Last updated July 18, 2025

Glossary of Astronomy and Astrophysics: Key Definitions and Fundamental Concepts

Technical and precise definitions of key terms

This glossary brings together concise, factual, and rigorous definitions of the main terms used in astronomy and astrophysics. Each term is presented with its technical definition to better understand the underlying physical and mathematical concepts.

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Glossary of astronomy and astrophysics terms
Term	Technical Definition	Physical Comments
Cluster of galaxies	Large ensemble of galaxies bound by gravity.	Large-scale structure, indicative of the distribution of dark matter and cosmology.
Anisotropy of the Cosmic Microwave Background	Small temperature variations in the residual radiation of the universe.	Provides clues about the density, curvature, and history of the universe.
Planetary rings	Disks composed of ice and rock particles orbiting a planet.	Gravitational stability governed by tidal forces and orbital resonance.
Light-year	Distance traveled by light in a vacuum in one Earth year (~9.46×10¹⁵ m).	Fundamental unit of measurement in astrophysics, related to the speed of light \(c\).
Aphelion	Point in the orbit of a celestial body farthest from the Sun.	Applies Kepler's laws; orbital speed is minimal.
Apoapsis	Orbital point farthest from the body around which an object orbits (generalization of aphelion).	Important in orbital mechanics to characterize elliptical orbits.
Astrology	Belief claiming that the position of celestial bodies influences human destiny.	Non-scientific; not based on the empirical and physical methods of astronomy.
Astrophotography	Technique of capturing images of the night sky and celestial objects.	Exploits the optical properties of telescopes and cameras sensitive to low light flux.
Asteroid	Small rocky body mainly orbiting in the belt between Mars and Jupiter.	Residue from the formation of the solar system, notable local gravitational interaction.
Atmosphere	Gaseous envelope surrounding a planet or celestial body.	Protects and modulates the thermal, chemical, and radiative environment of the surface.
Big Bang	Cosmological model describing the origin of the universe by an initial very dense and hot expansion.	Basis of modern cosmology, supported by background radiation and primordial nucleosynthesis.
Blazar	Active galactic nucleus with a relativistic jet directed towards Earth.	Very energetic non-thermal emission phenomenon, linked to a supermassive black hole.
Kuiper Belt	Disk of small icy bodies beyond the orbit of Neptune.	Probable source of some short-period comets.
Night sky	View of the sky observed during the Earth's night, with objects visible to the naked eye or instruments.	Allows the study of light and astrophysical phenomena without solar disturbance.
Comet	Icy body developing a coma and a tail as it approaches the Sun.	Sublimation/radiation interaction with the Sun, primitive material from the solar system.
Cosmological constant	Term added to Einstein's equations representing a repulsive vacuum energy.	Associated with the acceleration of the expansion of the universe, often linked to dark energy.
Hubble constant	Rate of expansion of the universe measured in km/s/Mpc.	Value in tension according to methods: CMB observations vs supernovae.
Curvature of spacetime	Geometric deformation of spacetime caused by mass-energy according to general relativity.	Source of gravitational effects; mathematical basis of modern gravitation.
Blueshift	Decrease in the wavelength of an electromagnetic signal due to relative approach.	Inverse of redshift, classic Doppler phenomenon in astrophysics.
Redshift	Increase in the wavelength of radiation due to relative distance or cosmological expansion.	Fundamental measurement for the study of the speed of galaxies and the expanding universe.
Doppler effect	Apparent change in the frequency of a signal due to the relative motion of the source and observer.	Used in astrophysics to measure the radial velocities of celestial bodies.
Dark energy	Hypothetical component of the universe responsible for the acceleration of its expansion.	Unknown nature, influences global cosmological dynamics.
Star	Celestial body producing energy through nuclear fusion in its core.	Dynamic astrophysical object with life cycles defined by mass and composition.
Binary star	System of two stars gravitationally bound in mutual orbit.	Allows the direct measurement of stellar masses through orbital kinematics.
Neutron star	Compact remnant of a collapsed massive star, composed mainly of neutrons.	Extreme density, compact object with intense magnetic field and rapid rotation.
Shooting star	Luminous trail due to the combustion of a meteoroid entering the Earth's atmosphere.	Atmospheric phenomenon related to the impact of extraterrestrial objects at high speed.
