Observatoire de Paris

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Image without iridescences. The achromatism is a property of an optical system which the response is independent from the wavelength of the incident radiations, and therefore from their colours.

small body that orbits the Sun, located between Mars and Jupiter, with a diameter up to a few hundred kilometers.

The study of the internal structure of the Sun is the heliosismology, the extension of this study to other stars is the asterosismology.

Astronomical Unit
The astronomical unit (AU) is the mean distance between the Earth and the Sun. One AU is about 149,6 million kilometers, i.e. 500 light-seconds.

- B -

binary systems
a binary system is made up of two stars orbiting around their center of mass.

brown dwarf
Small body with a low mass, star too cold to start nuclear reactions. The brown dwarves do not shine and can not be directly observed. They could be a component of the dark matter of the universe and explain, at least partly, the problem of the missing mass in the universe.

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Mini-satellite (in collaboration with the USA) dedicated on the study of the effects of clouds and aerosols on the climate

Charge Coupled Device. Photon detector : integrated circuit of millions of detectors.

clean room
Room where the atmospheric conditions (dust, temperature, pression, dampness, ...) are controled and automatically regulated. A class system is used to measured the amount of dust allowed. An air flux from the ceiling to the ground can be set up in a clean room. Concerning the pression, it can be useful to choose a slight high pressure, or a slight low pressure (compared to outside), depending on the goal pursuited. The clean rooms have an airlock.

Small body made of rocks and ice, on an eccentric orbit around the Sun (it is an elongated ellipse that makes the object come very close to the Sun, melting and sublimating the comet's ices)

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The Doppler-Fizeau effect is the shift between the frequency of the wave emitted and the wave received, when the emitting body moves compared with the observer. This effect was proposed by Christian Doppler in 1842 (sound waves) and by Hippolyte Fizeau in 1848 (electromagnetic waves). This effect explains why the pitch of a sound is different depending on whether the vehicle gets closer to (the sound gets more high-pitched) or moves away (the sound gets deeper) from the receptor.

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conic parameter which represents the flattening of an ellipse. A null eccentricity corresponds to a circle.

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Fourier (Joseph)
Joseph Fourier (March, 21, 1768 in Auxerre - May, 16, 1830 in Paris), mathematician and french physician, kniwn for his work on the decomposition of periodic functions in convergent trigonometric series called Fourier series.

fusion of the atomic nuclei, which provides energy.

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galactic plane
Symmetry plane of the galaxy. The major part of the stars of the galaxy are in the galatic plane.

set of a few million or a few billion stars. A galaxy also contains some dust, some gas and probably a halo of dark matter.

The galaxy that hosts our solar system (the Milky Way)

globular cluster
Spherical cluster of stars, denser than an open cluster, containing about a hundred thousand stars.

Global Oscillation Group

- H -

The harmonics are the decomposition of a vibration in periodic functions, which are easier to analyze. The frequencies of these harmonics are some multiples of the frequency of the initial function, and the infinite sum of all the harmonics recreates the initial function.

American space telescope in orbit around the Earth.

- I -

assembly of the elements of an instrument

interstellar medium
Set of extremely diffuse materials (gas and dust), dispatched up into several more or less dense clouds between the stars.

- J -

program of space oceanography

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- L -

A light-year is a unit of distance. It is the distance that light can travel in one year, corresponding to 9.463.10^15 m.

When a quantity strongly varies, the usual scale is not suitable. The logarithmic scale is an alternative to the linear scale. It expands low values and move high values closer. At a fixed length, it makes correspond the multiplication by a given number (example : 2 graduations on the x-axis correspond to a multiplication by 100, 3 graduations a multiplication by 1000, etc.)

- M -

Mayor (Michel)
Swiss astrophysicist, born on January 12, 1942. Member of Geneva Observatory and Professor at the University of Geneva, he is, with Didier Queloz, the discoverer of the first extrasolar planet around a star of the main sequence,51 Pagasi, in 1995.

1micron = 0,000001 m

- N -

neutron star
A neutron star is a very dense object of several solar masses and a diameter of the order of 10 kilometers, which is the final stage of the evolution of some stars. It is made up of neutrons stuck together, and its density is about one hundred million tons per cubic centimeter (100 000 000 T/cm3).

star which suddenly becomes 10000 to 100000 times brighter during a very short time of few hours. It is probable that the novae are binary systems combining a white dwarf and a red giant. They become very luminous, following the mass transfer between two components of the system.

