Through a combination of quantum chromodynamics theory and astronomical observations, researchers have established that neutron stars—the culmination of the gravitational collapse of certain massive stars—pack about one to two solar masses into spheres 20–30 km across. Pinning down the properties of such small objects located so far away is an impressive achievement, but astrophysicists and nuclear physicists want to do even better.
The compression in a neutron star is so high that it may force the nuclear matter into exotic phases (see the Quick Study by Nanda Rea, Physics Today, October 2015, page 62). For a 1.4-solar-mass neutron star, a few-kilometer difference in radius could determine whether the nucleons exist as hyperons, free quarks, or something else. To help pinpoint the parameters of the densest matter in the universe, Sabrina Huth of Technical University of Darmstadt in Germany, Peter T. H. Pang of Nikhef in Amsterdam, and their colleagues have incorporated data from the densest matter on Earth: heavy ions that collide in particle accelerators.
The researchers strove to home in on the nuclear equation of state, which encompasses the relationship between neutron stars’ masses and radii and quantifies the stiffness of nuclear matter. The stiffer the matter, the greater its resistance to gravitational collapse and the larger the neutron star radius. Similarly, the nuclear stiffness dictates the dynamics of the compression and subsequent expansion when heavy nuclei such as those of gold slam into each other at relativistic energies inside particle colliders. The expansion of the post-collision nucleons is sensitive to the nuclear symmetry energy, a measure of how the nuclear binding energy changes with the neutron-to-proton ratio of the nucleus. That energy, in turn, is related to symmetry pressure, an important factor in the determination of the equation of state (see the article by Jorge Piekarewicz and Farrukh Fattoyev, Physics Today, July 2019, page 30).
Huth, Pang, and colleagues analyzed data from accelerators at the GSI Helmholtz Centre for Heavy Ion Research in Germany and at the US’s Lawrence Berkeley and Brookhaven National Laboratories. After combining the heavy-ion data with nuclear-theory calculations, the researchers set constraints on the radii of and the typical pressures within 1.4-solar-mass neutron stars that are consistent with those based on astrophysical measurements. They then merged the astrophysical and heavy-ion data to tighten the constraints for both properties. The data suggest a slight increase over the prior predicted value for the radius, which is supported by recent observations from NASA’s x-ray-observing Neutron Star Interior Composition Explorer mission.
The study highlights the value of looking beyond astrophysics and theory to understand the dense matter in neutron stars. Accelerators at GSI and elsewhere should soon be able to achieve particle densities comparable to those in neutron star cores, providing even more useful data for pinning down the equation of state. (S. Huth et al., Nature 606, 276, 2022.)
