An Etymological Dictionary of Astronomy and Astrophysics
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A body other than a → star or → substellar object located in → interstellar space and not → gravitationally bound to a star. Its → hyperbolic orbit would indicate an object not bound to the Sun. The first known ISO is → 1I/'Oumuamua. ISOs are icy → planetesimals that are expected to behave like the → long-period comets of the solar system; volatile ices sublimate when the ISO approaches the Sun, developing a → coma and a → dust tail -- features that should make them bright and therefore easy to spot. The rocky ISOs, on the other hand, only reflect sunlight. As their → albedo is expected to be extremely low they become dark (after eons of bombardment by high-energy cosmic rays), they would be extremely faint and hard to detect (Hainaut et al., 2018, The Messenger 173, 13).

→ interstellar; → object.

Initial name of → quasars.

→ quasi-; → stellar; → 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.

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.