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A parsec (from [par]allax and arc[sec]ond) is the distance from Earth at which 1 AU is equal to 1 arcsecond on the sky. 1pc=3.26light-years.

the interval of time taken by the planet to complete one orbit around the star.

Techniques used to determine the magnitude of stars in different colours (wavelength intervals). This technique complement the spectrometry, which measures the light from the stars dispersing it according to the wavelength.

Device used to detect and count photons

fundamental particles, vehicle for the electromagnetic interactions. The photons are light "grains".

this is the region of the atmosphere of a star (the Sun in particular) where almost all the optical radiation comes from. It is located between the convective zone and the chromosphere, it is about 200 km thick.

Image element, the smallest homogeneous surface of an image recorded by a computing system.

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

prebiotic molecule
Assembly of carbon, hydrogen, oxygen, and nitrogen atoms. The prebiotic molecules are the origin of life on Earth.

The rotation axis of the Earth is not always in the same direction, it slowly moves describing a cone-shaped motion. This is the precession of the equinoxes. More generally, the precession is the conical movement described around the rotation axis of a body

Acronym of "Plateforme Reconfigurable pour l'Observation, les TElécommunication et les Usages Scientifiques", that is a platform that can be set up for observing, telecommunications and scientific purposes.

Particle of the atomic nucleus. The mass of the proton is 1,672 623 1.10^-27 kg, i.e. 1800 times heavier than the electron.

a neutron star (extremely dense) that rotates rapidly. The observer sees it as a source of radiation that fluctuates in intensity.

pure sound
The simplest sound has only one harmonic, the fundamental frequency.

- Q -

- R -

radial velocity
Component of the velocity of a star parallel to the line of sight. The radial movement of the star leads to a variation of the frequency of the lines emitted by the star (doppler effect). A periodic variation of the radial velocity can indicate the presence of a planet.

- S -

In astronomy, a satellite is an object, natural or artificial (space vehicle), which orbits another body.

semi-major axis
half of the distance between the two most distant points of the orbit.

The sismology, or seismology, studies the seisms (earthquakes) and more generally the propagation of waves in the interior of the Earth. The tools of seismology can be applied to study the waves generated by the turbulent convective movements in the interior of the stars or planets.

spectral line
A spectral line is a dark line (absorption) or luminous (emission) in an electromagnetic spectrum, which characterizes a radiation of given frequency and coreesponds to a given object.

Instrument which analyzes the light from the Sun at a given wavelength. A sweeping system allows to reconstitute an image (monochromatic) of the solar disk.

Techniques and methods of the study of the light of a body dispersed into a spectrum depending on the wavelength. The study of the spectrum allows to determine the chemical composition and the physical features of the radiation sources.

STEllar PHotometry International

The supernova phenomenon is the result of the global explosion of a star, so they are very bright. They are two types of supernovae : Either they are the result of a transfer of mass between the two components of a binary system (as the novae). Or they correspond to the normal end of life of a star with a mass greater than 9 solar masses, which the core collapses to form a neutron star or a blackhole, and which the external layers are violently evicted.

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a telescope is an instrument dedicated to astronomical observations with a mirror as objective

The telluric planets are like the Earth : they are made of rocks and have a solid surface. These planets differ from the gas giant planets in their lower diameter and mass, and in their higher density values. The telluric planets (Mercury, Venus, Earth and Mars) orbit the Sun with radii less than 400 million kilometers.

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- V -

variable stars
stars for which the luminosity varies in a perceptible way on short periods, contrary to the Sun which do not have any measurable variation (about 0.1% on a cycle of 11 years).

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white dwarf
Star which has consumed all its nuclear fuel. The pressure exerted by the nuclear combustion and the gravity forces do not balance each other, and the star collapses. The matter reaches a degenerate state. The white dwarves are extremely dense. A white dwarf with the mass of the Sun can have the size of the Earth, and a density of the order of one ton per cm3. This is the ultimate step of the evolution of the low mass stars.

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