An Etymological Dictionary of Astronomy and Astrophysics
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A concept of → star formation in which → high-mass stars and → low-mass stars form in different physical conditions involving different → molecular clouds. Following the pioneering suggestion of Herbig (1962), successive investigations have generally supported the idea that star formation proceeds bimodally with respect to stellar mass. The star formation rate appears to differ both spatially and temporally for low mass and → massive stars. This is of considerable importance for galactic evolution, since the low-mass stars lock up mass and are long-lived, low luminosity survivors to the present epoch, whereas massive stars are short-lived, recycle and enrich interstellar gas, and leave dark remnants while producing a high luminosity per unit of mass (Silk, J., 1988, in Galactic and Extragalactic Star Formation, p. 503, eds. R. E. Pudritz and M. Fich).

→ bimodal; → star; → formation.

Two stars gravitationally bound to each other, so that they revolve around their common center of gravity. → double star; → spectroscopic binary.

→ binary; → star.

Setâré, → star; dorin, → binary.

A star showing the → Blazhko effect.

→ Blazhko effect; → star.

A star belonging to a variety of stars located above the → horizontal branch and blueward of the → red giant branch in the → Hertzsprung-Russell diagram of the → halo population.

→ blue; → halo; → star.

Setâregân plural of setâré, → star, âbi, → blue, hâlé, → halo.

Same as → blue horizontal branch star.

→ blue; → horizontal; → branch; → star.

A rare class of → horizontal branch (HB) stars that so far have been found in only very few Galactic → globular clusters. These stars are such called because they form a blue hook at the hot end of the HB in → far ultraviolet (FUV) → color-magnitude diagrams. The physical mechanism that produces blue hook populations is still uncertain. At least two scenarios have been proposed.
In the first scenario these stars are explained as a consequence of extreme → mass loss during the → red giant branch phase and late helium flashing while descending the → white dwarf cooling track. Due to the thin residual hydrogen envelope, helium is mixed into the envelope and hydrogen is mixed into the core during the late → helium flash. As a result, the stars are hotter and UV-fainter than canonical → extreme horizontal branch stars (EHB).
By contrast, in the He self-enrichment scenario the EHB and blue hook stars are produced via the normal evolution of He-enriched sub-populations in globular clusters. These sub-populations might have formed from the ejecta of intermediate-mass → asymptotic giant branch (AGB) stars of the first generation of stars. For the same age and → metallicity, He-enriched HB stars have smaller masses than normal HB stars, resulting in bluer → zero age horizontal branch star (ZAHB) locations. They are also brighter in the FUV, but this effect is reversed for very hot He-enriched HB stars with → effective temperatures larger than 19000 K.
See Dieball A., et al., 2013, arXiv:0901.1309v1, and for blue hook stars in ω Cen cluster, M. Tailo et al., 2015, Nature 523, 318.

→ blue; → hook; → star.

A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0, they have evolved past the → red giant stage and are burning helium in their core.

→ blue; → horizontal; → branch, → star.

A → post-AGB star that undergoes a last → thermal pulse when it is already on the → white dwarf  → cooling track. The thermal pulse will expand the hot central star, whereby hydrogen will be ingested into the → helium burning shell. This will temporarily return the star to the → AGB phase it has previously left.

→ born; → again; → asymptotic giant branch; → star.

An obsolete designation used in early objective-prism studies to denote → B-type stars with "abnormal spectra" characterized by → forbidden emission lines. → B[e] star.

→ star.

An intense → star formation activity in a region of → interstellar medium or, more globally, in a → galaxy. It is characterized by a → star formation rate which is much higher than the corresponding average. Same as → starburst.

→ burst; → star; → formation.

A → Be star with → forbidden lines in emission in its spectrum. B[e] stars show large → infrared excess due to → circumstellar dust emission. See also → supergiant B[e] star, → pre-main sequence B[e] star, → compact planetary nebula B[e] star, → symbiotic B[e] star, and → unclassified B[e] star.

B, referring to the spectral type; e for emission lines, brackets for distinction from Be; → star.

A class of → red giant stars whose spectra show strong → molecular bands of → carbon compounds.

→ carbon; → star.

A star that presents very low → iron  → abundances [Fe/H] < -4 but an → anomalous richness in carbon. CEMP stars have been defined as a subset of → metal-poor stars that exhibit elevated [C/Fe] ≥ +1.0. It has been recognized that ~15-20% of stars with [Fe/H] < -2.0 are carbon enhanced. This fraction rises to 30% for [Fe/H] < -3.0, to 40% for [Fe/H] < -3.5, and ~75% for [Fe/H] < -4.0. This increasing trend of CEMP-star frequency with declining [Fe/H] is confirmed by the observation of many thousands of CEMP stars (Daniela Carollo + ApJ 2014, 788, 180). See also → extremely metal-poor star (EMPS)

→ carbon; → enhance; → metal; → metal; → poor; → star.

A → main sequence star of → spectral type A or B (→ A-type star, → B-type star) identified by the presence of anomalously strong or weak → absorption lines of certain elements in their spectra. CP stars have been divided into four main classes on the basis of their spectra: 1) non-magnetic metallic-lined (CP1, → Am star), magnetic (CP2, → Ap star), non-magnetic mercury-manganese (CP3, → HgMn star), and helium-weak (CP4, → He-weak star).
See also → Ap/Bp stars.

→ chemical; → -ly; → peculiar; → star.

Star that, from a given observer's → latitude, does not rise or set, but circles around the → celestial pole. To be circumpolar, a star must have a polar distance that is less than the observer's latitude. Whether a given star is circumpolar at the observer's latitude (φ) may be calculated in terms of the star's → declination (δ). The star is circumpolar if φ + δ ≥ +90° (observer in northern hemisphere), or φ + δ ≤ -90° (observer in southern hemisphere).

→ circumpolar; → star.

A → T Tauri star in which → accretion from a → circumstellar disk is responsible for ultraviolet and infrared excess emission and for a moderate to strong emission line spectrum superimposed on the photospheric spectrum. Classical T Tauri stars probably evolve into → weak-line T Tauri stars when their disks are fully accreted by the stars.

→ classical; → T Tauri star.

A binary system in which the separation of the component stars is comparable to their diameters, so that they influence each other's evolution most commonly by the tidal forces.

→ close; → binary; → star.

A late → O-type star or an early → B-type star in whose spectrum the lines of some of the elements → carbon (C), → nitrogen (N), and → oxygen (O) are present.

→ CNO; → star.

A star hidden in a dense envelope of gas and dust which is a strong source of infrared emission.

→ Cocoon nebula; → star.

A star that has undergone → collapse.

Collapsed p.p. of → collapse; → star.

Setâré, → star; rombidé p.p. of rombidan, → collapse.