronomers wondered how these galaxies managed to produce so many stars in just a few hundred million years.

Now it turns out that the question is probably misguided. In the journal "Astronomy & Astrophysics," an international working group presents evidence that the Little Red Dots are not galaxies at all, but exotic stars of a kind never before seen. Unlike the sun, these black hole stars are not caused to glow by nuclear fusion, but by an active black hole at their center.

The red color of the Little Red Dots reveals that these objects must be very distant. The wavelength of their light has been stretched by the expansion of the universe. Ultraviolet light has thus become infrared light, which can be analyzed with the James Webb Telescope.

The study of these objects has revealed numerous similarities. Among other things, the brightness of the Little Red Dots drops abruptly below a characteristic wavelength. This step is particularly striking in an object nicknamed "the Cliff." It was discovered as part of a program on the James Webb Telescope initiated by Anna de Graaff of the Max Planck Institute for Astronomy in Heidelberg.

De Graaff's research group compared the Klippe's light spectrum with the spectrum of compact and star-rich model galaxies. None of the simulated galaxies could explain the abrupt jump in brightness. Even the assumption that an active black hole lies at the center of the galaxy did not produce the desired agreement. The striking step in the Klippe's spectrum could not be reproduced. "For the first time, we were able to rule out that the Little Red Dots are star-rich galaxies," says de Graaff.

But what are the Little Red Dots? De Graaff's group noticed that the cliff's light spectrum resembles the spectrum of certain stars. This led the astronomers to believe that the Little Red Dots could be a hybrid of a star and a black hole. A supermassive black hole, according to the hypothesis, is surrounded by a dense shell of hydrogen gas. The intense radiation from the black hole heats the gas and causes it to glow. This compact black hole star is so bright that it outshines all the stars in the surrounding galaxy.

Using this model, the researchers were not only able to recreate the striking step in the cliff's light spectrum. The model also provides an explanation for why the object does not appear to emit X-rays, as black holes in the center of galaxies typically do. The gas envelope surrounding the black hole is so dense that most of the X-rays are shielded.

The cliff is not an isolated case. In a yet-to-be-reviewed publication, a group led by Rohan Naidu of the Massachusetts Institute of Technology analyzed another Little Red Dot with a pronounced step in its spectrum. The researchers also concluded that a black hole star best fits the data.

"These two Little Red Dots are extreme examples," says Pascal Oesch of the University of Geneva, who is involved in both studies. In the other Little Red Dots, the light from the black hole star overlaps with the light from normal stars in the surrounding galaxy. This results in a less pronounced step in the light spectrum.

Black hole stars aren't the only explanation for the Little Red Dots. As astrophysicist Avi Loeb of Harvard University recently demonstrated , the reddish spots could also be stars a million times more massive than the Sun. This could also explain the spectral signatures of the Little Red Dots.

Such massive stars can only form in the early universe. And they are extremely short-lived. After just a thousand years, their core collapses into a black hole. In such a supernova explosion, the outer layers of a star are usually ejected. This may have been different for the supermassive stars in the early universe. Their gaseous envelope may have been so dense that it survived the explosion. According to astrophysicist Mitchell Begelman of the University of Colorado in Boulder, this could be how black hole stars formed.

For now, these are hypotheses that need to be tested through further investigations. This could reveal a great deal about the early universe. As astronomers using the James Webb Telescope have discovered, black holes with masses several million times that of the sun existed just a few hundred million years after the Big Bang. How these objects managed to grow so large so quickly is a mystery. There are theoretical limits to their growth rate.

Black hole stars, which emerged from the collapse of supermassive stars, may have given black holes a head start. Their growth would then begin with a relatively large mass. Furthermore, the black hole in black hole stars is surrounded by dense gas. It therefore finds abundant nourishment in its surroundings and can grow accordingly rapidly.

In a recently published paper, Begelman argues that nearly every supermassive black hole in the universe could have passed through the stage of a black hole star. This is not necessarily the case. There is evidence that supermassive black holes can also form directly from a collapsing gas cloud, i.e., without the prior formation of a star.

The key to answering these questions is the James Webb Telescope. De Graaff's group has already been granted additional observing time to study Little Red Dots like the Cliff in more detail.