Active galactic nucleus

An active galactic nucleus is a compact, exceptionally luminous region at a galaxy’s center, powered by matter accreting onto a supermassive black hole.

Accretion disk: A rotating structure of gas and dust spiraling around a central object as it loses angular momentum. Infalling gas forms a hot disk that releases much of an active nucleus’s radiation.
Supermassive black hole: A black hole with a mass of millions to billions of times the Sun’s mass. Its gravity anchors the central engine and sets the scale of the accretion flow.
Quasar: An exceptionally luminous active galactic nucleus whose central source can outshine its host galaxy. Quasars are the most conspicuous high-luminosity examples of active nuclei.
Starburst galaxy: A galaxy undergoing an unusually rapid rate of star formation. Its intense luminosity comes from young stars rather than primarily from a central accretion flow.
Relativistic jet: A narrow outflow of particles moving near the speed of light from an astrophysical object. Some active nuclei channel energy into jets that extend far beyond their host galaxies.
Electromagnetic spectrum: The full range of electromagnetic radiation, ordered by wavelength or frequency. Active nuclei radiate across the spectrum, from radio waves to gamma rays.
Seyfert galaxy: A galaxy whose nucleus is exceptionally bright and shows strong emission lines, while its host remains visible. Seyfert galaxies provide nearby, comparatively accessible examples of nuclear activity.
Radio galaxy: A galaxy whose active nucleus produces strong radio emission, often through extended jets and lobes. It is a radio-loud active nucleus whose jets are not aimed directly at Earth.
Black hole accretion: The process by which matter falls toward a black hole and converts gravitational energy into radiation or outflows. This energy source powers the nucleus without requiring matter to cross the event horizon.
Doppler beaming: The apparent brightening and frequency shift of radiation emitted by matter moving relativistically toward an observer. It can make a jet pointed toward Earth appear unusually luminous.

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