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 is observed to consist of a regular series of pulses, believed to be in synchrony with the rotation of the star.

Astronomers classify pulsars according to the source of energy that powers the emission of radiation. There are three presently accepted classes:

• Rotation-powered pulsars, where the loss of rotational energy of the star powers 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 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 begun 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 pulsar with planets, PSR B1257+12
• The first double pulsar binary system, PSR J0737−3039