Binary black hole

A system of two black holes orbiting a shared center of mass. Gravitational-wave emission can shrink the orbit until the black holes merge.

Gravitational-wave radiation reaction: The loss of orbital energy and angular momentum caused by emission of gravitational waves. It drains energy from the orbit, causing the black holes to spiral together.
Gravitational-wave astronomy: The study of cosmic sources through gravitational waves rather than electromagnetic radiation. Merging black-hole binaries are its first directly detected source population.
Stellar evolution: The sequence of physical changes a star undergoes from formation through its remnant stage. Massive stars can leave black holes that later form binaries.
Neutron star–black hole binary: A compact binary consisting of one neutron star and one black hole. Unlike a pair of black holes, it may produce electromagnetic signals if the neutron star is disrupted.
Inspiral: The late orbital phase in which a compact binary loses energy and its separation steadily decreases. This is the shrinking-orbit stage before a binary black hole merges.
LIGO: A pair of laser interferometers in the United States designed to detect gravitational waves. LIGO made the first direct detection of gravitational waves from a binary black-hole merger.
Common-envelope evolution: A binary-star phase in which one star's envelope engulfs both stellar cores. Envelope drag can shrink a stellar binary enough for its black-hole remnants to merge.
Binary neutron star: A compact binary containing two neutron stars in orbit around their shared center of mass. Its merger can eject matter and produce light, unlike the usually dark merger of two black holes.
Post-Newtonian expansion: An approximation scheme that extends Newtonian gravity with successive corrections from general relativity. It models binary orbits when gravity is strong but the objects remain well separated.
Virgo interferometer: A European laser interferometer that detects gravitational waves in coordination with other observatories. Its detections help locate binary black-hole mergers across the sky.

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