There may be stars as compact as black holes

A study points to the existence of stars as compact as black holes. Some of the objects cataloged as black holes could actually be ultracompact stars.

Although stars and black holes are a priori clearly different objects, from the aforementioned theoretical study a strange question emerges: Are we confusing a very compact type of stars with black holes? The key lies in a factor, the polarization of the vacuum, which allows much more compact stars to exist than previously thought, with densities similar to those of black holes.

The study has been published in the academic journal Scientific Reports.

Stars can range from medium densities, like that of our Sun, to the very high densities of white dwarf stars, which can contain masses like that of the Sun in volumes equivalent to that of the Earth. In the case of a star like the Sun, it is the balance between gravity, which tends to sink the star, and thermal pressure, which tends to expand it, that keeps the structure stable. In the case of white dwarfs, the structure remains stable because another mechanism acts, the degeneracy of the 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 how compact a star can be, known as the Buchdahl limit. Thus, any object that exceeds this limit must be a black hole, because for such compact objects there is no known material structure that can support its own gravity,” says Julio Arrechea, co-author of the study and researcher at the Institute of Astrophysics of Andalusia (IAA). , dependent on the Higher Council for Scientific Research (CSIC), in Spain.

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

“In our study we have 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 an energy density and which, according to the central idea of ​​general relativity, it should also be a source of gravity”, points out Carlos Barceló, an IAA researcher who has participated in the research.

Vacuum polarization behaves as if there were an additional cloud of matter, which is called semiclassical matter, in addition to the conventional, or classical, matter. This cloud of semiclassical matter has very peculiar 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 close to the Buchdahl limit.

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

Comparison between a black hole and a semiclassical (ultracompact) star. (Image: IAA / CSIC)

“This type of semiclassical stars could be perfectly confused with black holes, since they can be almost as compact as them. This leads us to ask whether the objects that in astrophysical practice we call black holes are really black holes and not semiclassical ultracompact stars. The new observational capabilities promise an exciting decade to discern the true nature of these objects”, concludes Julio Arrechea. (Source: IAA / CSIC)