Glossary

Proportion of these elements in the universe, often reported in relation to the quantity of hydrogen, the most abundant element.

Capture of neighbouring matter by the gravitational attraction of an object, for example a star in formation.

Image without iridescence. Achromaticity is a property of an optical system whose response is independent of the wavelength of incident radiation and therefore of its colour.

An observational technique that corrects in real time for distortions in the image of a celestial object by eliminating defects caused by atmospheric turbulence.

Fraction of incident light reflected by a body at a given wavelength.

The point furthest from the Sun in a heliocentric orbit.

A small celestial body orbiting the Sun with a diameter of up to a few hundred kilometers. The vast majority is located in the Main Belt ^? , between Mars and Jupiter.

The measurement of the position, distance and movement of celestial bodies (also known as positional astronomy).

Unit of distance based on the semi-major axis ^? of the Earth’s orbit around the Sun. It is equal to 149,597,870,700 metres, or approximately 500 light seconds. Noted: au ^? .

System consists of two stars orbiting around their common centre of gravity.

A small, low-mass body, too cold to undergo nuclear reactions. Brown dwarfs ^? do not radiate like stars and cannot be observed directly.

Light photon ^? detector: integrated circuit comprising millions of detectors.

Stony meteorite (less than 35% metal) containing chondrules ^? .

These small submillimetre-sized beads, usually composed of silicates, are embedded in a chondrite ^? . These droplets, which have melted and then cooled, are the oldest materials in the Solar System.

A room in which atmospheric conditions (dust, temperature, humidity, pressure, etc.) are controlled and regulated automatically. A classification system is used to measure the amount of dust permitted. For pressure, it may be appropriate to choose a slight overpressure or slight underpressure (relative to the outside) depending on the desired objective. Clean rooms ^? are equipped with an airlock.

Small body composed of rock and ice that orbits the Sun in a highly eccentric orbit (a very elongated ellipse that brings the body very close to the Sun, causing the ice it is composed of to melt and sublimate).

The moment when the position of a planet, as seen from Earth, is very close to the Sun. Mercury and Venus have inferior conjunctions ^? (when the planet is in front of the Sun) and superior conjunctions ^? (when the planet is behind the Sun).

An astronomical technique that involves masking the central object (usually the Sun or a star) to reveal its immediate surroundings (the solar corona ^? or possibly a circumstellar disc or exoplanets). This technique is widely used for direct imaging of exoplanets.

A process similar to volcanism that occurs on certain icy satellites ^? (Titan, Triton, etc.). The ejected material is water, ammonia or methane under pressure, which recondenses immediately upon reaching the surface.

Shift between the frequency of the emitted wave and that of the received wave when the transmitter and receiver are moving relative to each other. This effect was proposed by Christian Doppler in 1842 (sound waves) and by Hippolyte Fizeau on electromagnetic waves in 1848. This explains why the pitch of a vehicle’s sound differs depending on whether you are inside it (the sound emitter is stationary relative to the receiver), whether the vehicle is approaching the receiver (the sound becomes higher pitched) or whether it is moving away (the sound becomes lower pitched).

Parameter of a conic section representing the flattening of the ellipse. Zero eccentricity corresponds to a circle.

The apparent and temporary disappearance (or decrease in brightness) of a celestial body when another celestial body passes between it and the source of light that illuminates it (as in the case of a lunar eclipse ^? , when the Moon passes into the Earth’s shadow) - or between that celestial body and the observer (as in the case of a solar eclipse ^? for an observer in the shadow of the Moon). This second configuration is more precisely called an occultation ^? or a transit for an exoplanet passing in front of its star.

Used to locate a point in the sky relative to the plane of the Earth’s equator. Equatorial coordinates ^? are right ascension (RA, alpha) and declination (DEC, delta).

Plane of symmetry of the Galaxy ^? . Most of the stars in the Galaxy ^? lie in the galactic plane ^? .

The galaxy ^? in which the Sun is located (the Milky Way).

A collection comprising several million to several billion stars, as well as a greater or lesser amount of gas and dust, and most likely also an extended halo of dark matter.

A spherical cluster of stars denser than an open cluster, typically containing hundreds of thousands of stars.

A phenomenon that occurs when a massive object is located between a distant object and the observer. The deviation of the background object’s light rays by the massive object concentrates them on the observer, producing an amplification of the signal.

Process leading to the warming of a planetary atmosphere when its constituents are transparent to visible solar radiation but absorb infrared radiation. The most effective greenhouse gases are CO₂, H₂O and CH₄. Solar radiation (maximum in the visible range) heats the surface, which, as it warms up, emits infrared radiation that is absorbed by the atmosphere, amplifying the warming effect. The mechanism can run out of control; the best example is the planet Venus.

