If the integrated colours of a star cluster mainly depend on chemical composition and age, then theoretical calibrations of colours as function of age for different chemical compositions are very useful to obtain quantitative determinations of the age and composition of individual clusters, and thus to trace the chemical history of nearby galaxies. Several calibration curves exist in the literature which rest on the standard theory of stellar evolution. However, a growing amount of observational evidence seems to indicate that overshooting from convective cores may be an important phenomenon in stellar evolution. In fact models computed with overshooting are significantly different from the standard ones. The aim of this preliminary investigation is to study the effects of convective overshooting on the integrated colours of clusters whose turnoff mass is in that range in which convective overshooting is effective.

Advances in both theoretical understanding and observational capabilities in the past few years have made possible the determination of the effective temperature, surface gravity and chemical abundance of massive stars with unprecedented accuracy. These data are in turn important for the study of galaxies, as stars are important sources of information concerning the evolutionary state, past and present chemical composition, and distance of the parent galaxy. In addition to this diagnostic role, stars are crucial as sources of light, matter, and metals in the galaxy. Thus an improved understanding of massive stars makes possible a better determination of the physical conditions in a galaxy as well as a deeper understanding of how it functions.

Because the term “luminous blue stars” is relative, let me begin by delineating what region of the H-R diagram I am considering. For the purpose of this talk, I will discuss primarily 0-type stars, with Teff > 30,000 K and Mbol > − 7, but I will also mention their evolved descendants the B supergiants with 30,000 K > Teff > 10,000 K and Mbol > −8, and their even later evolutionary form, the Wolf-Rayet (W-R) stars with Teff > 30,000 K and uncertain Mbol. Although these stars are not among the visually brightest stars, they are the bolo-metrically most luminous as well as the hottest stars. They are also the most massive. They are an important channel in the metal enrichment in a galaxy through the action of mass loss via stellar winds and their ultimate disruption as supernovae (Maeder 1981). They also contribute in a major way to the energy balance of the interstellar medium (Abbott 1982).

The current status of the absolute magnitude calibration of the brightest blue and red supergiants in galaxies of different absolute magnitudes shows trends of M(star) with M(parent galaxy). Red supergiants show a more shallow correlation than the blue stars for galaxies brighter than MB = −14. For fainter galaxies, the red supergiant method appears to become totally degenerate.

Four areas of application of the brightest star data are discussed as (1) determining MB of the Eddington limit to be ∼−10 for blue supergiants, (2) calibration of MB(max) = −20.0±0.4 for type I supernovae, leading to a Hubble constant of HO = 43±10 km s−1 Mpc−1, (3) detection of the deceleration of the cosmological expansion by the Local Group leading to a Local Group mass of 4×1011 M⊙ and a mass-to-blue light ratio of 3, and (4) use of the brightest stars to map the Virgo cluster velocity perturbation of the Hubble flow.

The intermediate and late-type supergiants are the visually brightest stars. They are among the first stellar objects observed in other galaxies and provide our first clues to the conditions of massive star evolution in galaxies of different types. They are not as massive as the hottest and most luminous stars in the upper left of the HR diagram. Nevertheless, these somewhat lower mass stars (≈20−50 M⊙) with relatively cool temperatures play a major role in our efforts to understand massive star evolution. These supergiants are usually considered to be post hydrogen burning stars, and their relative numbers in the HR diagram provide essential comparisons with models for the later stages of massive star evolution. Most importantly, the most luminous cooler supergiants define the stability limit for massive stars in the HR diagram.

Although luminous stars are relatively rare, they can potentially be studied out to large distances. In our own Milky Way, this advantage is offset by obscuration due to dust in the plane of the Galaxy. In addition, distances to these individual stars are extremely difficult to determine. The study of external galaxies allows a panoramic view of the system and its individually brightest stars which are all at a common distance. The spatial distribution of star forming regions is immediately apparent, and the effects of obscuration are minimized. Nearby resolved galaxies therefore provide a rich resource for examining the properties of the intrinsically brightest stars and their relation to other components of the galaxy.

In this poster we illustrate how the Minnesota Automated Plate Scanner (APS) is being used to study the luminous stellar content of NGC 2403. Presented are a brief description of the APS, examples of photometric calibration, separation of stellar and non-stellar images, and a preliminary color-magnitude diagram. The eventual goal is to study the evolution of massive stars via color-magnitude diagrams, luminosity functions and star formation rates.

Preliminary results are presented of an observing programme aimed to obtain estimates of the stellar masses from studies of spectroscopic binary systems in the Large Magellanic Cloud. These are the first steps with the final purpose to determine an empirical mass-luminosity relation in a galaxy other than our own.

HR 8752 is one of the brightest stars of the Galaxy. Humphreys (1978) gives its absolute visual and bolometric magnitudes between −9.1 and −9.5, and between −9.2 and −9.5, respectively. The star received its variable star name, V509 Cas, in 1972 (Kukarkin et al., 1972). Radial velocity variations were first noticed by Harper (1923) and by Campbell and Moore (1928). In Harvard Ann. Vol. 44 (Pickering, 1899) there are indications for light variation at the end of the 19th century.

Yellow hypergiant, HD 217476, (HR 8752) is known to have extensive circumstellar envelope. Its unusual spectrum makes it enigmatic object. On the H-R diagram this hypergiant lies among the most luminous stars known in six Local Group galaxies (Humphreys et al., 1984). We have observed it with the same equipment during last fifteen years at the Dominion Astrophysical Observatory. Our present results arg based upon several hundreds of high dispersion spectra (6 and 10 Å/mm).

It is still a matter for some debate as to how far the most massive stars ever get towards the right-hand side of the HR diagram during the course of their evolution. The apparent absence of very luminous red stars suggests either that stars never reach this region at all, or that if they do, their sojourn there is very brief indeed. If the latter is the case, one might expect to see an increase in mass-loss activity with luminosity among the brightest stars that we actually do see - the cool galactic hypergiants. An attempt has been made to search for one of the signatures of mass-loss - thermal reemission from ejected dust - using recently released data from the InfraRed Astronomical Satellite (IRAS).

The inclusion of rotation as an ingredient in radiation pressure driven stellar wind models is a nontrivial undertaking. Those bold enough to attempt an investigation of its likely importance include Castor (1979), Abbott (1980), and Marlborough & Zamir (1984), whose work shows that the critical point is expected to move away from the stellar surface with increasing rotation. Observationally, the surface mass flux is predicted to be insensitive to rotation, but the wind acceleration is expected to be less than in a nonrotating star and the terminal velocity smaller.

The 1600–2500Å spectra of many early-type supergiants are dominated by a broad depression of the continuum near 1920Å. This depression, often called the 1920Å feature, was detected in S2/68 (TD-1) spectra of early B stars (Thompson et al. 1974) and was shown to be strongly luminosity dependent. It was also shown that its profile could be matched by the cumulative effects of FeIII absorption lines.

The causes of light variations in red supergiants are still discussed. In particular it is not clear if these variations result from deterministic or narrow-band stochastic (pseudo-periodic) processes or from a mixture of both types.

Statistics on the variability of 127 high-luminosity, intermediate spectral-type supergiants in the Magellanic Clouds are presented.

Photometric and recent spectroscopic studies of the SMC have shown that the differences observed in the SMC clusters and those of our Galaxy could be attibuted to differences in metallicity, star formation rate and/or the Initial Mass Function (IMF) (Humphries, 1983). The studied clusters NGC152 and KRON3 are located at the west side of the bar of the SMC and their adjoining fields represent the halo population of this galaxy.