| Abstract | UBV photographic photometry is used to select luminous stars in M33 with M(V) of less than about -7 and U-B of less than zero, and the efficiency of the U plates in examining blue luminous stars in nearby galaxies is assessed. The spatial distribution of the stars shows a general concentration at an intermediate distance of about 8 arcmin from the nucleus. The youngest association is roughly 5 million yr old and the oldest is roughly 10 million yr old. The true distance modulus is found to be 24.2, the position angle of the major axis is 22 deg, and the tilt angle of the M33 plane is 57 deg. There is no age gradient across the spiral arms, which differ from those in M31 in being diffuse and wide. The multiarm features of M33 support the stochastic self-propagating model of Gerola and Seiden (1978), in which the rate of star formation is due to a chain reaction process originating in a shock wave from a supernova. |
