Black Holes, Neutron Stars & Compact Objects

The idealised, spherically symmetric capture of surrounding gas by a compact object, worked out by Hermann Bondi in 1952. It sets a benchmark rate at which a black hole or neutron star can swallow the gas around it, and underlies estimates of how quiescent supermassive black holes such as Sgr A* are fed.

The twisting of spacetime by a rotating mass, which drags nearby matter and light around with it. Extreme near a spinning black hole, it creates the ergosphere; it is tiny near the Earth, where it was measured by the Gravity Probe B and LAGEOS satellite experiments — a direct confirmation of a subtle prediction of general relativity.

The stretching of light to longer wavelengths as it climbs out of a gravitational well, a prediction of general relativity. It is extreme near a black hole's event horizon, where light is redshifted without limit, and has been measured for the Sun, for white dwarfs, and even for atomic clocks at different heights on Earth.

A near-regular flickering in the X-ray brightness of matter orbiting a black hole or neutron star, revealed in the power spectrum as a broad peak rather than a sharp tone. QPOs probe the innermost accretion flow, close to the innermost stable circular orbit, and are used to study strong gravity — though their exact origin is still debated.

A narrow beam of plasma launched at nearly the speed of light from the vicinity of an accreting black hole — from stellar black holes in binaries and from the supermassive black holes powering active galaxies. Jets can span from light-years to millions of light-years and are thought to be powered by the black hole's spin and magnetic fields.

The stretching of an object into a long, thin shape by the difference in a black hole's gravity between its near and far sides. Around a small stellar black hole the tidal force is lethal well outside the horizon; around a supermassive black hole it is gentle at the horizon, so an infalling object could cross without being torn apart.

The leading explanation for how relativistic jets are powered: magnetic field lines threading a spinning black hole extract its rotational energy and fling plasma outward. Proposed by Roger Blandford and Roman Znajek in 1977, it lets a black hole act as a cosmic engine, converting spin into the enormous power of a jet.

The region just outside the event horizon of a rotating (Kerr) black hole where spacetime itself is dragged around so fast that nothing can stay still relative to the distant stars. Energy can, in principle, be extracted from a black hole's rotation here through the Penrose process. A non-rotating black hole has no ergosphere.

The closest orbit around a black hole in which matter can circle stably — at three Schwarzschild radii for a non-spinning black hole, and closer for a rapidly spinning one. Inside it, matter spirals inevitably inward. The ISCO sets the inner edge of an accretion disk and thus how much energy accretion can release.

The result that a black hole in equilibrium is fully described by just three numbers — its mass, its spin, and its electric charge — and retains no other detail of whatever formed it. All the complexity of the collapsing matter is lost to the outside universe, hidden behind the event horizon with the central singularity, giving black holes a remarkable simplicity.

The radius around a non-rotating black hole — one and a half Schwarzschild radii — at which gravity bends light so strongly that photons can orbit in unstable circles. It sets the size of the black hole's 'shadow' and the bright ring seen in the Event Horizon Telescope images of M87* and Sgr A*.

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.

The compact object of Cygnus X-1 — the first widely accepted black hole. A bright X-ray source in Cygnus discovered in 1964, it is a stellar-mass black hole of roughly twenty-one solar masses pulling gas from its blue-supergiant donor star (HD 226868). It was the subject of a famous bet between Stephen Hawking and Kip Thorne, which Hawking conceded in 1990.

The first pulsar ever discovered, found by Jocelyn Bell Burnell and Antony Hewish in 1967 as a startlingly regular radio signal once labelled 'LGM-1' for Little Green Men. Its 1.337-second pulses opened the study of neutron stars; the discovery is commemorated on the cover of Joy Division's album Unknown Pleasures.

One of the most massive neutron stars known, at about 2.08 solar masses, weighed through the Shapiro delay of its pulses. Its radius has been measured by NASA's NICER X-ray telescope, and together the mass and radius are among the tightest constraints on the neutron-star equation of state and how dense matter behaves.

The young neutron star at the heart of the Crab Nebula, formed in the supernova recorded by observers in 1054. It spins about thirty times a second and is the archetypal rotation-powered pulsar, its wind lighting up the surrounding nebula across the spectrum, from radio to gamma rays.

A young, bright pulsar in the Vela supernova remnant, spinning about eleven times a second. It is the classic glitching pulsar — famous for sudden small speed-ups in its rotation that reveal the superfluid interior of a neutron star — and one of the brightest gamma-ray sources in the sky.

A stellar-mass black hole of about nine solar masses in a binary system in Cygnus, and one of the nearest black holes with a precisely measured distance from radio parallax. Normally quiet, it erupted in a dramatic X-ray and radio outburst in June 2015 — a nearby microquasar caught devouring gas from its companion.