parâš-e Fresnel (#)
Fr.: diffraction de Fresnel
The diffraction effects obtained when either the source of light or observing screen, or both, are at a finite distance from diffracting aperture or obstacle. → Fraunhofer diffraction.
Named after Jean Augustin Fresnel (1788-1827), French physicist, a key figure in establishing the wave theory of light. His earlier work on interference was carried out in ignorance of that of Thomas Young (1773-1829), English physician and physicist, but later they corresponded and were allies; → diffraction.
Fr.: interaction fondamentale
Any of the four interactions in nature between bodies of matter and that are mediated by one or more particles. Also called the → fundamental force. In order of decreasing strength, the four fundamental interactions are the → strong interaction, the → electromagnetic interaction, the → weak interaction, and the → gravitational interaction.
Adjective of → galaxy.
Fr.: anticentre galactique
Fr.: barre galactique
An elongated bar-shaped structure composed of stars present in some spiral galaxies. About two-third of such galaxies contain bars that cross their centers. Bars, like → spiral arms, result from a → density wave in which stars take very elliptical orbits. They form when the → galactic disk dominates the → galactic bulge, → Ostriker-Peebles criterion. Bars play an extremely important role in a galaxy's evolution. The gravity from a bar is the mechanism that drives → interstellar gas from the outer parts of a → spiral galaxy inward toward the central regions, and into the galactic nucleus itself. This causes tremendous bursts of star formation. Therefore, a majority of massive stars are born in such starbursts in the nuclei of galaxies. Bars may also channel the material that falls into black holes within active galactic nuclei, releasing enormous power in radiation and particles from tiny regions at the centers of some galaxies. Bars disappear as galactic centers grow more massive (after some 2 to 8 Gyr).
Fr.: bulbe de la Galaxie
markaz-e kahkešân (#)
Fr.: centre galactique
1) The rotational center of the → Milky Way galaxy located
in the direction of the → Sagittarius constellation at a
distance of 7.62 ± 0.32 kpc (2005, ApJ 628, 246). Its equatorial coordinates
(J2000 epoch) are: R.A. 17h45m40.04s, Dec. -29° 00' 28.1''.
The Sun orbits around the Galactic center once every 200 million years
at a speed of 220 km per second. It is believed that
there is a → supermassive black hole at the
Galactic center cluster
xuše-ye markaz-e kahkešân
Fr.: amas du centre galactique
One of the three massive clusters located toward the → Galactic center: → Quintuplet cluster, → Arches cluster, → Central cluster. Heavily extinguished by the presence of dust clouds and only accessible at infrared (and longer) wavelengths or in X-rays, each of these clusters has a population of more than a hundred → massive stars. The three clusters are similar in most respects, each containing about 104 solar masses in stars. The Arches cluster is younger than the two others.
xuše-ye kahkešâni, ~ kahkešânhâ
Fr.: amas galactique
Fr.: coordonnées galactiques
A system of astronomical coordinates using → latitude (bII) measured north and south from the → Galactic equator and → longitude (lII), measured from the → Galactic Center in the sense of increasing → right ascension from 0 to 360 degrees. In the old system (lI,bI), the Galactic center was at lI = 327°41'. Same as → galactic system.
Fr.: disque galactique
Fr.: dynamique galactique
Fr.: équateur galactique
The great circle in the sky defined by the place of the → Galactic plane or the → Milky Way. At an angle of about 62°, the Galactic equator intersects the celestial equator at two points located in the constellations → Monoceros and → Aquila.
Galactic habitable zone
zonâr-e zistpazir-e kahkešân
Fr.: zone habitable galactique
A region of the Galaxy whose boundaries are set by its calm and safe environment and access to the chemical materials necessary for building terrestrial planets similar to the Earth. → circumstellar habitable zone; → habitable zone.
Fr.: halo galactique
A roughly spherical aggregation of → globular clusters, as well as the oldest stars and unseen mass that surrounds the Galaxy.
Fr.: latitude galactique
In the → Galactic coordinate system, the angle between the line of sight to an object and the → Galactic equator. Galactic latitude, usually represented by the symbol bII, ranges from +90 degrees to -90 degrees.
Fr.: longitude galactique
In the → Galactic coordinate system, the angle between the → Galactic Center and the projection of the object on the → Galactic plane. Galactic longitude, usually represented by the symbol lII, ranges from 0 degrees to 360 degrees.
Fr.: noyau de galaxie
A concentration of stars and gas in the innermost region of a galaxy, sometimes extending over thousands of light-years from the center of the galaxy.
Fr.: flot galactique
Fr.: plan galactique