On Physics: The golden age of neutron stars

Neutron stars, weighing up to twice the mass of the sun but only 15 miles wide, are the densest objects in the universe, with matter compressed beyond atomic nuclei. Their discovery was predicted in 1932 by Walter Baade and Fritz Zwicky at the California Institute of Technology, following the neutron’s identification, though they remained unobserved until 1967 when Jocelyn Bell discovered pulsars—rapidly rotating neutron stars emitting precise radiation beams. The structure of neutron stars resembles Earth’s, with a solid crust and liquid interior, but at extreme densities, neutrons and protons dissolve into quarks. Deep inside, matter may transform into a quark soup resembling conditions in the early universe, recreated in lab experiments by smashing heavy atomic nuclei. Understanding this dense matter has been a key challenge, but recent observations have advanced the field. Astronomers have identified neutron stars with masses near twice that of the sun, indicating their interiors generate enough pressure to prevent collapse into black holes. NASA’s NICER telescope, mounted on the International Space Station, has measured the masses and radii of several neutron stars by analyzing X-ray emissions bent by their gravity, offering insights into quark contributions to pressure. Gravitational wave detections, such as the 2017 neutron star merger, have provided direct probes of their internal stiffness. The merger emitted light across all frequencies and revealed how the stars deformed under mutual gravity before merging. Future observations of such events will further refine understanding of neutron star interiors and their resistance to gravitational collapse. Research at the Aspen Center for Physics has focused on these questions, with contributions from Nobel Prize physicist Murray Gell-Mann, who proposed quarks. The center’s work highlights the interplay between theoretical predictions and observational astronomy in unraveling the mysteries of neutron stars.