Stars as compact as black holes


Although stars and black holes are a priori clearly distinct objects, a strange question emerges from a recently published theoretical study: Are we confusing a very compact type of star with black holes? The key lies in one factor, vacuum polarization, which allows for the existence of stars that are much more compact than previously thought, with densities similar to those of black holes. The study was published in the latest edition of the journal Scientific reports.

Stars can range from medium densities, like that of our Sun, to very high densities of white dwarf stars, which can contain masses like that of the Sun in volumes equivalent to that of Earth. Whereas in the case of a star like the Sun it is the balance between gravity, which tends to make the star sink, and thermal pressure, which tends to expand it, which keeps the structure stable, in the case of white dwarfs, it is another mechanism, the degeneracy of electrons.

But the maximum limit would be found, according to our current knowledge, in neutron stars, which can contain two solar masses in a diameter of 24 kilometers and whose stability is due to the degeneracy of neutrons.


“General relativity predicts the existence of a limit to the compactness of a star, known as the Buchdahl limit. Thus, any object exceeding this limit must be a black hole, because for such compact objects it there is no known material structure capable of supporting its own gravity.underlines Julio Arrechea, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who is leading the work.

Stellar black holes are produced precisely by the collapse of the core of a very large star. They are the most compact objects known, with huge amounts of matter packed into a very small diameter and, in fact, their gravitational pull is so strong that not even light can escape.

“In our study, we worked with a factor that affects the compactness limit of stars: standard general relativity does not take into account a well-known phenomenon in electromagnetism, vacuum polarization, which is associated with a density of energy and which, according to the central idea of ​​general relativity, it should also be a source of gravity“explains Carlos Barceló, a researcher at the IAA-CSIC who is taking part in the work.

Vacuum polarization behaves as if there is an additional cloud of matter, which they call semi-classical matter, on top of conventional or classical matter. This cloud of semi-classical matter has very special properties that classical matter cannot reproduce, and can even harbor negative energies in certain situations. While in less compact stars like the Sun, vacuum polarization is a negligible phenomenon, it is relevant in situations near the Buchdahl limit.


In the recently published work, which analyzes the structure of stars made of classical and semi-classical matter, the scientific team finds, due to the peculiarities of semi-classical matter, relativistic stars that are more compact than what is established by the Buchdahl limit.

“This type of semi-classical star could perfectly be confused with black holes, since they can be almost as compact as them. This leads us to wonder if the objects that in astrophysical practice we call black holes are really black holes and not semi-classical ultracompact stars. New observational capabilities promise an exciting decade of discerning the true nature of these objects.”concludes Julio Arrechea (IAA-CSIC).

Character font: IAA, DICYT,

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