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.
Andreas Bauswein (HITS)
Janka, Hans-Thomas
Scientific Staff
Phone: 2228
Email: thj@mpa-garching.mpg.de
Oliver Just
Links:
- personal homepage (the institute is not responsible for the contents of personal homepages)
Original Publication
1. Bauswein, Andreas; Just, Oliver; Janka, Hans-Thomas; Stergioulas, Nikolaos
Neutron-star Radius Constraints from GW170817 and Future Detections
The Astrophysical Journal Letters, 850, L34, (2017)
Source/DO
More Information
Neutron Stars on the Brink of Collapse
HITS Press Release
