Quintuplet Cluster
A series of papers with A. Moneti and A. Moorwood explores the nature of this cluster of mainly young luminous stars near the Galactic Centre. It is one of three young clusters of luminous stars in the region, the other two being the Galactic Centre Cluster itself, which contains similar objects, and the `Arches' Cluster, which contains mainly hot stars. They constitute evidence for ongoing massive star formation near the centre of the Galaxy. I first became aware of it when searching for counterparts of IRAS sources near the Centre (Glass, MNRAS, 234, 115, 1988). With the crude methods then available, it appeared to consist of 3 sources. The name `Quintuplet' came from early infrared images made by Okuda et al.
Imaging using a 32 x 32 array at ESO showed it to be an open cluster of luminous objects, some of them very red "coccoon" stars (Glass, Moneti & Moorwood, MNRAS, 242, 55p, 1990; erratum [better reproduced K-band picture] MNRAS, 244, 767, 1990). The sources off the grey area were found by scanning. Positions were determined by offsets from the visible star "Ref".
Spectroscopic investigation showed up the existence of emission-line objects in the cluster and nearby (Moneti, Glass & Moorwood, MNRAS, 258, 705, 1992; MNRAS, 268, 194, 1994).
Photometric monitoring has shown that two of the cocoon stars are variable. In addition, two other intrinsically blue stars nearby show irregular variations and are thus confirmed as Luminous Blue Variables. Another apparent member of the Quintuplet is probably a Mira variable in the same line of sight (Glass, Matsumoto, Carter & Sekiguchi, MNRAS, 304, 10, 1999).
For more recent interpretations of this cluster and its contents , please see the Wikipedia article https://en.wikipedia.org/wiki/Quintuplet_cluster