Basic Properties

The largest open clusters are those near the centers of the more compact OB associations. Examples include NGC 2244, the group of diameter 11 pc within the Mon OB2 association, and IC 1396, a 12 pc cluster located inside Cep OB2 (see Figure 4.14). Despite appearances, systems of this type are rarely gravitationally bound, as few remain within older and larger associations. Most other open clusters are indeed bound and have diameters ranging from 2 to 10 pc, with a median value close to 4 pc. The total number of members in any one is always difficult to assess, but the population seen in photographs can be anywhere from 10 to 103. Within the Galaxy, open clusters concentrate strongly toward the plane; the scale height of the distribution in the solar neighborhood is 65 pc. There are currently over 1200 systems known, almost all of them less than 6 kpc from the Sun. The sample is essentially complete within 2 kpc, while dust extinction increasingly hampers visual observations at greater distances.

Table 4.3 lists all known open clusters within 300 pc. We shall discuss shortly how the actual distances are established. The diameters were obtained by eye from photographic plates, after applying the distances. While clearly subject to some uncertainty, these figures should represent the volume within which the majority of component stars reside. The tabulated ages follow from analyzing the main-sequence turnoff in the HR diagram. Finally, we note that the membership figures given in the last column represent true counts (or estimates) of the total number of objects for which there is optical photometry in the U-, B-, and V-bands.

Open clusters contain little molecular gas and are not currently forming protostars. They therefore represent groups whose members, all located at nearly the same distance, share a common age and chemical composition. These characteristics have made them invaluable research tools for over a century. In 1930, Trumpler first demonstrated the phenomenon of interstellar absorption by showing that clusters of smaller apparent diameter, which are more distant on average, are also systematically dimmer, above and beyond the usual inverse square falloff in flux. Over the next 30 years, it was the intercomparison of open clusters of different ages that provided the empirical foundation for the developing theories of pre- and post-main-sequence evolution. More recently, studies of stellar surface activity (such as X-ray emission), rotation, and the depletion of light elements have also relied heavily on these systems.

Table 4.3 The Nearest Open Clusters

Name

Distance

Diameter

Age

Members

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