How Large Are Neutron Stars?

Data from the first observation of a neutron-star collision combined with input from modern nuclear theory narrow the range of neutron star radii.

Image courtesy of ESO/University of Warwick/Mark Garlick.
An artist’s impression of the collision of two neutron stars. This collision causes gravitational waves, a gamma-ray burst, and a massive explosion. Scientists believe the explosions are the universe’s main source of heavier elements such as gold.

The Science

An interdisciplinary research team has identified new, narrower limits on the radii of neutron stars. The team included nuclear physicists, data scientists, and astronomers. Their novel approach combined two sources of information. These sources were the first gravitational-wave and electromagnetic observations of a binary neutron-star collision and modern nuclear-theory calculations of uncertainty. The team determined the radius of a typical neutron star to be close to 11 kilometers. The results suggest that neutron-star black-hole collisions can swallow neutron stars whole.

The Impact

Neutron-star collisions teach us about the nature of the densest matter in the Universe. The properties of this matter can be understood in part by measuring the radii of neutron stars. Until recently, observers inferred that typical neutron-star radii ranged from 10–14 km with large uncertainties. The present work determines the neutron-star radius independently and more accurately. The improved constraints will have implications for the interpretation of future observations of neutron stars and will help scientists better understand the universe.


Neutron stars are the remnants of supernova explosions and have extreme properties. In particular, the cores of neutron stars are made up of extremely dense nuclear matter. When two neutron stars collide, they give off gravitational waves, electromagnetic radiation, cosmic rays, and neutrinos, so-called multi-messenger signals. Scientists observed one of these binary neutron-star collisions with several multi-messenger signals in August 2017. Using a new approach, a research team combined observations with state-of-the-art nuclear theory and used supercomputers to calculate the properties of neutron star matter. Finally, using Bayesian statistical tools, the team combined these calculations with the multi-messenger observations to constrain the radius of the neutron stars.


Ingo Tews
Los Alamos National Laboratory  


This work was supported by the Department of Energy Office of Science, Office of Nuclear Physics, Los Alamos National Laboratory’s Laboratory Directed Research and Development program, the NUCLEI SciDAC program, the National Science Foundation, and the NASA Space Telescope Science Institute. Computational resources were provided by the ATLAS Cluster at the Albert Einstein Institute in Hannover, Germany, Los Alamos Open Supercomputing via the Institutional Computing (IC) program, which is supported by the Department of Energy National Nuclear Security Administration, the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility, the Julich Supercomputing Center, and Syracuse University.


Capano, C.D., et al., Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory. Nature Astronomy 4 (2020). [DOI: 10.1038/s41550-020-1014-6]

Related Links

Determination of neutron star radii enhances understanding of densest matter in the cosmos, Los Alamos Science, Technology & Engineering Highlights March 2020

Neutron star with eleven kilometres radius, Max Planck Gesellschaft Research News

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