With diameters of less than 30 km, neutron stars harbor the densest matter in the cosmos. Much of it is surely just neutrons packed together at nuclear densities. But there has been lively speculation that more exotic phases—nuclear matter with hyperons, Bose–Einstein condensates of mesons, or plasmas of free quarks—reside at neutron-star cores. Much of that speculation has now been laid to rest by just one precision mass determination at the National Radio Astronomy Observatory in Green Bank, West Virginia. The NRAO measurements of the millisecond pulse train from a rapidly spinning neutron star orbited by a white-dwarf companion revealed that the neutron star’s mass was a record 1.97 ± 0.04 times that of the Sun. This unexpected record mass exceeds the neutron-star mass limits prescribed by almost all of the proposed equations of state that yield the exotic phases. The precision measurement exploits an unusually strong manifestation of a general relativistic effect first pointed out by Irwin Shapiro in 1964: Light is not only bent but also delayed as it passes near a massive object. In this case, as shown in the figure, radio pulses along the line of sight from the neutron star are manifestly delayed once every nine days when they pass very close to the orbiting companion. The happy accident that the binary system’s orbits are seen almost perfectly edge-on makes possible the unusually clear Shapiro-delay signal that yields the precise measurement of both stellar masses. The white dwarf is about ¼ as heavy as the neutron star. (P. B. Demorest et al., Nature 467, 1081, 2010.)—Bertram Schwarzschild
