An Etymological Dictionary of Astronomy and Astrophysics
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1) The act of associating, → associate; the state of being associated.
2) A group of → astronomical objects physically or apparently gathered in a particular area of sky, for example an → stellar association.
3) An organization of persons having a common interest. → dissociation.

Association, noun from → associate.

Âhazeš, verbal noun of → âhazidan→ associate. Anjoman, for the meaning 3, from Mid.Pers. anjaman, from Av. han-jamana, from han- "together" + jamana, from gam- "to come;" cf. Skt. samgamana "gathering together."

A celestial body making part of an astronomical association.

→ association; → member.

One of nine → OB associations located in the Cygnus → constellation. The central association, → Cygnus OB2, is the most famous and the youngest of the Cygnus region. (L. Mahy et al. 2013, astro-ph/1301.0500 and references therein).

→ Cygnus; → OB star; → association.

A loosely bound grouping of O and B stars that typically stretches up to several hundred → light-years and may contain between a dozen and several hundred → O stars and → B stars. The members of an OB association are young and of roughly the same age. OB associations dissipate in a few tens of millions of years.

O and B, from spectral types; → association.

A large OB stellar association centered on the → Orion Nebula. Lying some 1500 → light-years away, it is about 400 light-years across and contains the main stars of Orion, except → Betelgeuse.

→ Orion; → association.

An → OB association consisting of several dozen → hot stars of → spectral types O and B (→ O star, → B star). The Orion OB1 association consists of several subgroups, first divided by Blaauw (1964) into four subgroups. The subgroups differ in age and gas and dust content:
Orin OB1a, which contains the stars to the northwest of → Orion's Belt stars. Within this group lies another subgroup, known as the → 25 Orionis group;
Orion OB1b, containing the group of stars located in and around the Belt;
Orion OB1c, including the stars around → Orion's Sword; and
Orion OB1d, which contains the stars in and close to the → Orion Nebula (including the → Trapezium cluster).

→ Orion; → OB association.

A → stellar association containing a number of → reflection nebulae. The stars are of low or intermediate mass and young, less than a million years old. They are still surrounded by patches of dust that reflect and absorb light from the interstellar cloud in which they formed. This type of association was first suggested by Sidney van den Bergh (1966, AJ 71, 900).

R, from → reflection; → association.

The nearest → OB association to the Sun. It contains several hundred stars, mostly → B stars which concentrate in the three subgroups: Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux. Upper Scorpius is the youngest subgroup, Upper Centaurus Lupus the oldest subgroup of the association. Isochrone fitting to the Hertzsprung-Russell diagram indicates that the star formation occurred some 5-20 Myr ago. Based on data from the → Hipparcos catalog, it turns out that the Sco-Cen association lies at a distance of 118-145 → parsecs, with the exact value depending on the subgroup of the association. The Sco-Cen association is probably a member of the → Gould Belt (Preibisch & Mamajek, 2008, astro-ph/0809.0407).

→ Scorpius; → Centaurus; → association.

1) A large, loose grouping of 10 to 1000 stars that are of similar spectral type and share a common origin. The members move together through space, but have become gravitationally → unbound. Stellar associations are primarily identified by their common movement vectors and ages. → OB association; → T association; → R association.
2) An → unbound stellar agglomeration for which the age of the stars is smaller than the → crossing time (Giels & Portegies Zwart, 2010, MNRAS Letters, astro-ph/1010.1720). See also → star cluster.

The concept of stellar association was first introduced by Viktor A. Ambartsumian (1908-1996), Armenian astrophysicist (1947, Stellar Evolution and Astrophysics, Armenian Acad. of Sci.; German translation, Abhandl. Sowjetischen Astron. Ser. 1. 33, 1951). → stellar; → association.

A → stellar association containing many → T Tauri stars. Examples include the Taurus-Auriga T association, the nearby → TW Hydrae association, and Vela T1 and T2.

T, letter of alphabet; → association.

A young (≤ 100 million years) association of stars (abbreviated TWA) with at least 27 members, from → intermediate mass stars to planetary mass objects. Five of them, including → TW Hydrae, display signatures of → T Tauri stars. TWA is the first moving group of young nearby (≤ 100 → paesecs) stars to be identified.

→ TW Hydrae; → association.