A neutron star located 4000 light-years away has broken a record: It's nearly twice the mass of the sun and about 20% more massive than any neutron star measured before. Such stars form when massive stars collapse in supernovas, leaving behind a dense, neutron-rich core. The record breaker—named J1614-2230—is a type of neutron star called a millisecond pulsar; it spins at a dizzying rate of more than 300 revolutions per second and beams radio pulses in the direction of Earth every few milliseconds. Researchers were able to gauge the mass of the star by measuring the slowing speed of those pulses as they passed through the gravitational field of its companion star—an effect predicted by Einstein's theory of relativity. The measurement, reported  online today in Nature, deals a death blow to several proposed models for the kind of matter that makes up a neutron star: exotic particles like hyperon, kaon condensates, and free quarks are out. The composition of a neutron star's dense core remains a mystery.
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