The galactic cluster NGC 4755 (κ Crucis)

Radial velocities and spectral types in NGC 4755 are listed in Table I. A rediscussion of the photometry of Arp and van Sant together with the present observations leads to a revised distance of kpc. The cluster contains 4 B-type supergiants and the present data are used together with other published data for similar stars in clusters to calibrate the intrinsic colours and luminosities for these stars. The absolute magnitudes agree well with Blaauw 5 calibration but the intrinsic colours are appreciably bluer ( orn.04) than those given by Johnson. The three clusters NGC 4755, NGC 3293, and h and Per are compared. The MR diagrams (Fig. 3) are very similar with vertical evolved main sequences near Bi and with M supergiant members. The three clusters together show a conspicuous gap between the top of the evolved main sequence and the supergiant branch. Earlier omparisons between these clusters and clusters in the Magellanic Clouds are mentioned as well as their importance in establishing a Hess diagram for bright supergiants. The radial velocities of stars in NGC 4755 and h and Per show a dependence on absolute magnitude similar to that found previously in NGC 3293. The Ca ii interstellar velocities are free of this effect.