Pulsar

A pulsar is a kind of neutron star: the extremely compact remnant left when the core of a sufficiently massive star collapses, usually after a supernova. Neutron stars are only tens of kilometres across but contain roughly stellar masses. They rotate and have powerful magnetic fields. When the magnetic axis is tilted relative to the rotation axis, charged particles and radiation emerge in beams near the magnetic poles. As the star turns, a beam sweeps through space; observers detect a pulse when it crosses their line of sight. The star is not switching on and off—the apparent blinking is produced by this rotating-beam geometry. Not every neutron star is observable as a pulsar: its beam might miss Earth or its emission might be too weak to detect.

Pulsar periods—the time between successive pulses—range from milliseconds to seconds. Some pulsars are observed at radio wavelengths; others emit detectable X-rays or gamma rays, and some are seen in more than one part of the electromagnetic spectrum. Pulsars lose rotational energy over time and generally slow down. A class called millisecond pulsars spins especially rapidly; many are in binary systems, and a leading account of their evolution is that matter transferred from a companion can speed up the neutron star's rotation. Pulses can be extremely regular, although individual sources may show small changes, including sudden spin-up events known as glitches.

The first pulsar was identified in radio data in 1967 by postgraduate researcher Jocelyn Bell Burnell, who was working with astronomer Antony Hewish at the University of Cambridge. The remarkably regular signal came from a celestial source and led to the recognition of a new class of astronomical object. Pulsars were first found through radio observations, but astronomers now study them with radio, optical, X-ray and gamma-ray instruments. Their timing, along with measurements of changes in the signal as it travels through space, also provides information about the stars and the interstellar material between them and Earth.

The regularity of many pulsar signals makes them useful astronomical clocks. When a pulsar orbits a companion, changes in pulse arrival times reveal features of the orbit and can help researchers estimate the masses of the objects. Measurements of binary pulsars provide tests of general relativity in strong-gravity systems. Long-term monitoring of many pulsars is also used in searches for very low-frequency gravitational waves: a passing wave would subtly alter the arrival times of pulses from different directions. These applications make pulsars valuable both for understanding the physics of dense stellar matter and for testing theories of gravity.