Composite Optical/X-ray image of the Crab Nebula pulsar, showing surrounding nebular gases stirred by the pulsar's magnetic field and radiation.

Pulsars are rotating neutron stars that are observable as sources of electromagnetic radiation. The radiation intensity varies at a regular period, believed to result from the rotation of the star. Pulsars were first detected at radio wavelengths by Jocelyn Bell and Antony Hewish in 1967, but have since been found in X-rays and gamma rays. Hewish received a 1974 Nobel Prize for the work.

Three distinct classes of pulsars are presently known to astronomers, according to the source of energy that powers the radiation:

• Rotation-powered pulsars, where the loss of rotational energy of the star powers the radiation
• X-ray pulsars, where the gravitational potential energy of accreted matter is the energy source, and
• Magnetars, where the decay of an extremely strong magnetic field powers the radiation.

Although all three classes of objects are neutron stars, their observable behaviour and the underlying physics are quite different. There are, however, connections. For example, X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions expanded and began transferring matter on to the neutron star. The process of accretion can in turn transfer enough angular momentum to the neutron star to "recycle" it as a rotation-powered millisecond pulsar.

Significant pulsars

• The first radio pulsar, CP 1919 (now known as PSR B1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04 second, was discovered in 1967 (Nature 217:709-713, 1968). A picture entitled "100 consecutive pulses from the pulsar CP 1919" appears on the front of Joy Division's album Unknown Pleasures
• The first binary pulsar, PSR B1913+16 , confirming general relativity and proving the existence of gravitational waves
• The first millisecond pulsar, PSR B1937+21
• The first X-ray pulsar, Cen X-3
• The first millisecond X-ray pulsar, SAX J1808.4-3658
• The first pulsar with planets, PSR B1257+12
• The first double pulsar binary system, PSR J0737−3039
• The magnetar SGR 1806-20 produced the largest burst of energy in the Galaxy ever experimentally recorded on 27 December 2004