Dictionnaire de l’Astronomie
Définitions claires et accessibles des principaux concepts astronomiques — de l’énergie noire aux trous noirs, en passant par le Big Bang et la matière noire.
Que vous soyez débutant ou passionné, ce dictionnaire vous aide à comprendre le vocabulaire de l’astronomie moderne. Chaque terme renvoie vers un article détaillé avec exemples et explications approfondies.
Absolute magnitude
Absolute magnitude is a measure of a celestial body’s actual brightness, regardless of its distance from the observer.…
Accretion disk
An accretion disk is a flat, disk-shaped structure composed of gas and dust that orbits in a spiral…
Active galactic nucleus
An active galactic nucleus, often abbreviated as AGN (Active Galactic Nucleus), refers to the center of a galaxy…
Age of the Universe
The age of the universe— the time that has elapsed since the Big Bang until today — is…
Anisotropy
Anisotropy is the property whereby a physical quantity varies depending on the direction in which it is measured.…
Aphelion and perihelion
In an elliptical orbit around the Sun, two specific points define the extreme distances: the aphelion is the…
Apparent magnitude
Apparent magnitude is a measure of a celestial body’s brightness as observed from Earth, regardless of its actual…
Asteroid
An asteroid is a small rocky or metallic body orbiting the Sun. Asteroids are mainly concentrated in the…
Astronomical filter
An astronomical filter is a transparent optical element that allows only a portion of the light spectrum to…
Astronomical unit
The astronomical unit, abbreviated as AU (or AU for astronomical unit), is a unit of length used in…
Baryogenesis
Baryogenesis refers to the set of physical processes that produced the asymmetry between matter and antimatter in the…
Baryonic matter
Baryonic matter is the scientific term for ordinary matter: everything composed of atoms, and therefore of protons, neutrons,…
Big Bang
The Big Bang is the cosmological model that describes the evolution of the universe from an initial state…
Black Hole
A black hole is a region of space where gravity is so strong that nothing can escape it—not…
Black Hole Evaporation
Black hole evaporation is a quantum phenomenon theoretically predicted by Stephen Hawking in 1974. According to this process,…
Brightness
The luminosity of a star is the total power it radiates in the form of light and radiation…
Brown Dwarf
A brown dwarf is a strange astrophysical object, halfway between a giant planet and a small star. With…
Collimation
Collimation is the process of precisely aligning all the optical elements of a telescope— mirrors or lenses—along a…
Comète
A comet is a small body in the solar system composed mainly of ice, dust, and rock. When…
Cosmic inflation
Cosmic inflation is a phase of nearly instantaneous and colossal expansion of the universe that occurred in the…
Cosmic Microwave Background
The cosmic microwave background (CMB), also known as fossil radiation, is the oldest thermal radiation in the universe.…
Cosmic radiation
Cosmic radiation refers to a stream of very high-energy subatomic particles that constantly reach Earth from space. Contrary…
Cosmic Time
Cosmic time is the universal measure of time used in cosmology to describe the evolution of the universe…
Cosmological constant
The cosmological constant, denoted by λ, is a term that Einstein introduced in 1917 into his equations of…
Critical density
The critical density is the average density of matter and &energy that the universe must have in order…
Dark energy
Dark energy is the most abundant component of the universe—and the most mysterious. It is estimated to account…
Dark Matter
Dark matter is one of the greatest mysteries of modern physics. It neither emits nor absorbs light, making…
Doppler Effect
The Doppler effect is the change in the frequency of a wave—sound, light, or other—perceived by an observer…
Dwarf planet
A dwarf planet is a celestial body that orbits the Sun, has sufficient mass for its gravity to…
Elliptical orbits
An elliptical orbit is the elliptical path followed by a celestial body— planet, comet, or satellite—follows around another…
Equatorial Mount
An equatorial mount is a type of mechanical telescope mount in which one axis of rotation—called the polar…
Event Calendar
The event horizon is the invisible boundary that defines a black hole. It is the ultimate point of…
