Steps toward the Hubble constant. II. The brightest stars in late-type spiral galaxies.

Identification and photometry of the brightest blue and red supergiant stars are given for the highly resolved galaxies NGC 2366, NGC 4236, IC 2574, Ho I, and Ho II in the NGC 2403-M81 group. The data are based on plates taken with the Hale 5-meter telescope and on new photoelectric sequences, or on photographic transfers to other photoelectric sequences. Brightest stars in the Large Magellanic Cloud, the Small Magellanic Cloud, and the solar neighborhood have been selected by analysis of recent data in the literature. This material, together with less complete data for NGC 6822, IC 1613, and M33, gives a calibration of the absolute blue magnitude of the brightest blue stars in terms of either the luminosity of the parent galaxy or the luminosity class. The absolute magnitudes range from Mn(lst) = -8.0 for M (ga = - 14.5 to ME(15t) = - 10.0 for M (gal) = - 20.7. The standard deviation of a single galaxy about the mean correlation line is a(M ) 0.5 mag. The correlation of M(star) and M(galaxy) is significant at the 2.5 a level, and can be understood as a consequence of the normalization of a common luminosity function to the total luminosity of each parent galaxy. A test of the assumption that six galaxies of the NGC 2403 group are closely at the distance of the parent galaxy was made by determining the individual distances by the H ii region calibration of Paper I and by the bright-star criterion here. The mean distance to the six galaxies is 3.09 1 0.13 Mpc (the standard deviation of a single galaxy distance is 10.30 Mpc). There are no significant differences (larger than 1.5 a) in our distances to any of the six members of the group. The brightest red supergiants [chosen such that (B - V)0 > 2.0] have a remarkably small dispersion in absolute magnitude, and the magnitudes do not depend on the luminosity of the parent galaxy. The calibration gives (max) = - 7.9 1 0.1 mag. Brighter stars are guillotined either by downward sloping evolutionary tracks from their brighter main-sequence progenitors, or by the sharp onset (at Mv -8) of some physical process that drastically shortens the life of a brighter red supergiant. Bright blue irregular variables are not so well defined in absolute luminosity. Although they are very luminous at maximum light [ MB -9.4 1 0.5], some members of the class, such as Car, become so much brighter (ME - 14) at certain times that the stars, as a class, are apparently unsuitable as accurate distance indicators. Subject headings: galaxies - galaxies, clusters of - luminosities - luminous stars