An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A galaxy with stellar masses ≤ 10⁹ → solar masses (Dawn K. Erb, 2015, Nature, 9 July).

→ low; → mass; → galaxy.

A star whose mass is around that of the Sun. See also: → intermediate-mass star; → high-mass star; → star formation.

→ low; → mass; → star.

A member of one of the two main classes of → X-ray binary systems where one of the components is a → neutron star or → black hole and the other component a → low-mass star with a spectral type A or later. LMXBs mainly emit → soft X-rays. The ratio of their optical to X-ray luminosities is less than 0.1. They belong to → old stellar populations with ages 5-15 × 10⁹ years and are found in → globular clusters and in the → bulge of our → Milky Way galaxy; some are also found in the disk. Hercules X-1 is an example of LMXBs.
See also: → high-mass X-ray binary.

→ low; → mass; → X-ray; → binary.

A medium in which chemical elements have abundances smaller than the solar values.

→ low; → metallicity; → environment.

Relatively low in position, rank, or order.

Comparative of → low.

Generally and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the → troposphere and lower → stratosphere); hence used in contrast to the common meaning for the → upper atmosphere. In other contexts, the term implies the lower troposphere (Meteorology Glossary, American Meteorological Society).

→ lower; → atmosphere.

The instant of culmination when the star passes between the pole and the horizon, having an hour angle of 12h. Lower culmination for non-circumpolar objects occur below the horizon and is thus unobservable. Same as → inferior culmination. See also → upper culmination.

→ lower; → culmination.

A → main sequence star whose mass is less than 1.5 Msun. Lower main sequence stars generate their energy chiefly through the → proton-proton chain. The core is surrounded by a → radiative zone above which lies a → convective envelope. In such stars the → opacity at the surface is high because of the low → surface temperature. Therefore, radiation cannot carry all the radiation because of high opacity. Thus, energy transfer takes place by → convection to the outer layer.

→ lower; → main; → sequence.

The part of the Earth's → mantle extending from about 660 km below the surface to above the → outer core at about 2,900 km.

→ lower; → mantle.

Same as → Russell-Saunders coupling.

L referring to the total → orbital angular momentum and S to the total → spin angular momentum; → coupling.

Moderately warm; tepid.

M.E. lukewarme "tepid," from luke "tepid," of unknown origin, + → warm.

Velarm "lukewarm, tepid," of unknown origin.

The → SI unit of → luminous flux, equal to the luminous flux emitted per unit solid angle by a standard point source having a → luminous intensity of 1 → candela.

L. lumen (gen. luminis) "light," from lucere "to shine," related to lux "light," lucidus "clear," luna, "moon;" Fr. lumière "light;" cf. Pers. ruz "day," rowšan "bright, clear," rowzan "window, aperture;" foruq "light," afruxtan "to light, kindle;" Mid.Pers. rôšn "light; bright, luminous," rôc "day;" O.Pers. raucah-rocânak "window;" O.Pers. raocah- "light, luminous; daylight;" Av. raocana- "bright, shining, radiant;" akin to Skt. rocaná- "bright, shining," roka- "brightness, light;" Gk. leukos "white, clear;" O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- "light, brightness."

Lumen loanword, as above.

The luminous intensity in a given direction of a small element of surface area divided by the orthogonal projection of this area onto a plane at right angle to the direction. It is measured in candelas per square meter. Luminance is often called surface brightness of the object.

From lumin-, combining form of → lumen "light," + -ance a suffix used to form nouns either from adjectives in -ant or from verbs.

Tâbâni, from tâbidan "to shine," → luminous.

The emission of light at low temperatures by any process other than → incandescence, where a substance emits light without being strongly heated. Luminescence is a collective term for different phenomena, for example: → phosphorescence, → fluorescence, → chemiluminescence, → photoluminescence.

From lumin-, from → lumen; → -escence.

Capable of, suitable for, or exhibiting luminescence.

From lumin-, from → lumen; → -escent.

The → total → brightness of a star or other astronomical object. It is expressed in watts and represents the total amount of → energy that the object radiates each → second over all wavelength regions of the → electromagnetic spectrum. Because this quantity is independent of distance, it is an → intrinsic brightness.
See also:
→ absolute luminosity, → anomalous luminosity effect, → bolometric luminosity, → color-luminosity diagram, → Eddington luminosity, → H II region luminosity, → intrinsic luminosity, → luminosity class, → luminosity distance, → luminosity function, → luminosity problem, → luminosity-size relation, → mass-luminosity ratio, → mass-luminosity relation, → peak luminosity, → period-luminosity relation, → solar luminosity, → stellar luminosity, → wind luminosity.

Verbal noun of → luminous.

A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star's → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars, or → main-sequence stars). Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.

→ luminosity; → class.

1) Distance derived by comparison of → observed and → intrinsic luminosities. If an object has a known luminosity L, and the observed flux is S, the luminosity distance is defined by D_L = (L/4πS)^1/2.

2) In cosmology, the → expansion of the Universe results in a diminution of the photon flux and the above equation fails. The reason is that for a homogeneous and isotropic Universe (→ Robertson-Walker metric), the luminosity decreases by a factor (1 + z)⁴. Therefore, the luminosity distance is related to the → angular diameter distance (D_A) by: D_L = (1 + z)².D_A.

→ luminosity; → distance.

Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.

→ luminosity; → function.

Low-mass → protostars are about an order of magnitude less luminous than expected. Two possible solutions are that → low-mass stars form slowly, and/or protostellar → accretion is episodic. The latter accounts for less than half the missing luminosity. The solution to this problem relates directly to the fundamental question of the time required to form a low-mass star (McKee & Offner, 2010, astro-ph/1010.4307).

→ luminosity; → problem.