The brightest stars in nearby galaxies. III - The color-magnitude diagram for the brightest red and blue stars in M81 and Holmberg IX. IV - The color-magnitude diagram for the brightest red and blue stars in NGC 2403

Abstract

Candidates for the brightest blue and red stars in M81 and its companion galaxy, Holmberg IX, are identified and photometered relative to a photoelectric sequence that is extended to B = 23, V = 22. Data that cover the time interval from 1910 to 1977 are given for seven irregular blue variables in M81. At maximum light, the brightest of these define the upper limit of luminosity on the blue main sequence at apparent magnitude B(2) = 19.0, one magnitude fainter than similar stars in NGC 2403, and 3.6 mag fainter than such stars in M33. The brightest Cepheids found in M8l are 0.8 mag fainter than those in NGC 2403, giving an apparent blue modulus of (m - M)<SUP>AB</SUP><SUP>M8l</SUP> = 28.8, which is 1 mag fainter than the value currently in use. The brightest candidate stars for red supergiants in Holmberg IX occur at apparent magnitude V(3) = 20.6 or 21.1 depending on the choice of members versus foreground field stars. The resulting absolute magnitudes are M<SUB>V(3)</SUB> = - 8.1 or - 7.7. The corresponding numbers are V(3) = 19.7 or 20.2 for M8l, giving MV(3) = - 9.1 or - 8.5. These values for the brightest red super- giants are combined with earlier data for other galaxies to provide a new calibration that shows the luminosity of the brightest red supergiants to increase with the luminosity of the parent galaxy, rather than to remain constant as previously believed.