An Etymological Dictionary of Astronomy and Astrophysics
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In an optical system, a collection of points which actually serves as a source of light rays.

→ real; → object.

An object which has a retrograde orbit around its primary.

→ retrograde; → object.

A → post-asymptotic giant branch star that in 1995 underwent sudden re-brightening due to a → helium shell flash, or → very late thermal pulse (VLTP), before embarking on a → white dwarf cooling track. Such an outburst is very rare, and in this case it is the first seen in modern times. Stellar outbursts observed in 1670 (nova CK Vul) and 1918 (nova V605 Aql) may have been caused by the same phenomenon. Since 1995, Sakurai's Object has undergone observable changes on time-scales of weeks to months. Several phases of dust production followed the outburst, with a deep optical minimum beginning in early 1999, such that any changes in the central star have since been inferred from radio and infrared observations. Subsequent observations and modeling have revealed much about the dust shell formation and the outer regions of the ejecta. This object is also the central star of an extended very faint planetary nebula (→ CSPN), confirming that the latest large mass ejection during the planetary nebula phase occurred several thousands years ago (see, e.g. H. L. Worters et al. 2009, MNRAS 393, 108 and references therein).

Named after Yukio Sakurai, a Japanese amateur astronomer, who serendipitously discovered it on February 20, 1996, when searching for comets; → object.

Any of a class of → astronomical objects which is thought to evolve into a → star or is a descendant of a star.

→ stellar; → object.

An object with a mass too small to sustain the → proton-proton chain and thus become a true star. See → brown dwarf.

→ substellar; → object.

A theoretical class of → stellar objects in which a → neutron star core is surrounded by a large and diffuse envelope. TŻOs are expected to form as a result of the evolution of two → massive stars in a → close binary, with the neutron star forming when the more massive star explodes as a → supernova. During subsequent evolution of the system, the expanding envelope of the companion may lead to a common envelope state and the spiral-in of the neutron star into the core of its companion. Alternately, a TŻO may be produced when a newly-formed neutron star receives a supernova "kick" velocity in the direction of its companion and becomes embedded. Supergiant TŻOs are predicted to be almost identical in appearance to → red supergiants (RSGs). The best features that can be used at present to distinguish TŻOs from the general RSG population are the unusually strong → heavy element and → lithium lines present in their spectra, products of the star's fully → convective envelope linking the → photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. These objects are thought to be extremely rare, with as few as 20-200 of them predicted to exist in the Galaxy at present, though some authors have doubted whether such an object could survive the merger with the envelope intact. A candidate is HV 2112 (Levesque et al., 2014, MNRAS, arXiv:1406.0001; Beasor et al., 2018, MNRAS, arXiv:1806.07399).

Thorne K. S., Żytkow A., 1975, ApJ 199, L19.

A member of a class of objects in orbit around the Sun at a larger distance than the distance between Neptune and the Sun. This includes several → dwarf planets, such as → Eris, → Pluto, and many small solar system bodies.

→ trans-; → Neptune; → object.

Any flying object or phenomenon that cannot be identified by the observer.

→ unidentified; fly; M.E. flien; O.E. fleogan (cf. O.H.G. fliogan, O.N. flügja, M.Du. vlieghen, Ger. fliegen); → object.

Any star that has evolved past the → protostar stage, but has not yet arrived on the → main sequence. There is a variety of YSOs depending on their age, mass, and environment, including → Herbig stars, → T Tauri stars, and, in general, compact infrared sources embedded in molecular clouds.

→ young; → stellar; → object.