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

A variation of planetary orbital elements which is always in the same direction as time increases.

→ secular; → perturbation.

Same as → secular perturbation.

→ secular; → variation.

The process of organizing society or aspects of social life around non-religious values or principles.

Verbal noun of secularize "giyânidan" (گیانیدن); → secular

The act or practice of segregating. The state or condition of being segregated. → mass segregation.

Verbal noun of → segregate.

Any of a set of rules specifying the relationships between the → quantum numbers that characterize the initial and final states of a quantum-mechanical system in a → discrete transition. Transitions that do not agree with the selection rules are called → forbidden and have considerably lower probability. There are several types of selection rules (→ rigorous selection rule, → LS coupling, etc.) for → electric dipole transition (→ permitted), → magnetic dipole (forbidden), electric → quadrupole (forbidden), etc.

Selection, verbal noun of → select; → rule.

Absorption which varies with the wavelength of radiation incident upon an absorbing substance.

Selective, verbal noun of → select; → absorption.

Same as → horizontal eclipse.

From Gk. selene "Moon," related to sela "light, brightness, flame," + helion, → sun.

→ horizontal eclipse.

Same as → horizontal eclipse.

From Fr. selenelion, contraction of → selenehelion.

→ horizontal eclipse.

A parameter representing the product of the → gravitational constant by the → lunar mass. It is 49.03 x 10¹¹ m³ s^-2.

→ selenocentric; → gravitational; → constant.

The decrease in the radiation from a material caused by the absorption of a part of the radiation by the material itself.

→ self-; → absorption.

The generation of a voltage in a circuit due to self-inductance, the polarity of which tends to oppose the changing current in the circuit.

→ self-; → induction.

An empirical relation between the → refractive index of a medium and the wavelength of light passing through the medium: n² - 1 = Σ (A_iλ²/(λ² - λ_i²)), where n is the refractive index at wavelength λ, and A_i and λ_i are constants.

Named after Wolfgang Sellmeier who derived the equation in 1871; → equation.

An → atomic transition whose probability is reduced by a factor of the order of 10⁶ because of → selection rules. Same as → interconnection line.

→ semi-; → forbidden; → transition.

In a semiconductor device, a region of transition between semiconducting regions of different electrical properties.

→ semiconductor; → junction.

An instability occurring in the region just outside the → convective core of a → massive star. The instability occurs when a → superadiabatic layer is stabilized by a chemical gradient. In fact, semiconvection takes place if → Schwarzschild's criterion for convection is fulfilled but at the same time → Ledoux's criterion is not fulfilled. The time-scale of semiconvection is the thermal time-scale, which is short compared to the nuclear time-scale in → main sequence stars but long compared to the time-scale of convection. However, semiconvection has a profound influence on the → post-main sequence star evolution. It affects the convective mixing above the hydrogen shell source, determines the appearance and extent of → blue loops in the → Hertzsprung-Russell diagram during core → helium burning, and is essential for defining the extent of the convective cores during core helium burning (See, e.g., N. Langer, 2012, Ann. Rev. Astron. Astrophys. 50, 107).

→ semi-; → convection.

The perception or awareness of stimuli through the senses.

From M.L. sensationem (nominative sensatio), from L.L. sensatus "endowed with sense, sensible," from L. sensus "feeling," → sense.

Hess-kard, hesseš, verbal nouns of hess kardin, hessidan, → sense.

General: The act or process of separating. The place at which a division or parting occurs.
Astro.: The angular distance between the two components of a visual binary or optical double star.

M.E., from O.Fr. separation, from L. separationem, from separare "to pull apart," from se- "apart" + parare "make ready, prepare."

Jodâyi state noun of jodâ "separate," from Mid.Pers. yut "separate, different;" Av. yuta- "separate, apart."

The energy required to remove a particle (a proton or a neutron) from a particular atomic nucleus.

→ separation; → energy.

The formation of second-generation stars in a → molecular cloud, as triggered by the presence of → massive stars. The observation that some nearby → OB associations contain distinct, spatially separate subgroups of → OB stars in a sequence of monotonically changing age led Blaauw (1964, ARA&A 2, 213) to suggest that star formation in fact occurs in sequential bursts during the lifetimes of the corresponding molecular clouds. The first quantitative model of this mechanism was presented by Elmegreen and Lada (1977, ApJ 214, 725), who showed that the powerful ultraviolet photons of the massive star create an → ionization front which advances in the molecular cloud and is preceded by a → shock front. The compressed neutral gas lying between the ionization and shock fronts is gravitationally unstable and collapses in time-scales of a few million years to form a new generation of massive stars. The propagation of successive births of OB groups would produce a chain of associations presenting a gradient of age. Elmegreen and Lada estimated the propagation velocity to be 5 km s^-1. For a region with a length larger than 100 pc, this would imply an age difference of the order of 20 million years between the extremities. See also → stimulated star formation, → triggered star formation; → collect and collapse model.

→ sequential; → star formation.

A collection of → nonempty subsets of a set A such that every element of A is in exactly one of the subsets. In other words, A is the → disjoint  → union of these subsets.

→ partition; → set.