A black hole as light as 40 tons can exist inside a star if dark matter helps

September 5, 2026 dialog A black hole as light as 40 tons can exist inside a star if dark matter helps Gaby Clark Scientific Editor Robert Egan Senior Editor In 1974, Stephen Hawking made a prediction that transformed our understanding of black holes. Black holes, he showed, are not truly black: They slowly lose energy through a quantum process now known as Hawking radiation. The lighter the black hole, the faster it evaporates. For primordial black holes formed in the early universe, the implications are dramatic. A primordial black hole with a mass of around 1012 kg has an evaporation timescale comparable to the age of the universe. Significantly lighter primordial black holes would therefore not be expected to survive until today. The tiny black holes considered here, however, need not have existed since the early universe. They can form much later inside compact stars. Neutron stars and white dwarfs can capture hypothetical ultraheavy asymmetric dark matter particles. Because these particles do not efficiently annihilate, they can accumulate at the stellar core, become self-gravitating and eventually collapse to form a tiny endoparasitic black hole. Once formed, the black hole's fate is determined not by Hawking evaporation alone. Surrounded by dense stellar matter, it can gain mass by accreting ordinary matter from its host star and through the continued supply of dark matter. Under favorable conditions, the calculations show that a black hole with an initial mass of only about 40 metric tons, roughly the mass of a loaded semitruck, can overcome Hawking evaporation and continue to grow. The key is dark matter. A race between growth and evaporation A newly formed endoparasitic black hole does not automatically consume its host star. Its fate is decided by a competition: Accretion of stellar matter and continued dark matter feeding increase its mass, while Hawking radiation decreases it. There is also a quantum complication. For an extremely small black hole, the usual description of accreting matter as a continuous fluid can break down. The wavelength of an incoming particle can become comparable to the characteristic scale of the black hole, requiring a quantum description of particle absorption. As the black hole grows, the familiar fluid-like accretion regime eventually takes over. The calculation follows both regimes while simultaneously accounting for stellar matter accretion, continued dark matter feeding and Hawking evaporation. How small can a growing black hole be? The competition defines a critical black hole mass. Below this mass, Hawking evaporation wins and sustained growth is impossible. Above it, the incoming supply of matter can overcome evaporation, and the black hole can continue growing. Continued dark matter feeding can lower this critical mass dramatically. If dark matter feeding is neglected and only stellar matter accretion and Hawking evaporation are considered, the characteristic critical mass can be around 1010 kg in representative compact-star cases. This should not be confused with the 1012 kg scale for primordial black holes. The latter concerns survival for a cosmic time in isolation. The critical mass inside a star instead asks whether a newly formed black hole can grow rather than evaporate. Once continued dark matter feeding is included, the critical mass can fall much further. For a white dwarf in the Galactic disk, where the solar system resides, the critical initial mass is about 10,000 metric tons. In the much more dark matter-rich Galactic bulge environment, however, it can fall to just 40 metric tons. For a neutron star in the Galactic bulge, the corresponding critical mass is about 600 metric tons. This does not mean that a 40-ton black hole has been observed. Rather, under the conditions considered, a black hole with such a small initial mass can have a positive growth rate instead of evaporating through Hawking radiation. Given sufficient time, it can continue to grow, eventually consuming its host star and transforming it into a black hole. What this means Hawking evaporation remains essential to the calculation, but the surrounding environment can dramatically change the fate of a tiny black hole. The black holes studied here form inside compact stars through the accumulation of dark matter. Once formed, they can receive new mass from their surroundings. If this supply exceeds the mass lost through Hawking radiation, even an extraordinarily small black hole can continue to grow. The exact critical mass depends on the properties of the star and its environment. Additional effects, such as rotation and magnetic fields, could also modify the result. Nevertheless, the central result is striking: Under suitable conditions, a black hole as light as about 40 metric tons can grow inside a compact star instead of evaporating, with continued dark matter feeding helping to sustain its growth. The argument can also be reversed. If dark matter with certain properties could rapidly form a growing black hole and destroy its host, very old compact stars should no longer exist. The survival of millisecond pulsars and white dwarfs for billions of years therefore constrains the mass of ultraheavy dark matter particles and the strength of their interactions with ordinary matter. In this sense, the stars have performed the experiment for us, acting as natural dark matter detectors over astronomical timescales. These findings are published in the journal Physical Review D. This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate. Publication details H. A. Adarsha et al, Transmutation timescales for the dark matter induced collapse of compact stars into black holes, Physical Review D (2026). DOI: 10.1103/zb1m-762n. On arXiv: arxiv.org/abs/2608.10594 Journal information: Physical Review D , arXiv Key concepts Astronomical black holesHawking radiationAccretionGravitational collapseWhite dwarf starsChandrachur Chakraborty is an Assistant Professor at the Manipal Center for Natural Sciences (MCNS), Manipal Academy of Higher Education (MAHE), India, and an Associate at the Inter-University Center for Astronomy and Astrophysics, Pune, India. His research focuses on General Relativity with its applications to Astrophysics: sites.google.com/view/chandrachur H. A. Adarsha is a PhD student at the MCNS-MAHE. Sudip Bhattacharyya is a Professor in the Department of Astronomy and Astrophysics at the Tata Institute of Fundamental Research, Mumbai, India. His research spans neutron star astrophysics, X-ray binaries, and compact object physics, with a focus on using observations to probe fundamental physics.