Our final category of young systems includes those most easily recognized by the unaided eye. These are called open clusters because their individual members can often be clearly distinguished. In contrast, only the largest telescopes can resolve the central regions of the much more massive and dense globular clusters, very old systems residing far outside the plane of the Milky Way. Open (or "galactic") clusters are the most evolved aggregates we have discussed. Half of them reach ages of 1 x 108 yr, while about 10 percent survive as long as 1 x 109 yr. Even at 108 yr, however, all stars of less than 0.5 Mq are still in the pre-main-sequence phase, so most of these systems are young enough for our purposes.
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