Heaviest neutron star to date is a 'black widow' eating its mate

Millisecond pulsars spin far more rapidly than expected for a collapsed star. The best chance to study these neutron stars is to find a black widow system where the pulsar has evaporated and eaten much of its companion star. The Keck I telescope was just able to capture spectra of one such companion, allowing astronomers to weigh its pulsar. It's the heaviest found to date, and perhaps near the upper limit for a neutron star.