The Glossary

Event HorizonAn event horizon is the boundary around a black hole beyond which nothing, not even... SingularityA singularity is the point at the center of a black hole where, according to general... Hawking RadiationHawking radiation is the faint thermal glow that black holes emit because of quantum... Accretion DiskAn accretion disk is the flattened, swirling structure of gas and dust that forms as... QuasarA quasar is the brilliant core of a distant galaxy, powered by a supermassive black hole... Neutron StarA neutron star is the collapsed core left behind when a massive star explodes as a... MagnetarA magnetar is a neutron star with an extraordinarily powerful magnetic field, trillions... SupernovaA supernova is the explosive death of a star, briefly outshining an entire galaxy. There... Gravitational WavesGravitational waves are ripples in the fabric of spacetime, produced when massive objects... Dark MatterDark matter is an invisible form of matter that makes up about 27 percent of the... Dark EnergyDark energy is the mysterious component driving the accelerating expansion of the... RedshiftRedshift is the stretching of light to longer, redder wavelengths as it travels through... Cosmic Microwave BackgroundThe cosmic microwave background, or CMB, is the faint afterglow of the Big Bang, released... Hubble ConstantThe Hubble constant is the rate at which the universe is expanding today, measured in... Cosmic InflationCosmic inflation is the theory that in the first tiny fraction of a second the universe... Big BangThe Big Bang is the prevailing model for the origin of the universe, describing how it... BaryogenesisBaryogenesis is the unsolved problem of how the universe came to contain matter but... Gravitational LensingGravitational lensing is the bending of light by gravity, predicted by general... Cosmic WebThe cosmic web is the largest-scale pattern of the universe: galaxies are not scattered... ExoplanetAn exoplanet is a planet orbiting a star other than the Sun. More than 5,800 have been... Habitable ZoneThe habitable zone is the range of distances from a star where a planet's surface could... Red DwarfA red dwarf is a small, cool, long-lived star, the most common type in the galaxy.... BiosignatureA biosignature is a substance or pattern that could indicate the presence of life, such... Protoplanetary DiskA protoplanetary disk is the rotating platter of gas and dust around a young star out of... Tidal LockingTidal locking is when an object's rotation period matches its orbital period, so it... Brown DwarfA brown dwarf is a substellar object that bridges the gap between the largest planets and... Rogue PlanetA rogue planet is a free-floating world that drifts through the galaxy bound to no star,... Planetary NebulaA planetary nebula is the glowing shell of gas a dying Sun-like star sheds as it... White DwarfA white dwarf is the dense, Earth-sized ember left when a star like the Sun exhausts its... Light-YearA light-year is the distance light travels in one year, about 9.46 trillion kilometers.... Fast Radio BurstA fast radio burst is a flash of radio waves lasting only a few thousandths of a second... Tidal HeatingTidal heating is the warming of a moon or planet by the flexing of its interior as... Subsurface OceanA subsurface ocean is a layer of liquid water hidden beneath the frozen surface of an icy... Interstellar ObjectAn interstellar object is a comet or asteroid that formed around another star and is... Orbital ResonanceOrbital resonance occurs when orbiting bodies exert regular, periodic gravitational tugs... SETISETI, the Search for Extraterrestrial Intelligence, is the scientific effort to detect... Fermi ParadoxThe Fermi paradox is the tension between the high apparent likelihood of extraterrestrial... Dyson SphereA Dyson sphere is a hypothetical megastructure an advanced civilization might build... Light CurveA light curve is a graph of an object's brightness over time, one of astronomy's most... Chandrasekhar LimitThe Chandrasekhar limit is the maximum mass a white dwarf can have, about 1.4 times the... SpacetimeSpacetime is the four-dimensional fabric, three of space and one of time, that Einstein's... NucleosynthesisNucleosynthesis is the creation of atomic nuclei. In the first few minutes after the Big... PulsarA pulsar is a rapidly spinning neutron star that beams radiation from its magnetic poles,... Space WeatherSpace weather refers to changing conditions in space driven by the Sun, especially solar...