Pulsars

Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. Their observed periods range from 1.4 ms to 8.5 s. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name. Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses are very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock.  Pulsars are known to have planets orbiting them, as in the case of PSR B1257+12. Werner Becker of the Max-Planck-Institut für extraterrestrische Physik said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."

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

Rotation-powered pulsars, A Rotation-powered pulsar is a rapidly rotating neutron star, whose electromagnetic radiation is observed in regularly spaced intervals, or pulses. It differs from other types of pulsars in that the source of power for the production of radiation is the loss of rotational energy.

Accretion-powered pulsars (accounting for most but not all X-ray pulsars), where the gravitational potential energy of accreted matter is the energy source (producing X-rays that are observable from Earth). The X-ray periods range from as little as a fraction of a second to as much as several minutes. An X-ray pulsar consists of a magnetized neutron star in orbit with a normal stellar companion and are a type of binary star system. The magnetic field strength at the surface of the neutron star is typically about 1012 gauss, over a trillion times stronger than the strength of the magnetic field measured at the surface of the Earth (0.6 gauss).

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.

At the centre of  the Crab nebula lies the Crab Pulsar, a rotating neuron star, which emits pulses of radiation from gamma rays to radio waves with a spin rate of 30.2 times per second. The Crab Pulsar is believed to be about 28–30 km in diameter; it emits pulses of radiation every 33 milliseconds. Pulses are emitted at wavelengths across the electromagnetic spectrum, from radio waves to X-rays.