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A sample of the “little red dot” (circle) discovered during the James Webb Space Telescope survey. . |Image source: Bangzheng “Tom” Sun
Astronomers may have found evidence of some mysterious ‘little red dots’ James Webb Space Telescope (JWST) is not a black hole as previously proposed, but a huge star at the beginning of the universe.
The team made the discovery by developing simplified models of supermassive ancient stars, the potential “parents” of the first supermassive stars. black hole in the universe.
The “little red dot” that existed during the first two billion years of the universe is one of JWST’s most surprising discoveries. Astronomers first suggested that the dense red objects might be active galactic nuclei (AGN), which are large galaxies powered by black holes that are rapidly accreting matter.
But the evidence is not simple. These objects are extremely tiny—smaller than expected for typical galaxies. So far, they have not shown significant X-ray emission, the main signature of actively feeding black holes. Their spectra also lack strong emission lines from metals other than hydrogen and helium, suggesting that the surrounding gas may be chemically primitive, unlike the metal-rich regions typically seen around active black holes.
This motivates Devesh Nandal Avi Loeb of the Harvard and Smithsonian Center for Astrophysics (CfA) explored another possibility: What if these compact objects are actually supermassive stars captured before they collapsed into black holes?
“If these little red dots don’t have X-rays now, they don’t show any other metallic lines, and if supermassive stars could form and exist, then we have shown that these stars would naturally produce the characteristics of these little red dots,” Nandal, a CfA postdoctoral researcher and lead author of the study, told Live Science. “For the first time we thought what we were seeing wasn’t a celebrity’s death signature.”
The team’s research was published on February 5 The Astrophysical Journal.
monster ancestor
The supermassive star discovered by Nandal and colleagues Formerly known as “Monster Star” – are extremely massive stars that formed primarily from primordial gas (mainly helium and hydrogen) in the early universe. They are classified as first generation stars, or Population III stars. Some models suggest that the mass of 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 Spot, astronomers developed a detailed model of a metal-free supermassive star with a mass approaching one million solar masses. The team paired their simulations with two small red dots, called MoM-BH*-1 and cliffdiscovered approximately 650 million and 1.8 billion years after the Big Bang, respectively. Models of supermassive stars match not only their extreme brightness, but also some important features in their spectra (the different wavelengths of light they emit).
A unique feature of the Little Red Dot is the distinctive “V-shaped” dip in its spectrum. Some explanations suggest that this shape occurs because dust absorbs light, giving the object a reddish appearance.
Illustration of supermassive stars in the early universe. |Image credit: James Webb Space Telescope (background), Nandal et al. (packing)
According to the new model, this shape is created by the star’s atmosphere, or outer layers. So it’s not the dust that changes the light, but the star’s own atmosphere that creates the effect.
“If supermassive stars are real, which we think they are because Population III stars are supposed to be real, then a little red dot would be the perfect place for them to hide,” Nandal said.
He thinks the V-shaped tilt and reddish appearance may also be related to the star’s mass loss, something like coronal mass ejection From the sun. But in this case, the material expelled from the star forms a dense shell-like structure around it. The mechanism of this mass loss is not fully understood. The team is working to improve models of the star’s outer atmosphere. They are also testing whether pulsations (rhythmic expansion and contraction) can lift material from the star’s surface, forming a separate shell of gas that cools and reddens the emitted light.
“This study works well as a theoretical exercise,” Daniel WhalenA senior lecturer at the Institute of Cosmology and Gravitation at the University of Portsmouth, who was not involved in the study, told Live Science. “It shows that supermassive stars can reproduce some of the features of the Little Red Dot spectrum.”
Astronomers estimate that a star this large can only stay bright for about 10,000 years. If the star is less massive – between 10,000 and 100,000 solar masses – it will shine for up to a million years. The reason is simple: the more massive a star is, the faster it burns nuclear fuel.
If the little red dot was a supermassive star in its final moments before collapsing into a black hole, the window for observation would be even shorter. The team pointed to the extreme mass and short life requirements as reasons why the new model cannot account for all the little red dots.
“It’s a very short window,” Whalen said. “If they were short-lived, it would be difficult to explain why around 400 to 500 small red dots were found.”
This or that?
The other leading explanation for the little red dot is the accretion of black holes, which may have formed from the direct collapse of hydrogen gas clouds in the early universe without the formation of normal stars first. Whalen is skeptical that supermassive star models have any advantages over this theory. “I don’t think it has a clear benefit over the black hole explanation,” he noted.
“If these objects were accreting a black hole, then at some point you might expect X-rays to leak out,” Nandal explained. “Detecting clear X-ray activity would be very beneficial for the interpretation of AGN.”
A black hole undergoing chaotic feeding or explosion should show some changes in its light output. However, so far no significant changes in brightness have been observed between the small red dots. Detecting some scintillation would favor the activity of AGNs and essentially rule out supermassive stars, since these stars would shine more steadily.
Detailed spectroscopic measurements reveal an abundance of chemicals around the small red spot, which could help support or rule out explanations for supermassive stars.
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“The answer really lies in the composition – what is this gas made of?” Nandal said. Previous simulations have shown that supermassive stars pollute their surroundings by producing large amounts of nitrogen through nuclear reactions. Strong neon lines, on the other hand, are more indicative of AGN activity.
Whalen points out that if black holes exist, any X-rays they produce may be absorbed by the surrounding dust. However, the radio emissions from these black holes can pass through dense clouds of hydrogen and dust and escape into space.
This means coming from something like Square Kilometer Array Or a next-generation Very Large Array could provide a decisive test. “If the little red dot was indeed powered by an obscured direct collapsing black hole, then radio waves would escape and we would detect them,” Whalen said.
