MIT researchers have simulated what happens when a primordial black hole—a theoretical remnant from the universe's first moments—slips inside a star, and the answer is surprisingly stark: the star faces one of two terminal fates, each as dramatic as the other.
Primordial black holes, or PBHs, remain unproven but captivate astrophysicists because they might help solve one of science's deepest mysteries: what is dark matter? Unlike stellar black holes formed from dead stars, these hypothetical objects would have emerged directly from the intense, chaotic physics of the early universe. Some models suggest they could be as small as asteroids, making them small enough to be engulfed by stars. For decades, theorists have wondered what would happen if such an encounter occurred—until now.
Ore Gottlieb and his colleagues at MIT's Kavli Institute for Astrophysics and Space Research developed the first comprehensive framework to model this scenario. Using three-dimensional magnetohydrodynamic simulations combined with stellar evolution models, they discovered that PBH capture doesn't happen the way intuition might suggest. A black hole drifting through space cannot simply drift into a star through gravitational friction alone—that outcome is "negligibly rare," according to the researchers. Instead, three-body systems provide the crucial pathway: a planetary companion can nudge a black hole into a star-crossing orbit, causing it to spiral inward through repeated passages.
Once a PBH reaches the stellar core, it begins accreting material from the star's interior, creating what the researchers poetically call a "Hawking star"—a hybrid object that becomes a natural laboratory for studying how black holes grow and feed in dense, rotating environments. But this is where the star's story takes a turning point, governed almost entirely by a single physical process: disk formation.
The fate of the captured star hinges on a threshold of angular momentum. If the black hole accretes material rapidly enough, an accretion disk forms around it. This disk becomes a furnace of feedback, generating powerful disk winds and relativistic jets that blast outward with such force they tear the star apart within minutes. The result is a catastrophic explosion—a "Hawking-star transient" powered by a rapidly spinning black hole, bright and violent and utterly destructive. "Disk formation is the point of no return," the researchers write.
But if accretion remains slow and steady, angular momentum prevents disk formation entirely. In this quieter scenario, the black hole consumes the star gradually and almost imperceptibly, altering the star's luminosity and internal structure as it feeds. The star can exist in a strange quasi-steady state, slowly shrinking, with the black hole eventually acquiring a mass comparable to the stellar mass it has consumed. There is no dramatic explosion—only the patient, invisible erasure of a star.
The research reveals something profound: whether a star burns in spectacular destruction or fades in gradual consumption depends entirely on one factor—whether its accreting black hole can spin up a disk. If dark matter includes primordial black holes, and if our galaxy harbors them in significant numbers, then close stellar encounters would be frequent enough that this scenario might not be purely theoretical. The sky could hold secrets written in the final moments of captured stars.