Black holes are famous for gobbling up anything that gets too close – but could they ever be swallowed whole? A new study suggests that it’s possible that stars could capture very small black holes and keep them in their cores. There might even be a way to find these stars, and if so, they could help us understand the elusive dark matter.
Black holes come in several well-documented forms – there are those born from the deaths of stars, and supermassive monsters that lurk at the center of most galaxies. But it’s long been proposed that a huge number of black holes should have been created in the first few seconds after the Big Bang, and they could still be drifting around the cosmos. While they’d be essentially invisible to us, we could still infer their existence from their gravitational effects on matter around them – and if that sounds familiar, you’d probably not be surprised to hear that these “primordial black holes” are a strong contender for dark matter.
Confirming the existence of invisible objects is, of course, tricky, but they might just give themselves up through their interactions with other objects. The late, great Stephen Hawking himself was the first to propose the idea that on very rare occasions, newly forming stars could capture small primordial black holes with about the mass of an asteroid.
The black hole would then sink to the center of the star, but it wouldn’t destroy its host. As stars perform nuclear fusion reactions in their interiors, the intense energy flows outwards and that pressure keeps the star from collapsing in on itself. But that would also limit the amount of gas that could feed a hypothetical black hole in the center.
It’s an intriguing idea, but would it really mean the two could live in harmony? For the new study, scientists at the Max Planck Institute for Astrophysics (MPA) modeled the evolution of these so-called “Hawking stars,” using different starting masses for the black hole. And to their surprise, Hawking stars were more stable than you might expect, and would be almost impossible to identify from the outside.
“Stars harboring a black hole at their center can live surprisingly long,” said Earl Patrick Bellinger, lead author of the study. “Our Sun could even have a black hole as massive at the planet Mercury at its center without us noticing.”
However, there is a way that astronomers might be able to detect stars with dark hearts. The black hole would create different convection patterns deep in the star’s interior, which could potentially be picked up through a technique called asteroseismology. Essentially, this involves studying how sound waves travel through stars and affect the surface brightness.
The implications of finding a Hawking star would be huge. The discovery would be the first confirmation that primordial black holes exist – and if they do, then suddenly we might have the answer to the dark matter enigma, one of the most perplexing puzzles facing cosmology.
“There are good reasons to think that Hawking stars would be common in globular clusters and ultra-faint dwarf galaxies,” said Professor Matt Caplan, an author of the study. “This means that Hawking stars could be a tool for testing both the existence of primordial black holes, and their possible role as dark matter.”
The research was published in The Astrophysical Journal.
