Black Holes or Black Hole Stars? Astronomers Spar Over Webb Telescope’s ‘Little Red Dots.’ | Quanta Magazine

Introduction Astronomers built the James Webb Space Telescope to pick up faint light from the first billion years after the Big Bang, a chaotic era when vast swaths of hydrogen and helium gas gathered into the chains of galaxies we see today. Even in the telescope’s first images, astronomers could see a whole zoo of mysterious smears of light. One batch of objects proved especially difficult to interpret. They glowed blindingly bright, emitting red light with long wavelengths and shining as brilliantly as a whole galaxy. They were tiny, spanning just a pixel. And they were everywhere. A couple appear in almost every image Webb takes. In 2023, researchers started calling them “little red dots.” Astronomers have repeatedly pointed Webb toward the little red dots, wringing precious new information from these pixels of light. Initially, researchers thought the dots looked kind of like galaxies. Later, they concluded that little red dots look more like the supermassive black holes that sit at the heart of most galaxies. These monstrous masses are themselves dark, but their formidable gravity violently vacuums up gas and other nearby matter, generating rings of hot, swirling detritus that completely outshine the stars around them. Then, in the spring of 2025, two teams of astronomers simultaneously announced observations of a pair of little red dots that were unlike all the rest. In fact, they were unlike any object ever seen. “In all the millions of [observations] we’ve taken with ground-based telescopes,” said Anna de Graaff, a researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany, and head of one group, “there’s nothing that looks like these sources.” The two teams of astronomers propose that they are looking at a new astronomical object: a topsy-turvy lump of hydrogen that shines with the light of billions of suns while hiding a black hole deep in its core. They call it a black hole star. In a paper posted last week, astronomers took this analysis a step further. They argued that the Webb telescope is witnessing the births of supermassive black holes inside the cores of colossal stars. “There is a fundamentally new phenomenon afoot,” said Rohan Naidu, an astronomer at the University of Hawai‘i. But not everyone agrees with this bold interpretation. It has sparked a flurry of follow-up research and reignited a fierce debate over the nature of these peculiar pinpricks of light. “The field has gotten very polarized,” said Anna-Christina Eilers, an astrophysicist at the Massachusetts Institute of Technology who studies little red dots. The Mystery of the Little Red Dots When all you can see is a speck, it’s hard to tell what you’re looking at. All you know about it is its color and brightness. Astronomers first argued that little red dots were distant galaxies on the cosmic horizon, mainly because of their brightness. But galaxies that bright would have to be huge — and there was no known way for them to grow so big in just hundreds of millions of years. Astronomers dubbed them “universe breakers” for the way they seemed to demolish the standard cosmic timeline. Then they took a closer look. De Graaff led one survey, called Red Unknowns: Bright Infrared Extragalactic Survey (Rubies), and Naidu co-led another survey, called Mirage or Miracle (MOM). These were two of a wave of surveys that trained Webb telescope on distant objects, including little red dots, for hours at a time. They tabulated precisely what shades of light were coming from each dot, and how bright the shades were. This detailed color breakdown, known as a spectrum, told astronomers a far more detailed story than the initial observations had. Different atoms shine in subtly different hues, so the spectrum provided a sense of what the object’s particles were doing. The bombshell discovery in the little red dot spectra was that the colors of hydrogen were smeared out across multiple shades. Usually, seeing such an effect means you’re looking straight at an exposed black hole. Black holes whip hydrogen clouds around them at furious rates, with the clouds emitting slightly different colors depending on their speed. The net effect is that instead of seeing just the hue of hydrogen, you see a range of colors called a broad line. The wider this range, the faster the fastest hydrogen clouds are flying — and the more massive the black hole. Many astronomers concluded that big black holes dotted the universe, washing out the light of the stars in their host galaxies. As black holes, the little red dots would appear red because dust — grainy stuff much more complicated than gas — was blocking their blue light. Yet they still seemed weird. Most supermassive black holes flicker as they gulp down chunky streams of gas around them. They also beam powerful X-rays across the universe. Most little red dots seemed to be doing neither of these things. But that