Fr.: association R
A → stellar association containing a number of → reflection nebulae. The stars are of low or intermediate mass and young, less than a million years old. They are still surrounded by patches of dust that reflect and absorb light from the interstellar cloud in which they formed. This type of association was first suggested by Sidney van den Bergh (1966, AJ 71, 900).
Fr.: processus r
A → nucleosynthesis process in which → chemical elements heavier than → zinc are created through the intense bombardment of other elements by → neutrons in rapid succession. The essential feature of the r-process is the release of great numbers of neutrons in a very short time (less than 100 seconds). The r-process is a "rapid" version of the → s-process, occurring in supernova → core collapse and possibly when a → neutron star merges with a → black hole in a → binary star.
r stands for rapid, since the process entails a succession of rapid neutron captures on iron seed nuclei; → process.
R-type ionization front
pišân-e yoneš-e gune-ye R
Fr.: front d'ionisation de type R
A spherical → ionization front of → H II regions that moves radially outward from the → exciting star at a velocity much higher than → sound speed in the surrounding cold neutral gas of uniform density (ahead of the front). R-type ionization fronts corresponds to early evolution of H II regions, and will eventually transform into → D-type ionization fronts. If the motion of the front is supersonic relative to the gas behind as well as ahead of the front, the front is referred to as weak R. The strong R front correspond to a large density increase across the front.
The central object of the → 30 Doradus nebula in the → Large Magellanic Cloud. Also known as HD 38268, it was thought to be a single star of several thousands → solar masses until → speckle interferometry techniques resolved it into a rich and compact star cluster. Recent high-resolution studies have shown that R136 contains 39 known O3 stars, which is more than known to be contained in the rest of the → Milky Way, → LMC, and → SMC combined. R136 is a prototype "super star cluster," with an estimated mass of 105 solar masses. Its most massive stars are less than 1-2 million yeas old, while its lower-mass stars formed 4-5 millions years ago.
The Radcliffe serial number 136 (Feast et al. 1960, MNRAS 121, 25).
1) nežâd (#); 2) tâz
Fr.: 1) race; 2) course
1a) A group of persons related by common descent or heredity.
1) From M.Fr. race "race, breed, lineage, family," from It. razza,
(cf. Sp. and Port. raza), of unknown origin.
1) Nežâd, literally "born," ultimately from Proto-Ir. *nizat-, cf.
Av. nizənta- "born," from → ni- + *zan-
"to give birth, to be born," cognate with âzâd, → free;
see also → generate.
1) A belief or doctrine that inherent differences among the various
human races determine cultural or individual achievement, usually
involving the idea that one's own race is superior and has the right
to rule others.
Shortened form roentgen absorbed dose; → roentgen.
An emitting/receiving device in which the echo of a pulse of microwave radiation is used to detect and locate distant objects.
From ra(dio) d(etecting) a(nd) r(anging).
Emanating from a common central point; arranged like the radii of a circle.
Fr.: migration radiale
The process whereby a → disk star changes its → galactocentric distance. Radial migration involves → angular momentum transfer, resulting from → resonances created by transient → density waves such as → bars or → spiral arms in → galactic disks. According to → galactic dynamics models, → churning is the main cause of radial migration. Radial migration of stars plays an important role in shaping the properties of galactic disks.
Fr.: mouvement radial
A motion away from or toward a central point or axis.
Fr.: spoke radial
Any of short-lived (generally lasting less than 24 hours) radial features that periodically appear over the outer half of → Saturn's → B ring, when the ring tilt angle is small. These features revolve at the same rate as the planet's → magnetic field and maintain their shape over much of the course of their existence even though they extend tens of thousands of kilometers across the rings. It is believed that the tiny particles that make up these spokes are electrically charged and temporarily "frozen" into the planet's magnetic field (Ellis et al., 2007, Planetary Ring Systems, Springer).
Fr.: vitesse radiale
The component of a three-dimensional velocity vector of an object directed along the line of sight. It is measured by examining the Doppler shift of lines in the spectrum of astronomical objects.
radial velocity curve
xam-e tondâ-ye šo'â'i
Fr.: courbe de vitesse radiale
A curve describing the variation of the radial velocity of a star, due to the Doppler effect, under the gravitational effect of a secondary body (companion or exoplanet). The amplitude of these variations depends upon the mass of the secondary and its distance from the star.
radial velocity method
raveš-e tondâ-ye šo'â'i
Fr.: méthode de vitesses radiales
The technique based on the analysis of the → radial velocity curve, used to detect the presence of an invisible secondary around a host star. This method holds the majority of exoplanet discoveries.
A unit of angular measure; one radian is that angle with an intercepted arc on a circle equal in length to the radius of the circle.
From radi(us) + -an an originally adj. suffix.
1) Generally, the → radiant energy
per unit → solid angle per unit of
→ projected area of the → source.
It is usually expressed in → watt per
→ steradian per → square
→ meter (W m-2 sr-1). Same as
Tâbešmandi, noun from tâbešmand "possessing radiation," from tâbeš, → radiation, + -mand a suffix denoting possession; Mid.Pers. -ômand suffix forming adjectives of quality.
1) tâbandé, tâbeši; 2) tâbsar
1) Sending out rays of light; bright; shining.
M.E., from M.Fr. radiant, from L. radiantem (nominative radians) "shining," pr.p. of radiare "to shine, radiate," → radiation.
Fr.: dérive de radiant
Fr.: énergie radiative