Astronomers have confirmed the existence of black hole stars in the universe's first billion years, based on observations from the James Webb Space Telescope. These objects, known as little red dots, are vast balls of gas powered by central black holes that glow like enormous stars. The finding resolves a key mystery about these compact, bright galaxies.
The James Webb Space Telescope (JWST) has revealed a surprising population of objects in the early universe, dating back to its first billion years. Dubbed little red dots (LRDs), these appear as extremely compact, red, and luminous galaxies, differing from those observed in the nearby universe. Initial theories suggested they might be supermassive black holes surrounded by dust or densely packed star-filled galaxies, but neither fully accounted for the detected light patterns.
A team led by Anna de Graaff at Harvard University proposed an alternative: black hole stars. These are dense spheres of gas with a black hole at the core. As material accretes onto the black hole, gravitational energy release causes the surrounding gas to glow, mimicking a star but on a massive scale—billions of times brighter than the sun. "When material falls into the black hole, a lot of gravitational energy is released, and this could make the whole ball of gas around it glow like a star," de Graaff explained.
Analyzing spectra from over 100 LRDs, the researchers found patterns matching blackbody radiation from a smooth surface, similar to stars, rather than the complex spectra of galaxies with mixed light sources. "The black hole star model has been around for a while but was thought to be so weird and out there, but it actually does seem to work and make the most sense," said Jillian Bellovary at the American Museum of Natural History in New York. Anthony Taylor at the University of Texas at Austin added, "It’s just a simple framework, but it explains [observations] really, really nicely, without needing any real exotic physics."
One standout LRD, nicknamed "The Cliff," showed spectral features unexplained by prior models, bolstering the case for black hole stars. "We saw certain features in the spectrum that truly could not be explained by any of our existing models," de Graaff noted. However, confirming the black hole presence remains challenging due to the dense, obscuring gas envelope. Light variability, a hallmark of accreting black holes, offers a potential test, though JWST's observation limits hinder long-term monitoring.
A study by Fengwu Sun at Harvard used a gravitational lens to observe one LRD across four images, spanning 130 years of light travel time. The brightness variations resembled those of pulsating stars but with greater amplitude, aligning with the black hole star idea. If verified, these objects could represent a novel phase in supermassive black hole growth, absent in the local universe. "This could essentially be like a new growth mode... of these supermassive black holes," de Graaff suggested, though their lifetime and mass contribution remain unclear.