An Etymological Dictionary of Astronomy and Astrophysics
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A hypothetical star so dense that it is composed of degenerate quarks, a matter denser than that of a neutron star.

→ quark; → star.

A star which is a source of emission at radio frequencies. Radio stars include pulsars, flare stars, binary star systems in which mass is transferred from one component to the other, and some X-ray stars.

→ radio; → star.

A chemically peculiar star characterized by the presence of high-frequency non-radial oscillations, with periods that range between about 4 and 16 min. These variations have periods from about 5 to 20 minutes and low amplitudes (B < 10 mmag). They are consistent with acoustic (→ p mode) pulsations of low degree and high radial overtone.

→ rapid; → -ly; → oscillate; → -ing; → Ap star.

A star on the → horizontal branch which results from the evolution of a → red giant with an initial mass of ~ 1 Msun.

→ red; → clump; → star.

Same as → red horizontal branch star.

→ red; → horizontal; → branch; → star.

A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.

→ red; → horizontal; → branch; → star.

A star whose light has undergone → reddening.

Reddened, p.p. of → redden; → star.

Same as → red horizontal branch star.

→ red; → horizontal; → branch; → star.

Same as → rapidly oscillating Ap star

→ rapidly oscillating Ap star

A star that has a non-zero → angular velocity. In a rotating star, the → centrifugal forces reduce the → effective gravity according to the latitude and also introduce deviations from sphericity. In a rotating star, the equations of stellar structure need to be modified. The usual spherical coordinates must be replaced by new coordinates characterizing the → equipotentials. See also → von Zeipel theorem.

→ rotating; → star.

A member of a large class of → pulsating stars of type A2-F6 with periods less than 1 day. They are similar to → Cepheids, except that their periods are much shorter and are less luminous. RR Lyrae stars belong to → Population II and are often found in → globular clusters (hence one of their older names cluster variables) or elsewhere in the → galactic halo. They are used as distance indicators (→ standard candle) out to more than 200 kpc.

→ Lyra; → star.

A massive, young, and hot star that is moving quickly through space. Runaways are probably propelled through space from a binary star when its companion has exploded as a supernova, or ejected from a stellar cluster by the dynamical interactions in the system.

→ run; away, from O.E. aweg, earlier on weg "on from this (that) place;" → star.

Setâré, → star; gorizân present participle of goriz-, gorixtan "to escape; to flee, run away;" Mid.Pers. virextan; Proto-Iranian *vi-raik, from vi- "apart, asunder" + *raik; Av. raek- "to leave, set free, let off;" Mid./Mod.Pers. reg/rig (in mordé-rig "inheritance"); Skt. ric- "to leave," rinakti "gives up, evacuates;" Gk. leipein "to leave;" L. linquere "to leave;" from PIE *linkw-, from *leikw- "to leave behind" (cf. Goth. leihvan; O.E. lænan "to lend;" O.H.G. lihan "to borrow;" O.N. lan "loan").

A type of massive, → blue supergiant, → variable star, also known as a → Hubble-Sandage variable or a → Luminous Blue Variable (LBV). S Doradus stars are the most luminous stars in the Galaxy and are easily identified in other nearby galaxies. They are named after the prototype, S Doradus, in the → Large Magellanic Cloud.

→ Dorado; → star.

A → red giant of → spectral type S whose spectrum is dominated by → molecular bands arising from → zirconium → oxide (ZrO). S stars also have strong → cyanogen bands and contain spectral lines of → lithium and → technetium. Almost all S stars are → long-period variables.

S, letter of alphabet; → star.

Same as → S star.

S, letter of alphabet; → type; → star.

A general whole-sky catalog compiled by the Smithsonian Astrophysical Observatory which results from the combination of several earlier catalogs. The compilation gives positions and proper motions for 258,997 stars, of which 8,712 are double and 499 variable, with an average distribution of 6 stars per square degree. The star positions have an average standard deviation of 0''.2 at their original epochs (0''.5 at epoch 1963.5). The equinox is 1950.0 and the system that of the FK4.

SAO acrynome of the Smithsonian Astrophysical Observatory; → star; → catalog.

The progressive increase of random motions of → disk stars with increasing stellar → ages. While some initial random motion seems likely in the disturbed conditions of disks when the oldest stars formed, the observation is generally attributed to scattering processes. Both massive gas → clumps and → spiral waves are considered as scattering agents (J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein).

→ scattering; → star.

A star whose formation is induced by an older star itself formed previously in the same region. See also → stimulated star formation, → sequential star formation, → triggered star formation.

→ second; → generation; → star.

In a → binary system, the star that revolves around the more massive → primary component.

→ secondary; → star.

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.