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 suggested, but rather giant 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 simple. The objects are extremely small – 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, suggesting 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 Harvard and the Smithsonian Center for Astrophysics (CfA) to explore another possibility: What if these compact objects are actually supermassive stars trapped just before they collapsed into black holes?
“If these little red dots now don’t have X-rays, they don’t show any of these other metal lines, and if supermassive stars can form and exist, then we’ve shown that such stars will naturally produce the features of these little red dots,” Nandal, a postdoctoral fellow at CfA and lead author of the study, told LiveScience. “For the very first time, we believe we are not looking at a dead signature of a star.”
The team’s research was published on February 5 in The Astrophysical Journal.
Monster’s ancestors
Supermassive stars — like Nandal and his colleagues have formerly 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 that 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 tiny 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 small red dots, called MoM-BH*-1 and The Clifffound respectively around 650 million years and 1.8 billion years after the Big Bang. The supermassive star model matched not only their extreme luminosity, but also some important features of their spectra (the different wavelengths of light they emit).
A unique feature of the small red dots is a characteristic “V-shaped” dip in their spectra. Some interpretations suggest that this shape occurs because dust absorbs light, giving 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 changing 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 that the V-shaped dip and reddish appearance could also be linked to the star’s mass loss, something analogous to coronal mass ejections from the sun. But in this scenario, material ejected from the star forms a compact, shell-like structure around it. The mechanism for this mass loss is not fully understood. The team is working to improve models of stars’ outer atmospheres. They are also testing whether pulsations – rhythmic expansions and contractions – can lift material from 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 Whalena senior lecturer at the University of Portsmouth Institute of Cosmology and Gravitation who was not involved in the study, told LiveScience. “It shows that a supermassive star can reproduce some features in a small red dot spectrum.”
Astronomers estimate that such a massive star will remain bright for only about 10,000 years. If the star were less massive – between 10,000 and 100,000 solar masses – it would shine for up to a million years. The reason is simple: the more massive the star, the faster it burns through its core fuel.
If tiny 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 extreme mass and short lifetime requirements are why not all small red dots can be explained by the new model.
“It’s an extremely short window,” Whalen said. “That makes it difficult to explain how about 400 to 500 small red dots were detected if they are short-lived.”
This or that?
Another leading explanation for the small red dots involves the accretion of 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 offers a clear advantage over black hole interpretations,” he noted.
“If these objects are accreting black holes, at some point you can expect X-rays to leak out,” Nandal explained. “Detecting clear X-ray activity would strongly favor the AGN interpretation.”
Black holes undergoing chaotic feeding or explosions should show some variation in their luminosity. So far, however, no clear brightness variation among small red dots has been observed. Detection of some flicker would favor AGN activity and essentially rule out supermassive stars, as these stars would emit light more uniformly.
Detailed spectroscopic measurements showing chemical abundances around small red dots will help support or rule out the interpretation of supermassive stars.
“The answer really lies 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, the X-rays they produced could simply be absorbed by surrounding dust. However, radio emissions from these black holes can pass through dense hydrogen clouds and dust and escape into space.
This means very sensitive radio observations from facilities such as Square Kilometer Array or the next-generation Very Large Array could provide a decisive test. “If tiny red dots are really powered by shrouded black holes with direct collapse, the radio waves will come out and we will detect them,” Whalen said.
