What If There's A Black Hole Inside The Sun?

TL;DR

A primordial black hole captured inside a star can grow slowly, first creating a small convective region near the center and later taking over the star’s energy production.

Briefing Cornell Notes

Briefing

A captured primordial black hole inside a star would quietly change the star’s internal physics for billions of years—eventually forcing it into a “giant” phase early and in a distinctive way that could be detected through stellar vibrations. The payoff is bigger than one spooky scenario: if such Hawking stars exist, they could help pin down whether primordial black holes make up some or all of dark matter.

The idea traces back to Stephen Hawking’s 1971 work, which noted that a black hole with mass up to about 10^14 kg could hide in the Sun’s core without obvious immediate signatures. Hawking also raised the possibility that the early universe could have produced far smaller black holes—primordial black holes (PBHs). While a random black hole wandering into the solar system is extremely unlikely, a PBH already present in a star’s formation environment could be captured and then grow slowly from the inside.

The motivation for taking the scenario seriously comes from two longstanding puzzles. First, the “solar neutrino problem” once seemed to suggest the Sun might not be powered purely by fusion. A black hole engine was proposed as an alternative energy source: matter falling toward a black hole heats up and radiates, potentially reducing the need for fusion and therefore lowering neutrino production. That hypothesis lost its footing when neutrino oscillations were accounted for—detectors were only sensitive to one neutrino flavor—so the Sun’s neutrino output matches fusion expectations.

Second, dark matter remains unresolved. PBHs are one candidate class of dark matter, though observations rule out most PBH masses. A remaining window is roughly 10^14–10^20 kg (asteroid to medium-asteroid scale), where PBHs could still plausibly contribute. Gravitational-wave observations of black hole mergers also include features that some researchers have tried to interpret as possible PBH contributions.

New simulations—reported via two arXiv papers led by Earl Bellinger (Yale and the Max Planck Institute) and Matt Caplan (Illinois State)—model Sun-like stars that form around, or later capture, an asteroid-mass PBH. Because such a black hole is tiny (atom-sized in the model) with limited gravitational reach, it barely affects the star’s birth and early evolution. The PBH sinks to the center and begins feeding, heating a small central region (about 1% of the star’s radius, roughly Earth-sized). That region becomes convective and grows as the black hole grows.

Growth is throttled by feedback: energy radiated from the infalling material pushes back against new matter, and the black hole’s small “choke point” limits how quickly mass can be delivered. Over billions of years, the PBH can reach a size around 10 cm with a mass comparable to Uranus. A key transition follows when black hole power rivals fusion power in the rest of the core. The added energy disrupts the balance that normally keeps the core compact, causing the star’s outer layers to bloat and the core to expand and cool—shutting down fusion. The star then runs on black hole accretion rather than fusion.

The result is a Hawking star that can spend up to ~10 billion years looking like a rare “sub-subgiant” or “red straggler,” but with a maximum radius only about 4–5 times the Sun’s—smaller than the Sun’s typical red-giant expansion. Detection prospects hinge on asteroseismology: global oscillations depend on internal structure. A Hawking star’s deep convective motion would shift the harmonics compared with visually similar stars that have layered interiors. Current Sun observations don’t probe the deepest core well enough to rule out extremely small PBHs (down to around a millionth the Sun’s mass), but the team argues that any PBH large enough to matter would have altered neutrino production or core conditions beyond observational allowances.

Even if the Sun is almost certainly safe, the Milky Way’s hundreds of billions of stars make the broader test compelling. If PBHs exist in the allowed mass range, some stars should already be in the bloated Hawking phase. Finding them—or failing to—would tighten constraints on PBHs as dark matter, turning stars into detectors of warped spacetime.

Cornell Notes

Captured primordial black holes could form “Hawking stars” by growing slowly at a star’s center and eventually taking over the star’s energy budget. Simulations of Sun-like stars show that an asteroid-mass PBH initially has little effect, then creates a convective core region that expands as the black hole feeds. When black hole power matches fusion power, the star’s core expands, fusion shuts off, and the star enters a long-lived bloated phase (up to ~10 billion years) with a maximum radius only ~4–5 times the Sun’s. The key observational strategy is asteroseismology: the star’s vibrational harmonics would differ from visually similar giants because the interior structure is fundamentally altered. Non-detection would further limit how many primordial black holes could exist, including as a dark matter candidate.

Why did Hawking’s black-hole-in-a-star idea gain attention even though it was initially speculative?

Hawking’s 1971 discussion offered a potential alternative energy source for stars. That mattered because the solar neutrino problem once suggested the Sun might not be producing energy solely through fusion. A black hole engine could, in principle, provide luminosity while producing fewer neutrinos, since energy would come from accretion radiation rather than fusion. The later discovery that neutrinos oscillate between flavors resolved the mismatch, but the broader “hidden black hole” possibility remained relevant—especially when primordial black holes entered the conversation as a dark matter candidate.

