Substellar object
A celestial body whose mass is too low to sustain hydrogen fusion in its core. The category includes brown dwarfs and planetary-mass objects.
Hydrogen fusion: The nuclear reaction in which hydrogen nuclei combine to form helium, releasing energy. Sustained core hydrogen fusion marks the lower-mass boundary between stars and substellar objects.
Gravitational contraction: The shrinking of a self-gravitating body, converting gravitational potential energy into heat. Contraction supplies much of a young substellar object's early luminosity.
Molecular cloud: A cold, dense interstellar cloud of gas and dust in which stars and other objects can form. The collapse of cloud fragments can produce isolated substellar objects.
Direct imaging: An observational technique that separates an astronomical source's light from nearby glare to detect it directly. It can reveal young, luminous substellar companions beside their host stars.
Main-sequence star: A star that spends most of its lifetime converting hydrogen into helium in its core. Unlike substellar objects, it sustains core hydrogen fusion.
Brown dwarf: A substellar object massive enough to fuse deuterium, or sometimes lithium, but not sustain hydrogen fusion. Brown dwarfs occupy the higher-mass part of the substellar population.
Kelvin–Helmholtz mechanism: The release of gravitational energy as an astronomical body contracts and radiates heat. It describes how substellar objects shine without sustained hydrogen fusion.
Gravitational collapse: The inward contraction of matter under its own gravity, often leading to the formation of a compact object. It can form substellar bodies directly, without a protoplanetary disk.
Radial velocity method: A technique that detects orbiting bodies through periodic shifts in a star's spectral lines. It measures the masses of some substellar companions through their gravitational effects.
Gas giant: A giant planet composed mainly of hydrogen and helium, such as Jupiter or Saturn. Gas giants overlap substellar objects in composition but are usually lower in mass.