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... Black HoleA black hole is a region of spacetime where gravity is so strong that nothing, not even... Supermassive Black HoleA supermassive black hole is a black hole weighing between roughly a hundred thousand and... GalaxyA galaxy is a gravitationally bound system of stars, gas, dust, and dark matter, ranging... Milky WayThe Milky Way is the barred spiral galaxy that contains the Sun and every star visible to... Galaxy ClusterA galaxy cluster is the largest gravitationally bound structure in the universe, holding... NebulaA nebula is an interstellar cloud of gas and dust. Some are stellar nurseries, where... NeutrinoA neutrino is an electrically neutral elementary particle with a tiny but nonzero mass... AntimatterAntimatter is made of particles that share the mass of ordinary matter but carry opposite... LuminosityLuminosity is the total energy an object radiates each second, an intrinsic property that... SpectroscopySpectroscopy is the technique of splitting light into its component wavelengths to read... AsteroidAn asteroid is a rocky or metallic body orbiting the Sun, too small to count as a planet... CometA comet is a body of ice, dust, and rock that grows a glowing atmosphere and tail as it... Kuiper BeltThe Kuiper Belt is a ring of icy bodies beyond Neptune, reaching from about thirty to... Oort CloudThe Oort Cloud is a spherical shell of icy bodies believed to surround the solar system... Solar WindThe solar wind is the continuous stream of charged particles, mostly protons and... Electromagnetic SpectrumThe electromagnetic spectrum is the full range of light, ordered by wavelength, from... Andromeda GalaxyThe Andromeda Galaxy, catalogued as M31, is the large spiral galaxy nearest our own,... Relativistic JetA relativistic jet is a narrow beam of plasma launched at close to the speed of light... Super-EarthA super-Earth is a planet more massive than Earth but well below the mass of Neptune,... MagnetosphereA magnetosphere is the region around a planet where its own magnetic field, rather than... Stellar WindA stellar wind is the continuous outflow of gas from a star's surface into space. The... Astronomical UnitAn astronomical unit, abbreviated AU, is the average distance between Earth and the Sun,... Cosmic VoidA cosmic void is a vast, nearly empty region between the filaments and walls of galaxies... Solar CoronaThe solar corona is the Sun's outer atmosphere, a tenuous halo of plasma extending... KilonovaA kilonova is the glow produced when two neutron stars collide, powered by the... Radial VelocityRadial velocity is the speed at which an object moves toward or away from the observer,... MetallicityMetallicity is the proportion of a star or gas cloud made of elements heavier than... Gamma-Ray BurstA gamma-ray burst is the most energetic explosion known, releasing in seconds more energy... AuroraAn aurora is the glow produced when charged particles from the Sun are funnelled by a... ParallaxParallax is the apparent shift of a nearby object against a distant background when the... Dwarf PlanetA dwarf planet is a body that orbits the Sun and is massive enough for gravity to pull it... Molecular CloudA molecular cloud is a cold, dense region of interstellar gas where hydrogen exists as... ParsecA parsec is the distance at which one astronomical unit subtends an angle of one... Globular ClusterA globular cluster is a dense, roughly spherical swarm of hundreds of thousands to... Type Ia SupernovaA Type Ia supernova is the thermonuclear detonation of a white dwarf that has gained too... Cosmic RayA cosmic ray is a charged particle, usually a proton or atomic nucleus, travelling... Coronal Mass EjectionA coronal mass ejection is an eruption that hurls a billion tons or more of magnetized... Standard CandleA standard candle is an astronomical object whose intrinsic brightness is known, so that... Transit MethodThe transit method detects a planet by the small dip in a star's brightness as the planet... Doppler ShiftThe Doppler shift is the change in the observed wavelength of a wave when its source... Binary StarA binary star is a pair of stars bound by gravity and orbiting a common center of mass.... AlbedoAlbedo is the fraction of incoming light a surface reflects, running from zero for a... Population III StarA Population III star belongs to the first generation ever to form, condensing from gas... Great AttractorThe Great Attractor is a concentration of mass roughly 150 to 250 million light-years... PhotosphereThe photosphere is the thin layer of a star from which the light we see escapes,... Orbital EccentricityOrbital eccentricity measures how far an orbit departs from a perfect circle, running... Open ClusterAn open cluster is a loose group of a few dozen to a few thousand stars that formed...