Exoplanet
An exoplanet—or extrasolar planet—is a planet that orbits a star other than the Sun. For a long time,…
Expansion of the Universe
The expansion of the universe is the phenomenon by which space itself expands, gradually pushing galaxies farther apart…
Exposure time
The shutter speed, or exposure time, is the duration during which the camera sensor is exposed to light.…
Galactic dark matter
Galactic dark matter refers to the invisible component of matter that makes up the majority of a galaxy’s…
Galactic disk
The galactic disk is the main, flattened structure of spiral galaxies such as the Milky Way. It forms…
Galactic Halo
The galactic halo is the most extensive component of a galaxy. It is a spherical, diffuse region that…
Galaxy
A galaxy is a gigantic system composed of stars, gas, dust, and dark matter, all held together by…
Galaxy Collision
A galaxy collision is the gravitational interaction between two or more galaxies that move closer together and eventually…
Galaxy Formation
Galaxy formation refers to the set of physical processes through which the diffuse matter of the early universe…
Gamma-ray burst
A gamma-ray burst, or GRB (Gamma-Ray Burst), is an extremely intense flash of gamma rays that lasts from…
Globular cluster
A globular cluster is a spherical grouping of tens to a few hundred thousand stars that are strongly…
Gravitational lens
Gravitational lensing is the phenomenon by which light is deflected from its straight path when it passes near…
Gravitational singularity
A gravitational singularity is a point or region of spacetime where the equations of general relativity cease to…
Gravitational wave
Gravitational waves are ripples in the fabric of spacetime itself, produced by the accelerated movements of cosmic masses.…
Gravity
Gravity is one of the four fundamental forces of nature. It is the force of attraction that acts…
Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram, often abbreviated as the HR diagram, is one of the most important tools in stellar…
Hubble constant
The Hubble constant, denoted by H₀, quantifies the rate of expansion of the universe. It expresses the speed…
Intergalactic medium
The intergalactic medium (IGM) refers to the diffuse gas and matter that fill the vast space between galaxies.…
Interstellar matter
Interstellar matter (ISM) refers to all the gas and dust scattered throughout the space between the stars of…
Isotropy
Isotropy is the property of a system or medium in which the physical characteristics are identical regardless of…
Lambda-CDM Model
The Lambda-CDM model is the standard cosmological model of modern physics. Its name combines two fundamental elements: Lambda,…
Light pollution
Light pollution refers to the overall negative effects of excessive or poorly directed artificial lighting on the nighttime…
Light spectrum
A light spectrum is a representation of light broken down into its different wavelengths. Just as a prism…
Light-year
The light-year is a unit of distance equal to the distance traveled by light in a vacuum during…
Living area
The habitable zone, sometimes called the Goldilocks zone (like Goldilocks—not too hot, not too cold, but just right),…
Magnétar
A magnetar is a very rare type of neutron star characterized by an extraordinarily intense magnetic field, about…
Magnification
A telescope’s magnification is the factor by which the instrument enlarges the image of an object compared to…
Magnitude limit
The limiting magnitude refers to the apparent magnitude of the faintest celestial object that an instrument—or the naked…
Main sequence
The main sequence is the dominant stage in a star’s life. A star is said to be on…
Metallicity
Metallicity is a measure of the abundance of chemical elements heavier than helium in a star or an…
Meteor
A meteor is the luminous phenomenon produced when a fragment of solid matter— a meteoroid—enters Earth’s atmosphere at…
Meteorite
A meteoroid is a small solid body orbiting within the solar system, smaller than an asteroid. By convention,…
Meteorite
A meteorite is a fragment of extraterrestrial matter—from an asteroid,a comet, the Moon, or Mars—that has passed through…
Milky Way
The Milky Way is the name of our galaxy— that gigantic collection of stars, gas, dust, and dark…
Nebula
