belt. Significance Until the discovery of PSR J1748-2446ad in 2006, which spins 716 times per second, PSR B1937+21 was the fastest spinning neutron star known. At the time of its discovery, PSR B1937+21 extended the range of periods observed in pulsars by a factor of 20, it also extended the range of magnetic fields observed by a factor of 100, with a magnetic field of 4.2 gauss (42 kT). As the first discovered millisecond pulsar, PSR B1937+21 "sparked a 'theory frenzy'" by providing a new laboratory in which to study pulsars, neutron stars more generally, and perhaps even some other astrophysical problems such as gravitational waves.<ref name="encounters"></ref> For instance, as the density required to spin at such high rates are comparable to nuclear densities, the fastest spinning millisecond pulsars are important in understanding how matter behaves at such densities. The initially high estimate of the spin down rate was also intriguing, as it implied a signal that could be directly detected by gravitational wave detectors, but the actual spin down rate put the expected signal below the sensitivity of current detectors. The currently accepted value spin down rate corresponds to a change in the rotational period of 1.5 Hz over the course of one million years. The stability of rotation of PSR B1937+21 is of the same order of the stability of the best atomic clocks, and is thus a tool used in establishing ephemeris time. The discovery of B1937+21 launched "extensive pulsar surveys at all major radio observatories" and "happened to revitalize pulsar astronomy at a time when most people thought the field was moribund." References Vulpecula Pulsars 12 Hypothetical planetary systems