How do the simulations describe the early life of a star that forms around, or captures, an asteroid-mass primordial black hole?

The black hole is extremely small (atom-sized in the model) and has limited gravitational reach, so it doesn’t significantly change the star’s formation or early evolution. It sinks to the center and begins slow feeding. The infalling material heats up and radiates outward, altering a central pocket about 1% of the star’s radius (roughly Earth-sized). That region becomes convective and grows as the black hole grows.

What limits how fast the black hole can grow inside the star?

Two throttles dominate. First, feedback: radiation and energy output from near the black hole pushes back against additional infalling matter, reducing the accretion rate. Second, a “choke point” effect: the tiny black hole size limits how quickly matter can be packed into it. The starting rate is on the order of ~100 tonnes per second—small compared with the Sun’s mass-loss rate via the solar wind (~a million tonnes per second). As the black hole grows, its gravitational pull and effective feeding cross-section increase, but the star’s energy output from accretion also rises, reinforcing the transition dynamics.

What triggers the dramatic change into the Hawking-star bloated phase?

A transition occurs after billions of years when the black hole’s accretion power becomes comparable to the fusion power produced by the rest of the core. At that point, the extra energy disrupts the balance between gravity and outward energy flow. The star’s outer layers bloat early, and the core expands and cools enough that fusion shuts off entirely. The star then runs on black hole power rather than fusion.

How would astronomers distinguish a Hawking star from a normal giant that looks similar?

Through asteroseismology. Stars vibrate in global oscillation modes determined by internal structure. A Hawking star’s deep interior contains a region of vigorous motion (from the convective plasma around the growing black hole) and lacks the usual layered core behavior. That changes how oscillation waves propagate, shifting the harmonics compared with visually similar stars whose interiors produce more complex mode mixtures. The team notes that current Sun data don’t probe the deepest core well enough to exclude very small PBHs, but the method could identify Hawking stars elsewhere using existing data from ESO’s Gaia.

What observational constraints apply specifically to the Sun?

If the Sun contains a captured PBH, its mass must be below the point where the star would already be in the bloated transition phase—roughly less than the mass of Uranus (about one-10,000th of the Sun’s mass). For smaller PBHs, neutrino measurements constrain additional non-fusion energy contributions: up to about 1/1000 of the Sun’s energy could come from a non-fusion source without contradicting neutrino uncertainties. Because an embedded black hole would also alter core temperature and pressure (changing fusion rate and neutrino production), the simulations conclude that any solar PBH would need to be less than Mercury’s mass to avoid detectable deviations.

Review Questions

What physical mechanism causes fusion to shut off in a Hawking star, and when does that happen relative to the black hole’s growth?
Why does asteroseismology provide a stronger detection route than relying only on a star’s color and radius?
Which two observational puzzles—solar neutrinos and dark matter—help motivate the search for primordial black holes inside stars?

Key Points

1
A primordial black hole captured inside a star can grow slowly, first creating a small convective region near the center and later taking over the star’s energy production.
2
Accretion growth is self-limiting: feedback from radiation and a small “choke point” restrict how quickly matter can fall into an atom-sized black hole.
3
When black hole power matches fusion power, the star’s core expands, fusion stops, and the star enters a long-lived bloated phase powered by accretion.
4
Hawking stars should be detectable through asteroseismology because their internal structure changes the star’s global oscillation harmonics.
5
Neutrino observations constrain any black hole inside the Sun: a PBH massive enough to matter would likely have altered the Sun’s neutrino output beyond measurement uncertainties.
6
If primordial black holes make up dark matter in the remaining allowed mass window, some Milky Way stars should already be in the Hawking-star phase, enabling population-level tests.

Highlights

An asteroid-mass primordial black hole can remain effectively invisible during a star’s early life, then trigger a late transition after billions of years when accretion power rivals fusion.

The bloated Hawking-star phase can last up to ~10 billion years, but the star’s maximum radius is only about 4–5 times the Sun’s—smaller than a typical red giant’s expansion.

Asteroseismology is the central detection strategy: visually similar stars can vibrate differently because a Hawking star’s interior lacks the usual layered structure.

Neutrino constraints imply any black hole inside the Sun must be below roughly Mercury’s mass to avoid detectable deviations in fusion-driven neutrino production.

Topics

Primordial Black Holes
Hawking Stars
Asteroseismology
Solar Neutrinos
Dark Matter

Mentioned

Stephen Hawking
Earl Bellinger
Matt Caplan
PBHs
ESO
Gaia

What If There's A Black Hole Inside The Sun? | Hawking Stars