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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....
...at the geometric centre of the cloud. Large cores (which probably contain subcondensations) up to a few light-years in size seem to give rise to unbound associations of very massive stars (called OB associations after the spectral type of their most prominent members, O and B stars) or to bound clusters of less massive stars. Whether a stellar group materializes as an association or a cluster...
Stellar associations are generally classified into three types on the basis of their most prominent components: OB, R, and T associations. OB associations consist largely of very young, massive stars (about 10 to 50 solar masses) of spectral types O and B, which have an absolute luminosity about 100,000 times that of the Sun. In many cases, one or more small open star clusters lie near the...
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