didn’t trouble astronomers much; they expected to see some strangeness during the pandemonium of the early universe. And at least the black holes weren’t breaking any cosmological theories. Then, in the spring of 2025, de Graaff and Naidu’s teams unveiled the two strangest dots yet. A New Interpretation What made these two little red dots exceptional was how red they were. Webb picked up almost no light in the bluer hues of their spectra. And at a particular shade of red, the colors abruptly got much, much brighter. This feature, known as a Balmer break, is something you see when looking at a hot ball of hydrogen gas — typically, certain types of stars or galaxies (which are made of many stars). Deep in a star’s core, nuclear fusion pumps out heat and light, which slowly filters up to the star’s surface. There, hydrogen atoms can become energized in a way that blocks bluer light and lets through only redder light. These red colors have a hump-shaped spectrum that reveals the overall temperature of the star’s surface. But the new little red dots couldn’t literally be stars — they were way too bright. And they didn’t look much like black holes either. Black holes have an assortment of ringlike structures of different temperatures. They don’t typically produce a Balmer break, or the red, hump-shaped curve indicative of a stellar surface burning at a uniform 5,000 or so degrees Kelvin. Naidu and de Graaff concluded that they were looking at the first examples of something combining the vigor of a black hole with the outward appearance of a star: a black hole star. From the outside, a black hole star would appear as a huge agglomeration of hydrogen gas. If our sun were replaced with a black hole star, it would extend a dozen times farther than the orbit of Pluto. Out toward the edge, the star would boil unstably, sloughing off outer layers and explosively ejecting mass. “It’s going to be a very messy system where stuff is being blown out and falling back in,” de Graaff said. “I wouldn’t want to come too close.” Deep in the center, invisible to the outside world, the star would be powered by a black hole. This black hole would pull gas around it, dramatically heating it and pushing light and energy outward, which would keep the outer layers of hydrogen from collapsing inward. In this way, the black hole would form the “engine” of the star, analogous to the fusion-powered core of our sun. Moving outward, material swirling around the black hole would beam out a range of colors that would slowly make their way toward the surface. And as with certain stars, the hydrogen near the surface would stop the bluer light while letting the redder light pass through. The end result would be a gassy surface shining as brightly as a more exposed black hole but with the Balmer break and smooth red hump of a 5,000-kelvin star, de Graaff and Naidu theorized. As a bonus, the gas “cocoon” would also block X-rays, and the gas wouldn’t flicker much — which would explain two mysteries surrounding other little red dots. But what about the broad lines, supposedly caused by hydrogen swirling fast around a black hole? Another group provided a possible explanation. The group, which included Vadim Rusakov, an astronomer at the University of Manchester, had been scrutinizing the broad lines of the best-observed little red dots. Broad lines take the shape of a sharp mountain peak. But Rusakov and collaborators noticed that in many cases, these mountains sloped slightly more gently than would be expected if they came from fast-moving gas around a black hole. So they suggested that instead of coming from rotating gas, much of the spread of the hydrogen colors could come from light scattering off electrons. They digitally removed the effect of this electron-induced smudging from their data, Rusakov said. After that, the broad lines stopped looking quite so broad and started looking more like light passing through a sluggishly churning shell of hydrogen gas in a particular state, similar to what you’d expect to see from a black hole star. The three teams — de Graaff’s, Naidu’s, and Rusakov’s — posted their findings on March 20, 2025 — “black hole star date,” as some of the researchers called it. Black hole stars could represent a new stage in the development of a supermassive black hole: First, a black hole would form in the center of a shell of hydrogen, together with a baby galaxy of normal stars around it. Then, over time, the black hole would eat its way out of its cocoon, gaining mass as it cleared the hydrogen gas away. “We are seeing the seed,” Naidu said. “This is the birth of potentially every massive black hole in the universe.” The Argument Against The black hole star enthusiasts appeal to Occam’s razor, arguing that their theory gives the simplest accounting of these two little red dots, and perhaps of little red dots in general. But simple is subjective, and astronomers have spent the last year in a lively debate about what’s really going on.