Astronomers have a new explanation for mysterious “little red dots” observed in the early universe: Maybe they’re not crowded star clusters that upend everything scientists know about galaxies. Maybe they’re actually something new.
Researchers are calling the space objects “black hole stars” — giant spheres of gas wrapped around voracious black holes.
Normally, black holes are invisible, but gas and dust swirling toward them can get heated to millions of degrees, giving off very bright light — sometimes able to outshine a galaxy. These are usually called quasars, a portmanteau for “quasi-stellar objects.” The idea here is that the same process is happening but inside a massive gas shell, according to a new study by NASA‘s James Webb Space Telescope.
Perhaps the band Soundgarden, which irked a few astronomers in the ’90s with the dubiously named track “Black Hole Sun,” got it right after all.
“Little red dots instead appear to have much colder, and much denser, gas, so much so that they look just like the atmospheres on the surfaces of stars,” Joel Leja, a Penn State astrophysics professor and one of the researchers, told Mashable. “This is super surprising and why the first study on the little red dots thought they were galaxies full of old, cool stars — they effectively look like one very luminous, cold star.”
When the “little red dots” were first discovered in 2022, scientists thought the objects might be galaxies as mature as the Milky Way, which is about 13.6 billion years old. That’s because galaxies tend to get redder as the stars within them age.
But that interpretation was mind-boggling, given astronomers are seeing the dots as they were when the universe was just a few hundred-million years old. How, then, could they have grown up so fast?
The universe is believed to have begun 13.8 billion years ago, shortly thereafter forming the first stars and galaxies. Their ancient light still travels through space, albeit at infrared wavelengths — invisible to human eyes, but Webb can detect them with its sensitive instruments. By capturing light that has traveled for billions of years, Webb is essentially a time machine, giving scientists a glimpse at the past.
The new study, published in the journal Astronomy & Astrophysics, involved taking data from 4,500 distant galaxies. During that yearlong survey, the team nicknamed the most extreme object they found “The Cliff.”
The Cliff’s signal looked like that of a star, the researchers said, but Webb revealed it as twice as strong as anything seen before. The Cliff seemed too small for a galaxy — only about 40 light-years wide — and it wasn’t giving off X-rays, making the star cluster explanation even less likely.
The study instead showed the red glow could be a supermassive black hole in disguise, pulling in material so quickly that it surrounded itself in a hot ball of hydrogen gas. That ball changes the appearance of the black hole’s surrounding light so that it looks more like a star. That’s how the team came up with the name “black hole star.”
Credit: NASA / ESA / CSA / STScI / Dale Kocevski
But mysteries remain. The scientists also see signs of very hot, fast-moving material in the same small region.
“How can very hot gas and very cool gas all happily coexist in the same tiny volume of space?” Leja asked. “It’s a big puzzle, and the pieces don’t yet all fit together.”
Future observations, especially looking for atoms and molecules that can only form at these cooler temperatures, will help test the idea. If confirmed, black hole stars may hold the secrets to how the very first black holes were able to grow so fast in the early universe. No one yet knows the origins of gigantic black holes at the centers of galaxies. This may represent the first phase of their formation.
“This is why we do large projects like James Webb — we always hope to observe something shocking. It means we really are going to learn something new, in this case maybe a lot of new things,” Leja said. “It is just the most exciting thing for people like me.”