Neutron stars are the densest objects in the Universe, larger than the Sun in mass, but condensed into a relatively small sphere.
How big are neutron stars? Previous estimates of the radius ranged from eight to sixteen kilometers. Astrophysicists at the Goethe University in Frankfurt (Germany) were able to determine the size of neutron stars to within 1.5 kilometers using a sophisticated statistical approach based on measuring gravitational waves. The researchers' report is presented in Physical Review Letters.
Neutron stars are the densest objects in the Universe, with a mass greater than the Sun, but condensed into a relatively small sphere. For more than 40 years, sizing neutron stars has been the Holy Grail of nuclear physics, a discovery of which will provide important information about the fundamental behavior of nuclear densities.
Data on the detection of gravitational waves from a merger of neutron stars (GW170817) make an important contribution to solving this conundrum. In late 2017, Professor Luciano Rezzolla, along with his students Elias Most and Lucas Weich, already used them to answer a long-standing question about the maximum mass that neutron stars can have before collapsing into a black hole. After the first important result, the same team, with the help of Professor Jurgen Schaffner-Belich, set about setting tighter limits on the size of neutron stars.
An artistic representation of the collision of neutron stars that generated gravitational waves. Credit: Carnegie Institution for Science.
The bottom line is that the equation of state that describes the matter inside neutron stars is unknown. Physicists have chosen statistical methods to determine the size of neutron stars within narrow limits. They calculated over two billion theoretical models by solving the Einstein equation for them, and combined this large dataset with the constraints of GW170817's detection of gravitational waves.
As a result, the researchers determined the radius of a typical neutron star within a difference of 1.5 kilometers: it ranges from 12 to 13.5 kilometers, which can be further refined by future detections of gravitational waves.
“However, the problem might have had more than one solution,” comments Jurgen Schaffner-Belich. It is possible that at ultra-high densities, the substance dramatically changes its properties and approaches the so-called "phase transition". This is similar to what happens to water when it freezes and goes from liquid to solid. In the case of neutron stars, this transition supposedly converts ordinary matter into "quark" matter, creating stars that will have the same mass as their "twin", the neutron star, but are much smaller and therefore even more compact.
Promotional video:
While there is no evidence of their existence, they may be a plausible solution, and the Frankfurt researchers took this possibility into account, despite additional complications. The effort paid off: the twin stars were statistically unlikely. This is an important finding that now allows scientists to potentially rule out the existence of these very compact objects. Future observations of gravitational waves will reveal if neutron stars have exotic twins.