Decomposition of a vibration into periodic functions, which are easier to analyze. The frequencies of these harmonics ^? are multiples of the frequency of the initial function, and the infinite sum of all harmonics ^? reconstructs the initial function.

Diagram representing the luminosity ^? of a group of stars as a function of their effective temperature. It allows us to distinguish between different types of stars and understand their evolution.

Space telescope orbiting the Earth, built by NASA with contributions from the European Space Agency.

In a protoplanetary disc ^? , the distance beyond which simple volatile molecules are found in the form of ice. It depends on temperature and is specific to each molecule (H₂O, CH₄, CO). In general, the term refers to the condensation of water.

Assembly of the components of an instrument.

An instrument consisting of several interconnected telescopes or radio telescopes, enabling high spatial resolution ^? and the mapping of celestial bodies. In spectroscopy ^? , the interferometer ^? also enables high spectral resolution ^? .

All of the extremely diffuse materials (gas and dust) distributed in clouds of varying density between stars.

Energy flux received by a planet from the star and averaged per unit area of the planet’s surface. The irradiance ^? received by the Earth (above the atmosphere) from the Sun is 1360 W/m².

Empirical laws stated by Johannes Kepler to describe the motion of planets around the Sun. They were explained by Newton in the context of the law of gravitation.

A ring-shaped zone located beyond Neptune’s orbit, considered to be the reservoir of periodic comets ^? with low inclination. It is also the reservoir of Centaurs and certain types of trans-Neptunian objects ^? .

The distance travelled by light in one year, corresponding to 9.463 × 10¹⁵ meters.

When the quantity to be represented varies greatly, the usual scale is not suitable. The logarithmic scale ^? is an alternative to the linear scale. It expands low values and brings high values closer together. At a fixed length, it corresponds to multiplication by a given number (e.g. for 2 graduations on the x-axis, it corresponds to multiplication by 100, 3 graduations - multiplication by 1000, etc.).

Refers to a hot body, such as a star or planet, is the total amount of energy radiated per unit of time.
p = 4 × π × R² × σ × T⁴ where T = body temperature, R=body radius and σ is the Stefan-Boltzmann constant and is equal to 5.67 × 10^-8 W × m^-2 × K^-4.

Logarithmic scale ^? used to measure the brightness of a celestial body. The lower the brightness of the celestial body, the higher its magnitude ^? .

Main reservoir of asteroids ^? , located between the orbits of Mars and Jupiter.

1 micron ^? = 0.000001 meter. Noted: µ.

Very dense body, with several solar masses and a diameter of around 10 kilometres, which is the final stage in the evolution of certain stars. As its name suggests, it is made up of neutrons packed tightly together, and its density is around one hundred million tonnes per cubic centimetre (100 000 000 t/cm³).

A star that suddenly becomes 10 000 to 100 000 times brighter for a very short period of time, lasting a few hours. Novae ^? are likely to be binary systems ^? consisting of a white dwarf ^? and a red giant star. They become very bright as a result of mass transfer between the two components of the system.

The formation of the nuclei of different chemical elements through nuclear reactions. The lightest elements (H, He, Li, Be, B) were formed just after the Big Bang (primordial nucleosynthesis). The heavier elements were synthesised in stars.

Angle between the orbital plane of the planet and its axis of rotation.

A phenomenon whereby a celestial body (star, planet, asteroid ^? , etc.) is completely or partially obscured by another celestial body. Stellar occultations ^? are used to determine the properties of objects in the Solar System that occult a star.

A vast spherical shell located between 30,000 and 100,000 AU ^? (one to two AU ^? from the Sun), believed to be the reservoir of new comets ^? with high inclinations, formed closer to the Sun like other small bodies, but ejected into this region following gravitational interactions with the giant planets.

The moment when a planet in the Solar System, as seen from Earth, is exactly opposite the Sun. The distance between Earth and the planet is then at its minimum. Only Mars and the giant planets, whose orbits are outside that of Earth, can be seen at opposition ^? .

Angle between the normal to the plane of the exoplanet’s orbit and the Earth-star line of sight. An angle of 90 degrees indicates an orbit perpendicular to the plane of the sky.

The time required for a celestial body (planet, star, asteroid ^? , etc.) to return to the same point in its orbit.

The parsec ^? (from “parallax of one arcsecond”) is the distance at which 1 astronomical unit ^? subtends an angle of 1 arcsecond. 1 parsec ^? = 3,26 light years ^? .

The point in a heliocentric orbit closest to the Sun.

Techniques used to determine the magnitude ^? of stars in different colors (wavelength intervals). This technique complements spectrometry, which also measures the light from stars, but breaks it down precisely according to wavelength.

