What if there’s a star inside a black hole?

There is a question that often comes up at every public talk I do, and I have never had a good answer to it—well, not an entirely confident one. What’s inside a black hole? The textbook reply is a singularity. Cross the event horizon, and everything falls inward to a point of infinite density where our equations return nonsense. It isn’t so much an answer as an admission that we’ve run out of physics.

Enter Chen Tan and Yong-Qiang Wang from Lanzhou University, who have now published something rather more interesting. Their paper published on the arXiv preprint server describes a configuration they call, with wonderful directness, a neutron star in a black hole.

The setup starts with an idea from last year. Certain exotic forms of dark matter, arranged in a halo with a particular relationship between pressure and density, can produce black holes with no singularity at the center. These are so-called “regular black holes,” where the equations stay sensible all the way to the center. Tan and Wang asked what happens if you drop an ordinary neutron star into the middle of one of these halos.

The answer, worked through using the standard equations of stellar structure, is peculiar. Below a certain halo density, nothing dramatic occurs, and you get a neutron star with a dark matter cloud around it, slightly squashed. Above a certain density, the whole thing collapses, and no stable solution exists.

But between those two, something strange happens in the space just outside the star. A shell forms. In one of their models, it extends from about 10–12 kilometers (6.2–7.5 miles) out, with the star’s own surface sitting at 8.5 kilometers (5.3 miles). Here, the geometry flips and becomes that of a black hole interior. An event horizon, in other words, but one that closes over a star rather than being created by it. And inside, the neutron star simply carries on: regular, structured and entirely nonsingular.

The crucial point in this model is that nothing collapsed. The gravity making the horizon comes from the star and the halo combined, with the dark matter contributing roughly as much mass as the star itself. The team finds the same result using two different descriptions of neutron star matter, which suggests it isn’t an artifact of one particular assumption.

Now, the caveat, which the authors state plainly and which deserves stating plainly here, too: The dark matter densities this requires are far greater than anything astronomers actually expect dark matter to reach. This is not a claim about objects out there in the sky, but more of a thought experiment with the mathematics filled in. It says the inside of a black hole need not be the end of physics. There is at least one solution in which it’s a place, with something in it. The challenge remains the same as with any black hole: They need to be detected, and maybe one day one of these peculiar objects will be found.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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