millisecond pulsarsA class of pulsars that produce pulses with a period of only a few milliseconds, and are thus neutron stars rotating hundreds of times per second. Unlike the fast ‘normal’ pulsars (e.g. the Crab and Vela pulsars), the millisecond pulsars' fast rotation is not a result of youth; they have almost certainly been ‘spun up’ by mass transfer in a close binary star at an earlier evolutionary stage. Rapid rotation of a neutron star is normally slowed precipitously because the star's strong magnetic field radiates away the rotational energy, as with the Crab pulsar (see Crab nebula); the millisecond pulsars detected so far, however, have only a very gradual rate of slowing down, probably because their magnetic fields are comparatively weak (104–105 tesla).
PSR 1937+21 was the first millisecond pulsar to be found, 1982, and is currently the fastest known (period 1.56 ms, i.e. 642 rotations per second). The second to be found, PSR 1953+29 (6.1 ms), is a member of a binary pulsar system; it orbits its unseen companion in 120 days. The old but rapidly spinning binary pulsar PSR 1913+16 also belongs in this class, although its period is rather longer (59 ms).
Recent surveys have discovered large numbers of millisecond pulsars in globular clusters; more than half are close binary pulsars. It is generally believed that most of the single millisecond pulsars were initially members of binary systems, but lost their companion stars either because of a stellar collision (which is possible in globular clusters) or because the radiation emitted by the pulsars completely destroyed, or ‘evaporated’, the companion stars (see black-widow pulsars). One millisecond pulsar (PSR 1257+12) appears to be orbited by two planet-mass objects (see planet pulsar).