asymptotic giant branch (AGB)
šâxe-ye nâhamsâvi-ye qulân
Fr.: branche asymptotique des géantes
A region of the → Hertzsprung-Russell diagram populated by evolving → low-mass to → intermediate-mass stars. These stars have an electron → degenerate core of carbon and oxygen surrounded by two burning shells of helium and hydrogen. The H and He-burning shells are activated alternately in the deep layers of the star. An extended and tenuous convection envelope, having a radius of 104-105 times the size of the core, lies above these shells. The loosely bound envelope is gradually eroded by the strong → stellar wind, which forms a dusty → circumstellar envelope out to several hundreds of stellar radii. The convective envelope, stellar atmosphere, and circumstellar envelope have a rich and changing chemical composition provided by → nucleosynthesis processes in the burning shells in the deep interior.
Fr.: vitesse asymptotique
Atacama Large Millimeter Array (ALMA)
ârast-e bozorg-e milimetri-ye âtâkâmâ (ALMA)
One of the largest ground-based astronomy projects and a major new facility for world astronomy located on the plain of the → Chajnantor Chilean Andes, San Pedro de Atacama, some 5000 m above sea level. ALMA will initially comprise 66 high precision antennas, with the option to expand in the future. There will be an array of fifty 12 m antennas, acting together as an → interferometer to capture → millimeter and → submillimeter wavelengths of 0.3 to 9.6 mm. It will have reconfigurable baselines ranging from 15 m to 18 km. A compact array of 7 m antenna and few 12 m diameter antennas (ACA) will be used to measure the diffuse emission. Resolutions as fine as 0''.005 will be achieved at the highest frequencies. Construction of ALMA started in 2003 and will be completed in 2012. The ALMA project is an international collaboration between Europe, Japan, and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the → European Southern Observatory (ESO). The first 12 m diameter antenna, built by Mitsubishi Electric Corporation for the National Astronomical Observatory of Japan, was handed over to ESO in 2008. It will shortly be joined by North American and European antennas. ALMA will allow astronomers to study the cool Universe, i.e. the molecular gas and tiny dust grains from which stars, planetary systems, galaxies, and even life are formed.
Fr.: astéroïde Aten
yazdân-nâbâvari, xodâ-nâbâvari, a-yazdân-bâvari
1) The doctrine or belief that there is no → God.
Of or pertaining to the Atlantic Ocean.
M.E., from L. Atlanticum (mare) "the Atlantic (ocean)," from Gk. Atlantikos "of Atlas," adj. of → Atlas, in reference to Mount Atlas in NW Africa. So called because it lay beyond that mountain.
1) Atlas (#); 2) atlas (#); 3) Atlas
1) The second of → Saturn's known satellites. It has a
diameter of about 30 km and orbits Saturn between the outer edge of the A ring and
the F ring at a mean distance of about 137,600 km.
It was discovered by Richard Terrile in 1980 from Voyager 1 photos. Also known as
In Gk. mythology, Atlas a son of the Titan → Iapetus and the nymph Clymene. After the Titans revolted and lost a war against Zeus, Atlas was condemned by Zeus to stand forever holding up the heavens. He was identified with the Atlas Mountains in NW Africa.
javv (#), havâsepehr
1) The gaseous envelope surrounding a star, planet, or moon.
Several solar system planets
retain considerable atmospheres, due to their strong
gravitational force. The gas motions in the planetary
atmosphere, as a response to the heating, coupled with the rotation
forces, generate the meteorological systems. The planetary satellites
→ Titan and → Triton
also have atmospheres (M.S.: SDE).
New L. atmosphaera, from Gk. atmos "vapor" + spharia "sphere."
javvi, havâsepehri (#)
Pertaining to or existing in the atmosphere of an astronomical object such as a planet, moon, or star.
Fr.: absorption atmosphérique
The absorption of → electromagnetic radiation in the → atmosphere mainly by → water vapor, → carbon dioxide, and oxygen. The atmosphere introduces two more limiting factors in → remote sensing: → atmospheric scattering and → atmospheric turbulence.
Fr.: circulation atmosphérique
The large-scale movements of air around areas of high and low pressure whereby heat is distributed on the surface of the Earth. Atmospheric motion is driven by uneven heating of the planet. The atmosphere (and ocean) → transfers the excess heat from → tropics to → poles. The flow is determined by balance between → pressure gradients and the → Coriolis effect.
Fr.: dispersion atmosphérique
The splitting of starlight into a spectrum in the atmosphere because the atmosphere acts as a refracting prism. This phenomenon brings about a practical problem for spectroscopic observations using a slit. → differential refraction; → atmospheric refraction.
Fr.: émission atmosphérique
The emission of electromagnetic radiation from the atmosphere due to thermal and → non-thermal processes. → Thermal emission comes mainly from → water vapor. Non-thermal processes result in emission lines oxygen (optical) and OH (near-IR). Atmospheric emission is a very significant source of noise in astronomical observations. See also → airglow, → aurora.
Fr.: extinction atmosphérique
The decrease in the intensity of light from a celestial body due to absorption and scattering by Earth's atmosphere. It increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they do at the zenith.
muon-e javvi, ~ havâsepehri
Fr.: muon atmosphérique
A → subatomic particle produced when → primary cosmic rays, impinge on the Earth's atmosphere producing a particle cascade, in which secondary particles decay into → muons. In the energy range up to 100 → GeV atmospheric muons come mostly from the decay of secondary → pions: π±→ μ± + anti-νμ. At higher energies, the → kaon contribution to the muon flux become significant, reaching the asymptotic value of 27% at about 10 TeV: K±→ μ± + anti-νμ.
Fr.: neutrino atmosphérique
A neutrino produced in the collision of → cosmic rays (typically → protons) with nuclei in the → upper atmosphere. This creates a shower of → hadrons, mostly → pions. The pions decay to a → muon and a muon neutrino. The muons decay to an → electron, another muon neutrino, and an electron neutrino.
Fr.: bruit atmosphérique
Noise in radio wavelengths caused by natural atmospheric processes, mainly lightening discharges in thunderstorms. They can affect radio observations.
Fr.: réfraction atmosphérique
The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth's atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.
Fr.: diffusion atmosphérique
The → scattering of → electromagnetic radiation by various particles in the Earth's → atmosphere. The phenomenon is caused by collisions between photons and several scattering agents such as atoms, molecules, → aerosols, and water droplets in clouds. → Rayleigh scattering.
Fr.: turbulence atmosphérique
Random fluctuations of the atmosphere caused by the constant injection of energy into the atmosphere from solar and local sources, changing the temperature and pressure of the air where it is absorbed and leading to fluid instabilities. The development over time of the instabilities gives rise to fluctuations in the density of air, and therefore the → refractive index of the atmosphere. → turbulence; → seeing.