Pulsars

A pulsar is a neutron star that emits periodic pulsating radiation. Pulsars were discovered in 1967 by Anthony Hewish and Jocelyn Bell at the Radio Astronomy Observatory in Cambridge. Its characteristic radio emission is a uniform series of pulses, spaced very precisely, with periods between a few milliseconds and several seconds. Hundreds are known, but only some emit detectable pulses, such as the Crab pulsar and the Candle pulsar.

The pulses have great regularity: Observers can predict pulse times up to a year in advance, with accuracy better than a millisecond. How can a star behave like such a precise clock?

The only option for these precise repetitions is for the star to be rotating rapidly, emitting a beam of light that sweeps the sky like a lighthouse, pointing at the observer once for each rotation. You can see this effect graphed in the animation on the side.

Pulsars, the cosmic "lighthouses"

The only type of star that can rotate fast enough without exploding due to its centrifugal force is the neutron star. Pulsars are strongly magnetized neutron stars, with intensities of 100 million Tesla (1 million million Gauss, compared to less than 1 Gauss for the Earth's magnetic field).

The energy comes from the rotation of the star, which has to be slowing down. This decrease can be detected as lengthening of the pulsation period. Typically, the rotation rate of a pulsar decreases by one part per million each year: that of the Crab, the youngest energetic known, decreases by 1/2000 each year.

Pulsars have been found primarily in the Milky Way, within about 500 light years of the plane of the Galaxy. A complete search for pulsars is impossible, as the faintest ones are only detected if they are close. So far, probing the entire sky, more than 300 pulsars have been found. It is estimated that there are at least 200,000 in our Galaxy, but considering those whose beams do not sweep in our direction, there should be many more. Pulsars emit pulses about 4,000,000 years ago; They then lose so much rotational energy that they produce no detectable radio pulses.