Neutron Star Cores May Contain Exotic Quark Matter

by johnsmith

Massive neutron stars have sizable quark-matter cores, according to a study published in the journal Nature Physics.

An artist’s conception of a massive neutron star. Image credit: L. Calçada / ESO.

An artist’s conception of a massive neutron star. Image credit: L. Calçada / ESO.

Neutron stars, which are the result of a supernova explosion, are the smallest and densest stars in the Universe.

While these objects typically have a radius of about 10 km, they average between 1.4 and 2.2 solar masses.

Inside neutron stars, atomic matter is known to collapse into immensely dense nuclear matter, in which neutrons and protons are packed together so tightly that the entire star can be considered one single enormous nucleus.

Up until now, it has remained unclear whether inside the cores of massive neutron stars nuclear matter collapses into an even more exotic state called quark matter, in which the nuclei themselves no longer exist.

“Confirming the existence of quark cores inside neutron stars has been one of the most important goals of neutron star physics ever since this possibility was first entertained roughly 40 years ago,” said senior author Dr. Aleksi Vuorinen, a researcher in the Department of Physics at the University of Helsinki.

With large-scale simulations run on supercomputers unable to determine the fate of nuclear matter inside neutron stars, Dr. Vuorinen and colleagues proposed a new approach to the problem.

They realized that by combining recent findings from theoretical and nuclear physics with astrophysical measurements, it might be possible to deduce the characteristics and identity of matter residing inside neutron stars.

They found that matter inside the cores of the most massive stable neutron stars bears a much closer resemblance to quark matter than to ordinary nuclear matter.

Their calculations indicate that in these stars, the diameter of the core identified as quark matter can exceed half of that of the entire neutron star.

However, there are still many uncertainties associated with the exact structure of neutron stars.

“There is still a small but nonzero chance that all neutron stars are composed of nuclear matter alone,” Dr. Vuorinen said.

“What we have been able to do, however, is quantify what this scenario would require.”

“In short, the behavior of dense nuclear matter would then need to be truly peculiar.”

“For instance, the speed of sound would need to reach almost that of light.”


E. Annala et al. Evidence for quark-matter cores in massive neutron stars. Nat. Phys, published online June 1, 2020; doi: 10.1038/s41567-020-0914-9

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