Pulsars are rapidly rotating neutron stars with periods less than s. When they were first discovered at the radio telescope at Jodrell Bank, England, their origin was unknown and they were thought to possible be signals from extraterrestrials. As a result, the first pulsar was named LGM-1, with LGM standing for "Little Green Men." Pulsars were discovered by Jocelyn Bell in the course of her Ph.D. thesis, and appeared in the appendix of her 1968 dissertation. In 1974, Anthony Hewish,Bell's thesis advisor, shared the Nobel Prize in physics for Bell's discovery of pulsars.

The fastest known millisecond pulsar is PSR B1937+21, which has a pulsation period of ms.
A total of 26 millisecond pulsars were known in the beginning of 1995 (Kospi 1995). Millisecond pulsars are not believed to be formed spinning so fast, but to be sped up by mass dumped onto them by a companion star. In 1992, A. Wolszczan found two planets orbiting the millisecond pulsar PSR B1257+12. There are 3 known gamma-ray pulsars (Science News 1992).
The shortest possible period of a pulsar can be computed by considering the effective gravity as a result of the difference between the gravitational and centripetal accelerations,
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where G is the gravitational constant, M is the mass of the pulsar, r is its radius and v is the speed at the pulsar's surface. Using
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where T is the period, gives
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For a star not to lose mass as a result of spinning too rapidly,, so plugging in gives
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Solving for gives
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and plugging in for density and solving for T then gives
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