ESA	European Space Agency responsible for civilian space missions and research in Europe.	International collaboration, development of satellites, telescopes, and probes.
Exoplanet	Planet orbiting a star other than the Sun.	Essential study for understanding planetary formation and habitability.
Cosmic filament	Filamentous structure of matter on a large scale forming the cosmic web.	Distribution of dark matter and baryonic matter following gravity and the expansion of the universe.
Nuclear fission	Process of splitting a heavy atomic nucleus into lighter nuclei.	Source of energy in certain types of stars and terrestrial applications.
Gravitational force	Attractive force between masses, described by Newton's law of universal gravitation.	Foundation of orbital mechanics, dynamics of celestial bodies, and cosmic structure.
Frequency	Number of oscillations of a wave per unit of time.	Inverse of the period, characterizes electromagnetic and acoustic waves.
Nuclear fusion	Nuclear reaction where light nuclei combine to form a heavier nucleus.	Source of energy for stars, generating photons and subatomic particles.
Galaxy	Large ensemble of stars, gas, dust, and dark matter bound gravitationally.	Fundamental unit in the structure of the universe, with different morphologies.
Event horizon	Boundary around a black hole beyond which nothing can escape.	Defines the causal region, key concept in general relativity.
Cosmic inflation	Phase of extremely rapid exponential expansion of the primordial universe.	Explains the homogeneity and isotropy observed on a large scale.
ISS	International Space Station in low Earth orbit.	Scientific research platform in microgravity and Earth observation.
Gravitational lens	Deflection of light from a distant object by an intermediate massive body, according to general relativity.	Allows observation of very distant galaxies and detection of dark matter.
Libration	Apparent oscillation of a celestial body, revealing non-visible areas.	Phenomenon observed, for example, on the Moon due to elliptical orbit and synchronous rotation.
Wavelength	Distance between two successive crests of a wave.	Fundamental characteristic of waves, inverse of frequency.
Light	Electromagnetic radiation visible to the human eye.	Transport of energy and information, basis of optical astronomy.
Visible light	Part of the electromagnetic spectrum perceptible by the human eye (approximately 400–700 nm).	Used for classical astronomical observation, imaging, and spectroscopy.
Baryonic mass	Mass associated with particles composed of quarks (protons, neutrons).	Represents only about 5% of the total energy density of the universe.
Dark matter	Hypothetical component of the universe, interacting gravitationally but not electromagnetically.	Would represent about 27% of the content of the universe, without direct detection to date.
Meteor	Luminous trail created by a meteoroid entering the Earth's atmosphere.	Temporary phenomenon, often confused with a shooting star.
Meteorite	Fragment of a meteoroid that has reached the Earth's surface.	Object of study in geochemistry and the history of the solar system.
Meteoroid	Small rocky or metallic body in orbit in the solar system.	Source of meteors and meteorites, often cometary or asteroidal debris.
Brown dwarf	Substellar object not massive enough to trigger hydrogen fusion.	Characterized by low luminosity, infrared emission.
White dwarf	Final stage of evolution of a low to medium mass star.	Compact, composed of degenerate matter, cools slowly.
Red dwarf	Low mass and temperature star, extremely durable.	Slow fusion of hydrogen, very common in the galaxy.
NASA	American space agency responsible for civilian space programs and research.	Leader in space technologies, exploration, and astrophysics.
Nebula	Cloud of interstellar gas and dust visible by emission or reflection.	Site of star formation or supernova remnant.
Nova	Thermonuclear explosion on the surface of a white dwarf in a binary system.	Sudden increase in luminosity followed by rapid decline.
Primordial nucleosynthesis	Formation of the first light nuclei (H, He, Li) a few minutes after the Big Bang.	Predicts abundances consistent with current cosmological observations.
Observable universe	Part of the universe accessible to observation given the finite speed of light.	Limited by the cosmological horizon, about 46 billion light-years.
Gravitational waves	Ripples in spacetime generated by accelerated masses (e.g., merging black holes).	First detected in 2015 by LIGO; open a new window of observation.
Orbital	Relating to the trajectory of a body around another under gravitation.	Includes parameters such as eccentricity, inclination, periapsis, apoapsis.
Observatory	Facility equipped to observe astronomical phenomena.	Can be ground-based or space-based, equipped with various telescopes and instruments.
