Imagine a star one and a half times the mass of our Sun crammed into a sphere ten kilometers in diameter and spinning the size of a medium-sized city. Now, we set it to rotate so that it rotates on the order of a thousand times per second. It’s hard to imagine, but ‘it’ exists in our universe: it’s a neutron star. And although its existence was predicted in the mid-1930s, it remains an enigma.
What we know for sure is that the matter there is so concentrated (with a density a billion billion times that of water) and is under such high pressures that it does not exist in the form of atoms. What we have is a kind of soup of neutrons and other subatomic particles that have names as peculiar as pions . The structure of the star, which would collapse under the action of gravity, is supported by degeneracy pressure. To understand it, let’s think about what happens in bars and places to drink during parties: they are so crowded that, as they say, not even a pin will fit. If we wanted to enter, we would have to overcome the pressure exerted by the other people, who seem to be practically stuck. The same thing happens inside neutron stars: the weight of the star, which tends to concentrate all the mass of the star in the center, does not win because two particles of matter cannot occupy the same place at the same time.
So far everything is correct, but what kind of matter are we talking about? What is the internal structure of a neutron star like? What is it made of? Although theorists have advanced different models to explain it, it remains a mystery.
Knowing the interior of stars is complex because we only have information about what happens on the surface . In the case of neutron stars this is complicated by the fact that we have little idea what happens when the matter is so compressed. But still, theoretical astrophysicists have some pretty general idea of what goes on beneath its thin atmosphere of hydrogen and helium.
Beneath that atmosphere we have is a crust at most two centimeters thick made up of atomic nuclei and electrons circulating freely between them, followed by an innermost layer of heavy atomic nuclei and free neutrons and electrons. Beneath that inner crust the pressure is so high that protons and electrons join together to form neutrons. Below there we come to the core , a place where theoretical speculations run wild. It is thought to be divided into two zones. The outermost may consist of a neutron-rich quantum liquid while the inner… Well, here the physicists only venture to say that it is ultradense matter. As University of California astrophysicist Jocelyn Read says, “It’s one thing to know the ingredients and another to understand the recipe and how those ingredients are going to interact with each other.”
Some think that what you have are quarks and gluons (the particles with which protons and neutrons are built) wandering freely. Others believe that there may be other particles , such as hyperons , made up of three quarks (neutrons and protons only have two). These are very unstable and disintegrate quickly but perhaps at these inconceivable pressures they are stable. Another possibility is that in the center we have a Bose-Einstein condensate , a state of aggregation of matter that is below the solid. In it all the particles collapse to the same ground state and behave as if they were a single “superatom”. In this case, if the center is a Bose-Einstein condensate, the star must have a smaller radius than if it were made of ordinary material. And if it’s made of hyperons, the core could be even smaller. That is why measuring the radius accurately is very important. Unfortunately , these types of measurements are very complicated to make and have many inaccuracies : only a dozen pulsars have been calculated and the margin of error is one fifth of their size. Similarly, measuring the mass is also important. At present it can only be estimated in the case of pulsars in binary systems, since it is calculated from their orbital elements, but even so the estimates suffer from an experimental error of almost one solar mass.
That is why many hopes are pinned on the Neutron star Interior Composition Explorer (NICER), dedicated to collecting X-ray emissions originating from pulsars. It is equipped with 56 gold-coated detectors that also record the moment of arrival of the pulse with an accuracy of 100 nanoseconds, something essential to be able to determine its rotation period.
Astronomers are convinced that from their observations it will be possible to know both the mass and the radius with sufficient precision and they will be able to refine much more what is called the equation of state of neutron stars , which describes how matter is in its orbit. inside. The first breakthrough came in 2018, when two teams of scientists, one from the University of Helsinki and the other from Indiana University, improved the existing one based on observations made with the LIGO and Virgo gravitational wave detectors. These recorded the last minutes of the life of two neutron stars that were in the midst of orbital collapse. Thanks to the data collected, a much more precise equation of state was obtained, something that had not happened since 2012. With NICER, physicists are optimistic and think that the observations of this X-ray observatory will allow a cleanup between the different models of interiors that populate the scientific literature.
And furthermore, in combination with gravitational wave detectors and new X-ray space observatories, many astronomers hopefully hope that much of the secrets of the internal structure of neutron stars will be revealed in the next decade. .