The region of NGC 6522

Extract

The field of NGC 6522 is of especial importance because Baade considered that the cluster-type variable stars found in this field were located in a stellar concentration at the centre of the Galaxy. This particular field is a few degrees away from the actual centre, but is a region of comparatively small absorption, so that stars at considerable distances from the Sun can be seen and studied. Baade found a marked maximum in the frequency of these variables between 17^m and 18^m apparent photographic magnitude on plates which were exposed down to a limit of 20^m. He supposed that this maximum was due to an actual concentration of cluster-type variables round the centre of the Galaxy. Assuming 0m0 for the absolute magnitude, and allowing 3 magnitudes for absorption, Baade found a value of 8·2 kpc for R₀, the distance to the centre of the Galaxy. The absorption was computed from a rather high value for the reddening of the cluster NGC 6522 itself, which was given by Stebbins and Whitford.