An Etymological Dictionary of Astronomy and Astrophysics
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A peculiar → Mira variable with a → pulsation period of 387 days surrounded by an extended → emission nebula. R Aqr is a → symbiotic  → binary star with a mass-losing, pulsating → red giant and an accreting hot companion with a jet → outflow which ionizes an emission nebula. The orbital period of the R Aqr binary is about 44 years as inferred from periodic phases of reduced brightness observed around 1890, 1933, and 1977. These phases are interpreted as partial obscurations of the mira by the companion with its → accretion disk and the associated gas and dust flows. The inferred orbital period is supported by → radial velocity measurements (Schmid et al., 2018, A&A 602, A53 and references therein).

R, → Beyer designation; → Aquarius.

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).

R, from → reflection; → association.

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.

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.

R referring to a rarefied gas; → type; → ionization; → 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).

1a) A group of persons related by common descent or heredity.
1b) A population so related.
2) A contest of speed, as in running, riding, driving, or sailing (Dictionary.com).

1) From M.Fr. race "race, breed, lineage, family," from It. razza, (cf. Sp. and Port. raza), of unknown origin.
2) M.E. ras(e), from O.N. ras "running, race," cognate with O.E. ræs "a running, a rush, a leap, jump."

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.
2) Tâz, present stem of Tâxtan/tâz- "to run, rush upon, assault;" → surge.

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.
2) A policy, system of government, etc., based upon or fostering such a doctrine; discrimination (Dictionary.com).

→ race; → -ism.

A unit of energy absorbed from ionizing radiation, equal to 100 → ergs per gram, or 0.01 → joules per kilogram, of irradiated material. Rad has been replaced by → gray (Gy).

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).

Graphic display of measurements by a → radar of mineral deposits on a planetary surface.

→ radar; → chart.

Emanating from a common central point; arranged like the radii of a circle.

From L. radialis, from → radius; → -al.

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.

→ radial; → migration.

A motion away from or toward a central point or axis.

→ radial; → motion.

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).

→ radial; → spoke.

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.

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; → curve.

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.

→ radial velocity; → method.

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 steradiancy.
2) Of any particular → wavelength within the interval covered by a → spectral line, the → energy per unit → surface per steradian, per wavelength; denoted I_λ. The term radiance is often loosely replaced by "→ intensity." The radiance of the whole line is given by I = ∫ I_λ dλ. The radiance of an → emission line depends, among other things, upon the → number of → atoms per unit area in the → line of sight (the → column density) in the → upper level of the line.

From radia(nt), → radiant, + → -ance.

Tâbešmandi, noun from tâbešmand "possessing radiation," from tâbeš, → radiation, + -mand a suffix denoting possession; Mid.Pers. -ômand suffix forming adjectives of quality.

1) Sending out rays of light; bright; shining.
See also: → radiant energy, → radiant flux, → radiant intensity.
2) The point in the sky from which → meteors in a → meteor shower appear to radiate or come. See also: → radiant drift.

M.E., from M.Fr. radiant, from L. radiantem (nominative radians) "shining," pr.p. of radiare "to shine, radiate," → radiation.

1) Tâbandé, tâbeši adj. from tâbidan, → radiate.
2) Tâbsar, from tâb "light, radiation," → radiation, + sar "head, top, summit, point," → head.