Neutron Stars on the Brink of Collapse
Neutron stars are the densest objects in the Universe; however, their exact characteristics remain unknown. Using recent observations and simulations, an international team of scientists including researchers at the Max Planck Institute for Astrophysics (MPA) has managed to narrow down the size of these stars. Thus the scientists were able to exclude a number of theoretical descriptions for the neutron star matter.
When a very massive star dies, its core collapses in a fraction of a second. In the following supernova explosion, the star’s outer layer gets expelled, leaving behind an ultra-compact neutron star. For the first time, the LIGO and Virgo Observatories have recently been able to observe the merger of two neutron stars by detecting the gravitational waves emitted and to measure the mass of the merging stars. Together, the neutron stars had a mass of 2.74 solar masses. Based on these observational data, the international team of scientists from Germany, Greece, and Japan managed to narrow down the size of neutron stars with the aid of computer simulations. The calculations suggest that the neutron star radius must be at least 10.7 km.
In neutron star collisions, two neutron stars orbit around each other, eventually merging to form a star with approximately twice the mass of the individual stars. In this cosmic event, gravitational waves – oscillations of spacetime – whose signal characteristics are related to the mass of the stars, are emitted. This event resembles what happens when a stone is thrown into water and waves form on the water’s surface. The heavier the stone, the higher the waves.
The scientists calculated different merger scenarios for the recently measured masses to determine the radius of the neutron stars. In so doing, they relied on different models and equations of state describing the exact structure of neutron stars. Then, the team of scientists checked whether the calculated merger scenarios are consistent with the observations. The conclusion: All models that lead to the immediate collapse of the merger remnant can be ruled out because a collapse leads to the formation of a black hole, which in turn means that relatively little light is emitted during the collision. However, different telescopes have observed a bright light source at the location of the stars’ collision, which provides clear evidence against the hypothesis of collapse directly after the neutron-star collision.
The results thereby rule out a number of theories for neutron star matter, namely all model descriptions that predict a neutron star radius smaller than 10.7 kilometers. However, the internal structure of neutron stars is still not entirely understood. The radii and structure of neutron stars are of particular interest not only to astrophysicists, but also to nuclear and particle physicists because the inner structure of these stars reflects the properties of high-density nuclear matter found in every atomic nucleus.
While neutron stars have a slightly larger mass than our Sun, their diameter is only a few 10 km. These stars thus contain a large mass in a very small volume, which leads to extreme conditions in their interior. Researchers have been exploring these internal conditions for several decades already and are particularly interested in better narrowing down the radius of these stars as their size depends on the unknown properties of ultra-dense matter.
The new measurements and new calculations help theoreticians to better understand the properties of high-density matter in our Universe. The recently published study represents a significant scientific progress as it has ruled out some theoretical models. But there is still a large variety of other models with neutron star radii greater than 10.7 km.
However, the scientists have been able to demonstrate that further observations of neutron star mergers will continue to improve these measurements. The LIGO and Virgo Observatories have just begun taking measurements, and the sensitivity of the instruments will continue to increase over the next few years and provide even better observational data.