Parsec	Unit of distance equal to about 3.26 light-years, based on parallax.	Used to measure stellar distances in the galaxy.
Periapsis	Orbital point closest to the central body (generalization of perigee and perihelion).	Maximum orbital speed, defined by celestial mechanics.
Perigee	Orbital point closest to Earth in the orbit of a satellite or moon.	Important for artificial satellites and Earth observation.
Perihelion	Orbital point closest to the Sun.	Maximum orbital speed according to Kepler's second law.
Physical phenomenon	Observable manifestation resulting from physical interactions in the universe.	Includes gravitation, electromagnetism, thermodynamics, and nuclear physics.
Planet	Celestial body orbiting a star, spherical by gravity, having cleared its orbit.	Classification by composition (terrestrial, gaseous), atmosphere, and dynamics.
Pulsar	Rapidly rotating neutron star emitting regular radio pulses.	Phenomenon related to intense magnetic field and rotation, precise cosmic clock.
Quasar	Very luminous active galactic nucleus, powered by a supermassive black hole.	Extreme high-energy phenomenon, visible at very great distances.
Schwarzschild radius	Limiting radius from which a mass forms a black hole, according to general relativity.	Defines the event horizon of a black hole.
Cosmic Microwave Background (CMB)	Residual microwave radiation from the Big Bang, witness of the young universe.	Main source of information on primordial cosmology.
Cosmic radiation	Energetic particles, mainly protons, coming from space and bombarding the Earth.	Influences terrestrial detectors and provides information on violent astrophysical events.
Gamma rays	Very high-energy electromagnetic radiation, beyond X-rays.	Emissions linked to violent phenomena such as supernovae, pulsars, black holes.
X-rays	High-energy electromagnetic radiation used in astrophysics.	Observed in hot sources such as binary stars, nebulae, and black holes.
Rover	Mobile robot designed to explore planetary or lunar surfaces.	Direct study instrument, capable of analyzing soil, atmosphere, geology.
Singularity	Point where the density and curvature of spacetime become infinite.	Central characteristic of black holes, unresolved theoretical problem.
Space probe	Device sent into space to collect scientific data.	Allows the study of distant objects beyond the reach of Earth-based observations.
Electromagnetic spectrum	Set of electromagnetic waves classified by frequency and energy.	Basis of astrophysics for multi-wavelength analysis of celestial bodies.
Spectroscopy	Technique of analyzing light decomposed into wavelengths to characterize matter.	Allows identification of chemical elements, temperatures, velocities by Doppler effect.
Supercluster	Grouping of several galaxy clusters forming a megastructure.	Illustration of large structures and the distribution of matter in the universe.
Supergiant	Very massive and luminous star in an advanced stage of evolution.	Predecessor of supernovae, extreme mass and luminosity.
Supernova	Cataclysmic explosion of a star at the end of its life, releasing colossal energy.	Production of heavy elements, influences galactic dynamics.
Solar system	Set formed by the Sun, its planets, satellites, asteroids, and comets.	Typical example of a planetary system, fundamental study in astrophysics.
Telescope	Optical or radio instrument for collecting and amplifying light from celestial objects.	Basis of astronomical observation, design related to optics and detectors.
Hubble Space Telescope	Optical telescope in low Earth orbit, observations without atmospheric disturbance.	Revolution in astronomy through high-resolution imaging and spectroscopy.
James Webb Space Telescope	Infrared space telescope intended for the study of the primordial universe and exoplanets.	Allows observation of very distant objects thanks to increased sensitivity and resolution.
Ultraviolet	Electromagnetic radiation with a wavelength shorter than visible light.	Used to study atmospheres, hot stars, and energetic phenomena.
Universe	Set of everything that exists: space, time, matter, and energy.	Object of study of cosmology, expanding since the Big Bang.
Observable universe	Part of the universe whose light has had time to reach us.	Limited by the speed of light and the age of the universe.
Cosmic void	Region of space almost devoid of galaxies and visible matter.	Major component of the large-scale structure of the universe.
Milky Way	Spiral galaxy hosting the solar system.	Contains hundreds of billions of stars, visible and dark matter.
Habitable zone	Region around a star where conditions allow the presence of liquid water.	Key criterion for the search for potentially habitable exoplanets.