How big is a neutron star? Earlier estimates ranged from eight to 1
Neutron stars are the densest objects in the universe, with a mass greater than our sunshine compressed into a relatively small sphere, whose diameter is comparable to that in Frankfurt. This is really just a rough estimate. For more than 40 years, the determination of the size of neutron stars has been a sacred boundary in nuclear physics, whose solution would provide important information about the material's basic behavior at core densities.
The data from the gravity wave detection of neutron stars (GW170817) make an important contribution to solving this puzzle. At the end of 2017, Professor Luciano Rezzolla, Institute of Theoretical Physics at Goethe University Frankfurt and FIAS, together with his students Elias Most and Lukas Weih, already used these data to answer a long-term question about the maximum mass that neutron stars can support before they collapsed into a black hole – a result that was also confirmed by various other groups around the world. After this first important result, the same team, with the help of Professor Juergen Schaffner-Bielich, has worked to impose stricter restrictions on the size of neutron stars.
The question is that the state's equation describing matter in neutron stars is not known. Physicists therefore decided to drive another way: they chose statistical methods to determine the size of neutron stars within narrow limits. To determine the new limits, they calculated more than two billion theoretical models of neutron stars by solving the Einstein equations describing the equilibrium of these relativistic stars and combining this large dataset with the limitations derived from GW170817 gravity wave detection.
"Such an approach is not uncommon in theoretical physics," says Rezzolla, adding: "By exploring the results for all possible parametric values, we can effectively reduce our uncertainties." As a result, researchers could determine the radius of a typical neutron star within an area of just 1.5 km: it is between 12 and 13.5 kilometers, which can be further enhanced by future gravity wave detections.
However, there is a twist to all this because neutron stars can have twin solutions, says Schaffner-Bielich. It is actually possible that at extremely high densities, matter drastically changes its properties and undergoes a so-called "phase transition". This is similar to what happens to water when it freezes and passes from a liquid to a solid state. In the case of neutron stars, this transition is speculated to transform ordinary matter into "quark matter", which produces stars that will have the exact same mass as their neutron star "twin" but it becomes much smaller and therefore more compact. 19659005] Although there is no definite evidence of their existence, they are reasonable solutions, and the researchers from Frankfurt have taken this opportunity into consideration, despite the further complications that twin stars imply. This effort finally ended, as their calculations have shown an unexpected result: twin stars are statistically rare and can not be deformed much during the merger of two such stars. These are important considerations because it now allows researchers to potentially rule out the presence of these very compact objects. Future gravity-wave observations will therefore reveal whether neutron stars have exotic twins.
Neutron stars throw light on quark materials
Elias R. Most et al., New Restrictions on Radii and Tidal Deformabilities of Neutron Stars From GW170817, Physical Review Letters (2018). DOI: 10,1103 / PhysRevLett.120.261103