Fundamental particle, carrier of electromagnetic interactions. Photons ^? are ‘grains’ of light.

This is the region of a star’s atmosphere (particularly the Sun) from which almost all optical radiation originates. In the Sun and other solar-type stars, it lies between the convective zone and the chromosphere and is approximately 200 km thick.

Image element, the smallest homogeneous area of an image recorded by a computer system.

Movement of a planet within a protoplanetary disc ^? , following its interaction with gas, planetoids or a planet contained within the disc.

Agglomeration of matter ranging from a few micrometers to a few kilometers in diameter in the cloud of gas and dust surrounding a young star.

Assembly of carbon, hydrogen, oxygen and nitrogen atoms. Prebiotic molecules ^? are the origin of life on Earth.

The Earth’s axis of rotation does not always point in the same direction; it moves slowly in a conical motion. This is known as the precession ^? of the equinoxes. More generally, precession ^? is the conical motion described around a fixed direction by the axis of rotation of a body.

A particle that makes up the atomic nucleus. The mass of a proton ^? is 1,7 × 10^-27 kg, about 1 800 times heavier than an electron.

A circumstellar disc formed of gas and dust created by the collapse of a rotating nebula. The protostar forms in the central part, while the planets form within the protoplanetary disc ^? .

Mass of interstellar gas and dust from which the Solar System formed, following its collapse into a disc.

A rapidly rotating neutron star ^? (extremely dense) whose radiation is observed intermittently.

Component of a star’s velocity parallel to the line of sight. The radial motion of the star induces a variation in the frequency of the lines emitted by the star (Doppler effect ^? ). A periodic variation in the radial velocity ^? of a star may indicate the presence of a planet.

A low-mass star, 0,08 and 0,4 M_⊙, with low luminosity ^? . Red dwarfs ^? are the most numerous stars in the Universe.

Ability of spectrograph to separate the observed spectrum. The higher the resolution ^? , the more detailed the spectrum obtained.

Distance from a planet below which a satellite ^? begins to break apart under the effect of tidal forces induced by the planet.

The time required for a celestial body (star, planet, asteroid ^? , etc.) to complete one rotation on its axis.

Natural or artificial object (spacecraft) that orbits another object.

Half the distance between the two furthest points of the orbit.

A luminous trail associated with the fall of an interplanetary particle into the atmosphere. This can be associated with the Earth passing through the orbit of a comet ^? .

Region of the Sun above the chromosphere that extends over several million kilometres. It can be observed, in particular, during total lunar eclipses ^? .

Dark line (absorption) or bright line (emission) line in an electromagnetic spectrum that characterises radiation of a given frequency and corresponds to a given body.

Techniques and methods for studying the light from a body broken down into a spectrum according to wavelength. Among other things, studying the spectrum makes it possible to determine the chemical composition and physical characteristics of radiation sources.

The angle at which the semi-major axis ^? of the Earth’s orbit would be seen from a star. This measurement is used to estimate the distance of the nearest stars.

The region of the atmosphere above the troposphere and below the mesosphere, in which solar ultraviolet radiation is absorbed, causing the temperature to increase with altitude. The Earth and the giant planets have a stratosphere ^? , but Mars and Venus do not.

The phenomenon of supernovae ^? results from the global explosion of certain stars, which then become very bright. There are two types of supernovae ^? : some – like novae ^? – result from mass transfer between the two components of a binary system ^? . Others correspond to the normal end of life of a star with a mass greater than between 8 and 10 solar masses, whose core collapses into a neutron star ^? or black hole, and whose outer layers are violently ejected.

Planets that resemble Earth: they are made of rock and have a solid surface. They differ from giant planets in that they have a smaller diameter and mass and a higher density. They orbit the Sun within a radius of less than 300 million kilometres (Mercury, Venus, Earth and Mars).

Differential attraction exerted on a celestial body (satellite ^? , planet, star, galaxy ^? ) subject to the gravity of a nearby massive body. The tidal effect ^? can deform the body, fracture it, or accelerate or slow down its orbit.

Object in the Solar System orbiting the Sun beyond the orbit of Neptune.

A star whose brightness varies noticeably over short periods of time, unlike our Sun, which has virtually no measurable variation (approximately 0.1% over an 11-year cycle).

Star, that has consumed all its nuclear fuel reserves. The pressure exerted by nuclear combustion and gravitational forces no longer balance each other out and the star collapses in on itself, with the matter reaching a state known as degenerate. White dwarfs ^? are extremely dense. A white dwarf ^? with the mass of the Sun can be the size of the Earth, with a density of around one ton per cm³. This is the final stage in the evolution of low-mass stars.