3 The color-magnitude diagram

Figure 3ab presents the V-(B-V) and I-(B-I) diagrams based on our photometry of 3077 stars in the field of NGC 1978. Similar to earlier studies, we find numerous field stars as well as a moderately well-populated giant branch and a rich red horizontal branch. In all investigations, the scatter of stars around the giant branch of NGC 1978 was found to be considerable, probably due not only to crowding and field-star contamination but also to intrinsic reasons. Also, the intrinsic widths of the sequences are suspiciously broad in galactic globular clusters with red horizontal branches, and they are not always easily explained by crowding, differential reddening, contribution of field stars (see, for example, CMDs in Armandroff 1988; Ortolani et al. 1992; Sarajedini & Norris 1994). As for NGC 1978, some broadening of the upper giant branch may be, besides crowding, due to the contribution of stars on the asymptotic branch which must be quite close to the main giant branch in the CMD.

 

Figure 3: a) The V-(B-V) diagram and b) I-(B-I) diagram based on our photometry of 3077 stars in the $5'\times5'$ field around NGC 1978

Contrary to the metal-richest galactic globulars, where the reddest part of the giant branch curls back in itself and becomes fainter in V light than the giants at somewhat lower color index (see, for instance, Ortolani et al. 1992), the reddest giants of NGC 1978 are apparently the brightest ones, both in our photometry and in earlier studies.