A stellar association
is a very loose star cluster
, looser than both open clusters
and globular clusters
. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally unbound and are still moving together through space. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.
The conventional name for an association uses the names or abbreviations of the constellation (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.
Stellar associations were first discovered by the Armenian astronomer Viktor Ambartsumian in 1947. He categorized them into two groups, OB and T, based on the properties of their stars.
Young associations will contain 10–100 massive stars of spectral class O and B, and are known as OB associations
. These are believed to form within the same small volume inside a giant molecular cloud
. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. It is believed that the majority of all stars in the Milky Way were formed in OB associations.
O class stars are short-lived, and will expire as supernovae after roughly a million years. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within
10 million years. (Compare this to the current age of the Sun at about 5 billion years.)
The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun. The nearest OB association is the Scorpius-Centaurus Association, located about 400 light years from the Sun.
OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy. These associations can be quite sparse, spanning 1,500 light years in diameter.
Young stellar groups can contain a number of infant T Tauri
stars that are still in the process of entering the main sequence
. These sparse populations of up to a thousand T Tauri stars are known as T associations
. The nearest example is the Taurus-Auriga T association
(Tau-Aur T association), located at a distance of 140 parsecs
from the Sun. Other examples of T associations include the R Corona Australis T association
, the Lupus T association
, the Chamaeleon T association
and the Velorum T association
. T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. To summarize the characteristics of Moving groups members: they have the same age and origin, the same chemical composition and they have the same amplitude and direction in their vector of velocity.
Associations of stars that illuminate reflection nebulae
are called R associations
, a name suggested by Sidney van den Bergh
after he discovered that the stars in these nebulae had a non-uniform distribution. These young stellar groupings contain main-sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R-associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms. An example of an R-association is Monoceros R2
, located 830 ± 50 parsecs
from the Sun.
The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work. Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.
The Ursa Major Moving Group
is one example of a stellar association. (Except for α Ursae Majoris
and η Ursae Majoris
, all the stars in the Plough/Big Dipper
are part of that group.)
Other young moving groups include: