Astronomers may have found evidence that some of the mysterious “little red dots” discovered by the James Webb Space Telescope (JWST) are not black holes, as previously proposed, but rather gigantic stars from the beginning of the universe.
The team made the discovery by developing a simplified model of supermassive ancient stars — the potential “parents” of the first supermassive black holes in the universe.
But the evidence has not been straightforward. The objects are extremely tiny — smaller than expected for typical galaxies. And so far, they show no clear X-ray emission, which is the primary signature of actively feeding black holes. Their spectra also lack strong metal emission lines beyond hydrogen and helium, hinting that the surrounding gas may be chemically primitive, unlike the metal-rich regions typically seen around actively feeding black holes.
This motivated Devesh Nandal and Avi Loeb of the Harvard and Smithsonian Center for Astrophysics (CfA) to explore a different possibility: What if these compact objects are actually supermassive stars caught just before they collapsed into black holes?
“If these little red dots now have no X-rays, they don’t show any of these other metal lines, and if supermassive stars can form and exist, then we have shown that such stars will naturally produce the features of these little red dots,” Nandal, a postdoctoral researcher at CfA and lead author of the study, told Live Science. “For the very first time, we think we’re not looking at some dead signature of a star.”
The team’s research was published Feb. 5 in The Astrophysical Journal.
Monster ancestors
Supermassive stars — which Nandal and colleagues have previously called “monster stars” — are extremely massive stars formed mainly from primordial gas, mostly helium and hydrogen, in the early universe. They are classified as the first generation of stars, or Population III stars. Some models suggest these early stars could grow to thousands to a million times the mass of the sun. When these stars die, they transform into supermassive black holes.
To explain the extreme brightness of the little red dots, astronomers developed a detailed model of a metal-free supermassive star with close to a million solar masses. The team compared their simulations with the features of two little red dots, dubbed MoM-BH*-1 and The Cliff, found around 650 million years and 1.8 billion years after the Big Bang, respectively. The supermassive star model matched not only their extreme brightness but also some important features in their spectra (the different wavelengths of light they emit).
One unique feature of the little red dots is a distinctive “V-shaped” dip in their spectra. Some interpretations suggest this shape occurs because dust absorbs light, which gives the object a reddish appearance.
According to the new model, this shape is produced by a star’s atmosphere, or outer layer. So, instead of dust altering the light, the star’s own atmosphere creates the effect.
“If supermassive stars are real, which we think they are because Population III stars should be real, then a little red dot would be the perfect place for them to hide,” Nandal said.
He suggested the V-shaped dip and the reddish appearance could also be linked to the star’s mass loss, somewhat analogous to coronal mass ejections from the sun. But in this scenario, material expelled from the star forms a compact, shell-like structure around it. The mechanism of this mass loss is not fully understood. The team is working to improve the models of stars’ outer atmospheres. They are also testing if pulsations — rhythmic expansions and contractions — could lift material off the stars’ surfaces, creating a detached shell of gas that cools and reddens the emitted light.
“The study works well as a theoretical exercise,” Daniel Whalen, a senior lecturer at the University of Portsmouth Institute of Cosmology and Gravitation who wasn’t involved in the study, told Live Science. “It shows that a supermassive star can reproduce some features of a little-red-dot spectrum.”
Astronomers estimate that a star this massive would remain bright for only about 10,000 years. If the star were less massive — between 10,000 to 100,000 solar masses — then it would shine for up to a million years. The reason is simple: The more massive the star, the faster it burns through its nuclear fuel.
If little red dots are supermassive stars in their final moments before collapsing into black holes, that leaves an even shorter window for observation. The team noted that the requirements of extreme mass and a short lifetime are why not all little red dots can be explained by the new model.
“That’s an extremely short window,” Whalen said. “It makes it hard to explain how around 400 to 500 little red dots were discovered if they have short lives.”
This or that?
Another leading explanation for the little red dots involves accreting black holes, possibly formed from the direct collapse of hydrogen gas clouds in the early universe, without first forming normal stars. Whalen is skeptical that the supermassive-star model offers an advantage over that theory. “I don’t see that it provides a clear benefit over black hole interpretations,” he noted.
“If these objects are accreting black holes, at some point you might expect X-rays to leak out,” Nandal explained. “Detecting clear X-ray activity would strongly favor the AGN interpretation.”
Black holes that are undergoing chaotic feeding or explosions should exhibit some variability in their light output. So far, however, no clear brightness variability has been observed among little red dots. Detection of some flickering would favor AGN activity and essentially rule out supermassive stars, as these stars would emit light more steadily.
Detailed spectroscopic measurements showing the abundance of chemicals around little red dots would help support or rule out the supermassive-stars interpretation.
“The answer is really in the ingredients — what is this gas made of?” Nandal said. Previous simulations have shown that supermassive stars contaminate their surroundings with enormous amounts of nitrogen via nuclear reactions. On the other hand, strong neon lines would be more indicative of AGN activity.
Whalen noted that if black holes are present, any X-rays they produced could simply be absorbed by surrounding dust. Radio emissions from these black holes, however, could pass through dense hydrogen clouds and dust and escape into space.
That means highly sensitive radio observations from facilities such as the Square Kilometre Array or the next-generation Very Large Array could provide a decisive test. “If little red dots really are powered by shrouded direct-collapse black holes, the radio waves will get out, and we’ll detect them,” Whalen said.
