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Star Clusters

OB Associations

OB associations are sparsely populated groupings of stars, typically between a few tens and a few hundreds of light-years across. They consist mainly of very young stars that have formed in the relatively recent past -- a few million or tens of millions of years ago -- from the same large interstellar cloud. Their mutual gravitational attraction is insufficient to bind them permanently together, however, and eventually the grouping breaks up and the individual stars go their separate ways. Two distinct varieties of stellar association are recognized: T associations, which consist of numerous, low-mass T Tauri stars; and OB associations, which are
OB Association Antares loosely grouping of between 10 and 100 O stars and B stars, scattered across a region up to several hundred light-years across. The Scorpius OB association is a loose group of stars as shown in Figure 06-04a. This group contains many hot, extremely luminous OB-type stars. It is the site of recent star formation. The stars in such groups are mostly not gravitationally bound but are expanding away from some common center, which presumably marks their birthplace. A study indicates that the Scorpius association has had 20 supernova explosions over the past 11 million

Figure 06-04a Scorpius [view large image]

Figure 06-04b Dying Star, Antares [view normal image]

years. The bright, bloated Antares in Figure 06-04b (the yellow star in Figure 06-04a) is the next star likely to explode.

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