Black hole

A region of spacetime bounded by an event horizon, from within which nothing can escape to distant observers. Black holes form when mass and energy become sufficiently concentrated.

Event horizon: A boundary in spacetime beyond which events cannot influence distant observers. It marks the point of no return around a black hole.
Schwarzschild solution: Karl Schwarzschild’s 1916 solution to Einstein’s field equations for the exterior spacetime of a spherical, nonrotating mass. It supplied the first mathematical description associated with a black-hole horizon.
Karl Schwarzschild: German physicist and astronomer who found the first exact solution to Einstein’s field equations in 1916. His solution introduced the radius now associated with a nonrotating black hole.
Accretion disk: A rotating structure of gas and dust spiraling around a central object while losing energy and angular momentum. Hot accreting material often makes black holes detectable through intense radiation.
Neutron star: A compact stellar remnant supported against collapse by neutron degeneracy pressure and nuclear forces. Unlike a black hole, it has a material surface outside its compact interior.
Schwarzschild radius: The radius of the event horizon of a nonrotating, uncharged black hole. It sets the horizon’s size from the black hole’s mass.
Oppenheimer–Snyder collapse: A 1939 model showing how a homogeneous dust sphere can collapse through its Schwarzschild radius. It demonstrated that gravitational collapse could form a black hole.
Subrahmanyan Chandrasekhar: Astrophysicist who derived the maximum mass of a stable white dwarf, now called the Chandrasekhar limit. His work clarified how stellar remnants can progress toward gravitational collapse.
Relativistic jet: A narrow outflow of particles traveling near light speed from an energetic astrophysical system. Some black-hole systems launch jets that carry energy across enormous distances.
White dwarf: A dense stellar remnant supported primarily by electron degeneracy pressure. It is a stable endpoint for many stars, rather than an object with an event horizon.

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