Truck-Sized Black Hole Could Survive Inside a Star
Physicists calculated that a black hole weighing about 40 tons, formed from accumulated dark matter, can survive evaporation and grow inside the core of a neutron star or white dwarf.
A team of physicists from the Manipal Centre for Natural Sciences (MCNS) in India and the Tata Institute of Fundamental Research (TIFR) in Mumbai has calculated the smallest mass a black hole needs to form inside a star and actually survive, instead of evaporating before it gets the chance to grow. According to a paper published September 4 in the journal Physical Review D, that threshold is only about 40 tons, roughly the weight of a loaded truck, and only if the black hole is also being fed by dark matter accumulated in the star’s core. The result points to a striking conclusion in reverse: since millisecond pulsars and white dwarfs have existed for billions of years without turning into black holes, their continued survival places limits on the properties of hypothetical dark matter particles.
The idea that black holes evaporate goes back to 1974, when Stephen Hawking showed that every black hole loses energy through a quantum process, and does so faster the lighter it is. That is why primordial black holes from the early universe, lighter than about 10^12 kilograms, are thought to have evaporated away by now. The black holes in the new study are not primordial. They form much later, and in an entirely different place: inside a star itself. When a neutron star or white dwarf captures hypothetical ultra-heavy, asymmetric dark matter particles, ones that do not annihilate each other, those particles build up in the star’s core over time. Once the accumulated mass becomes dense enough, it collapses into what the authors call an “endoparasitic” black hole, since it forms inside the star and then feeds on it from within.
Why exactly 40 tons
The paper, titled “Transmutation Timescales for Dark Matter Induced Collapse of Compact Stars into Black Holes,” is authored by H. A. Adarsha and Chandrachur Chakraborty of the Manipal Centre for Natural Sciences, along with Sudip Bhattacharya of the Tata Institute of Fundamental Research. It was first posted to the arXiv preprint server on August 11, 2026, under the number 2608.10594, and was accepted and published in Physical Review D on September 4, with the identifier DOI 10.1103/zb1m-762n. The authors solved the growth equation for an endoparasitic black hole analytically, accounting simultaneously for three processes that change its mass: Bondi accretion of ordinary stellar matter, Hawking evaporation, and a steady inflow of dark matter from the surroundings.
The mass threshold above which a black hole keeps growing, rather than fading away, depends on where in the galaxy the host star sits. For a white dwarf in the galactic disk, the region that includes our own solar system, that threshold is around 10,000 tons. In the denser galactic bulge, closer to the galaxy’s center, where dark matter is more concentrated, the threshold for a white dwarf drops to only about 40 tons, while for a neutron star in the same environment it comes out to roughly 600 tons. What happens to the black hole after it forms depends on the balance between the three processes. If accretion of ordinary matter and dark matter outpaces evaporation, the black hole grows and eventually consumes the entire star, turning it into a black hole. If the two effects balance out, the mass stays stable. If evaporation wins, the black hole shrinks and disappears.
What the limit means for dark matter
The authors turn the argument around and use the existence of old stars to derive limits on dark matter itself. Because millisecond pulsars and white dwarfs survive for billions of years, the time it would take such a star to collapse into a black hole has to exceed roughly a billion years for pulsars, and ten billion years for white dwarfs. From that condition, the authors calculate limits on the scattering cross section between dark matter and ordinary particles, across a dark matter particle mass range of roughly 10^6 to 10^14 GeV (gigaelectronvolts). In other words, the very stars we observe today have effectively served as dark matter detectors over billions of years of their existence.
The result is theoretical and depends on the assumption that dark matter really does come in the form of ultra-heavy, asymmetric particles that don’t annihilate each other, particles that have not yet been directly confirmed in any experiment. The study does not claim that a 40 ton black hole has actually been observed inside any star, only that, under the stated conditions, a black hole of that mass could keep growing instead of evaporating. The authors note that additional effects, such as the star’s rotation and magnetic field, could shift the exact mass threshold, and that independent confirmation of the model still has to come from future calculations and, potentially, from observations that reveal stars behaving differently than expected.
