The Milky Way and Beyond by Britannica Educational Publishing

Author:Britannica Educational Publishing
Language: eng
Format: epub
Publisher: Britannica Educational Publishing
Published: 2010-04-10T04:00:00+00:00

OB AND T ASSOCIATIONS

The discovery of stellar associations depended on knowledge of the characteristics and motions of individual stars scattered over a substantial area. In the 1920s it was noticed that young, hot blue stars (spectral types O and B) apparently congregated together. In 1949 Victor A. Ambartsumian, a Soviet astronomer, suggested that these stars are members of physical groupings of stars with a common origin and named them O associations (or OB associations, as they are often designated today). He also applied the term T associations to groups of dwarf, irregular T Tauri variable stars, which were first noted at Mount Wilson Observatory by Alfred Joy.

The chief distinguishing feature of the members of a stellar association is that the large majority of constituent stars have similar physical characteristics. An OB association consists of many hot, blue giant stars, spectral classes O and B, and a relatively small number of other objects. A T association consists of cooler dwarf stars, many of which exhibit irregular variations in brightness. The stars clearly must be relatively close to each other in space, though in some cases they might be widely dispersed in the sky and are less closely placed than in the open clusters.

The existence of an OB association is usually established through a study of the space distribution of early O- and B-type stars. It appears as a concentration of points in a three-dimensional plot of galactic longitude and latitude and distance. More than 70 have been cataloged and are designated by constellation abbreviation and number (e.g., Per OB 1 in the constellation Perseus). In terms of dimensions, they are larger than open clusters, ranging from 100 to 700 light-years in diameter, and usually contain one or more open clusters as nuclei. They frequently contain a special type of multiple star, the Trapezium (named for its prototype in Orion), as well as supergiants, binaries, gaseous nebulae, and globules. Associations are relatively homogeneous in age. The best distance determinations are from spectroscopic parallaxes of individual stars—i.e., estimates of their absolute magnitudes made from studies of their spectra. Most of those known are closer than 10,000 light-years, with the nearest association, straddling the boundary between Centaurus and Crux, at 385 light-years.

Associations appear to be almost spherical, though rapid elongation would be expected from the shearing effect of differential galactic rotation. Expansion, which is on the order of 10 km/sec (6 miles/sec), may well mask the tendency to elongate, and this is confirmed in some. Tidal forces break up an association in less than 10 million years through differences in the attraction by an outside body on members in different parts of the association.

A good example of an OB association is Per OB 1 at a distance of some 7,500 light-years, which spreads out from the double cluster h and χ Persei. A large group of 20 supergiant stars of spectral type M belongs to Per OB 1. Associations with red supergiants may be in a relatively advanced evolutionary stage, almost ready to disintegrate.

The

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.