A nebula is a cloud of interstellar gas and dust, sometimes spanning tens or hundreds of light-years. The…
Neutron star
A neutron star is the stellar remnant left behind by the supernova explosion of a massive star. Despite…
Nuclear fusion
Nuclear fusion is the reaction in which two light atomic nuclei combine to form a heavier nucleus. This…
Nucleosynthesis
Nucleosynthesis refers to the set of processes by which atomic nuclei are forged from more elementary particles. It…
Observable Universe
The observable universe refers to the portion of the universe from which light has had time to reach…
Oort Cloud
The Oort Cloud is a hypothetical spherical region surrounding the solar system at considerable distances— between 2,000 and…
Open cluster
An open cluster is a relatively loose grouping of stars that share a common origin— they were born…
Orbital Resonance
Orbital resonance is a situation in which two bodies orbiting the same central object have orbital periods that…
Parsec
The parsec is the unit of distance preferred by professional astronomers for expressing distances on a stellar and…
Photon-Matter Decoupling
Photon-matter decoupling, also known as recombination, is one of the most important events in the history of the…
Planetary Nebula
A planetary nebula is a shell of luminous ionized gas ejected by a low- or intermediate-mass star at…
Pulsar
A pulsar is an extremely rapidly rotating neutron star that emits two beams of electromagnetic radiation along its…
Quantum fluctuations
Quantum fluctuations are random and spontaneous variations in energy that occur in a vacuum according to the laws…
Quasar
A quasar—a contraction of quasi-stellar object—is the active nucleus of a distant galaxy with extraordinary luminosity. Powered by…
Recombination
Recombination refers to the event that occurred approximately 380,000 years after the Big Bang, when theuniverse had cooled…
Red dwarf
A red dwarf is a star with very low mass and low luminosity, whose surface temperature is less…
Red Giant
A red giant is a star in an advanced stage of its evolution, characterized by a considerable expansion…
Redshift
Redshift refers to the shift toward longer wavelengths of light emitted by an object that ismoving away from…
Relativistic jet
A relativistic jet is a narrow, collimated stream of plasma—electrons, protons, and positrons— ejected perpendicular to the accretion…
Resolution Power
The resolving power ofan optical instrument is its ability to distinguish two very close light sources as two…
Right Ascension and Declination
The right ascension (RA, sometimes AD) and the declination (Dec) are the two coordinates of the celestial equatorial…
Satellite
A satellite is a celestial or artificial object that orbits a more massive body under the influence of…
Seeing
“Seeing” refers to the stability of Earth’s atmosphere from an astronomer’s perspective. When light from a star passes…
Solar mass
The solar mass is the unit of reference used in astronomy to express the mass of stars, black…
Solar System
The solar system is the system consisting of the Sun and all the celestial bodies that orbit it…
Solstice and Equinox
The solstices and equinoxes are four key astronomical events that structure the Earth’s year and mark the seasonal…
Speed of light
The speed of light in a vacuum is one of the fundamental constants of physics. It is exactly…
Spiral arms
Spiral arms are the characteristic spiral-shaped structures of spiral galaxies, which include our Milky Way. These arms are…
Spiral galaxy
A spiral galaxy is one of the three main types of galaxies, characterized by a flat, rotating disk…
Star Formation
Star formation is the process by which diffuse matter in the interstellar medium— mainly hydrogen and helium—condenses and…
Stellar Variability
Stellar variability refers to any change in a star’s luminosity over time. These variations can be periodic or…
Stellar wind
The stellar wind is the continuous flow of charged particles— primarily protons, electrons, and helium nuclei — that…
Supergiant
A supergiant is a massive star that has left the main sequence and is in the advanced stages…
Supernova
A supernova is the catastrophic explosion of a star at the end of its life. In a matter…
Surface temperature
A star’s surface temperature—also known as its effective temperature—is the temperature of its outer radiating layer, the photosphere.…