Stellar association

Types

Victor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars.{{cite journal

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.

OB associations

Main article: OB association

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.{{cite web | access-date = 2006-06-08 }} 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 one to fifteen million years, depending on the mass of the star. 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.{{cite journal

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.

T associations

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.

R associations

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.{{cite journal

Known associations

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:

• Local Association (Pleiades moving group)
• Hyades Stream
• IC 2391 supercluster
• Beta Pictoris moving group
• Castor moving group
• AB Doradus moving group
• Zeta Herculis moving group
• Alpha Persei moving cluster
• Cameleopardis OB1 association{{cite journal

References

References

1. (2011). "History of Astronomy: An Encyclopedia". [[Routledge]].
2. (15 August 1996). "Viktor A. Ambartsumyan, 87, Expert on Formation of Stars". [[The New York Times]].
3. (1999). "The Formation of Star Clusters". American Scientist.
4. (1999). "New proper motions of pre-main sequence stars in Taurus-Auriga". Astronomy and Astrophysics.