Neutron Stars & Pulsars

An old pulsar spun up to hundreds of rotations per second by accreting matter from a companion star — 'recycled' to spin periods of only a few milliseconds. Their extraordinary rotational stability makes them the most precise clocks known and the basis of pulsar-timing arrays searching for low-frequency gravitational waves.

The quantum pressure — arising from the Pauli exclusion principle acting on densely packed neutrons — that holds a neutron star up against its own gravity. It is far stronger than the electron degeneracy pressure that supports a white dwarf, but it too has a limit: above roughly two to three solar masses, no known pressure can prevent collapse into a black hole.

A rapidly rotating, magnetised neutron star observed as a source of regular pulses of radiation, usually in radio. The first was found in 1967 by Jocelyn Bell Burnell and Antony Hewish; thousands are now known, with periods from milliseconds to seconds. Pulsars are precise cosmic clocks used to test gravity and to search for gravitational waves.

A sudden, tiny speed-up in a pulsar's otherwise steadily slowing rotation, followed by a gradual relaxation. Glitches are thought to be caused by the sudden transfer of angular momentum from a superfluid deep inside the neutron star to its solid crust, giving a rare window onto matter at supernuclear density.

A pulsar whose radiation is powered by the gradual loss of its rotational energy as it slowly spins down — the classic young pulsar, of which the Crab is the archetype. This is distinct from an accretion-powered X-ray pulsar, which draws its energy from infalling matter rather than from its own spin.

The most powerful magnetic fields known in the Universe — around a hundred trillion to a quadrillion gauss — carried by magnetars, a class of young neutron star. The decay of this colossal field powers their X-ray and gamma-ray flares, including the giant flares bright enough to be detected across the Galaxy and beyond.

The still-uncertain relationship between pressure and density inside a neutron star, which decides how compressible its ultra-dense matter is and therefore the star's radius and maximum mass. Pinning it down — from radius measurements by NICER, from massive pulsars, and from the tidal signature in neutron-star mergers — is a central goal of modern astrophysics.

How a pulsar pulses: a rapidly rotating, strongly magnetised neutron star beams radiation from its magnetic poles, and if a beam sweeps across the Earth we see a regular pulse once per rotation, like a lighthouse. The precise physics of how the beam is generated in the star's magnetosphere is still not fully understood.

A neutron star in a binary system that pulls gas from its companion; the gas is funnelled by the star's magnetic field onto its poles, where it heats up and shines in pulsed X-rays as the star rotates. X-ray pulsars are how many neutron stars are weighed, and how millisecond pulsars are recycled.