Hubble Space TelescopeObservations of Star Clusters in M101

Hubble Space Telescope Advanced Camera for Surveys (ACS) images are used toidentify and study star cluster candidates in the nearby spiral galaxy M101.About 3000 round, slightly-resolved cluster candidates are identified in 10 ACSpointings covering an area of 106 arcmin^2. The cluster candidates' color andsize distributions are consistent with those of star clusters in other nearbyspirals. The majority of the M101 candidates are blue and more likely to beassociated with the galaxy's spiral arms, implying that they are young. Thegalaxy-luminosity-normalized number of `young massive clusters' in M101 issimilar to that found in other spirals, as is the cluster density at a fiducialabsolute magnitude. We confirm a previous finding that M101 has a large numberof faint, red star clusters: if these are old globular clusters then thisgalaxy has a very large globular cluster population. More plausible is that thefaint red clusters are reddened young clusters; their colors and luminositiesare also consistent with this explanation.