A millisecond pulsar is a pulsar with a pulse period in the range 1–10 milliseconds, equivalent to an axial rotation rate of between 100 and 1,000 revolutions per second. Such a high spin rate may suggest that these pulsars are young, but in fact the opposite is true. Millisecond pulsars are typically a billion years or more old. They have been rejuvenated by a "spin-up process" involving the accumulation of matter from a companion star and so are often referred to as recycled pulsars.
Not only are the periods of millisecond pulsars much shorter than those of normal, young pulsars – the Crab Nebula pulsar, which is the youngest known, has a period of 0.03 second – but their slowdown rate is very low because of their comparatively weak magnetic fields. Indeed, millisecond pulsars are so regular that their pulses can be averaged to create the most accurate clock known to man. Of the 2,000 or so pulsars identified to date, about 200 are of the millisecond variety and, of these, 130 are in globular clusters. The first to be discovered, in 1982, PSR 1937+211, is also the second fastest known, completing 642 revolutions per second. In recent years, several confirmed or suspected planets have been found, by pulsar timing, in orbit around millisecond pulsars (see pulsar planets).