tokens: can undergo extreme mass loss and develop dusty circumstellar shells, making them extend to fainter and extremely red optical colors. Below the AGB is the familiar RGB of evolved shell-hydrogen-burning low-mass stars, extending from F814W∼21 down through the AGB bump and red clump (RC) to the subgiants at F814W∼26. The AGB bump is comprised of early AGB (EAGB) stars, which are low-mass stars that undergo a pause in evolution when forming their double-shell structure (Gallart 1998 ). Below the AGB bump is the prominent RC of stable core-helium-burning low-mass stars. In addition, there is a small tail of stars departing from the RC toward the top left part of the CMD, which we tentatively identify as blends of RC plus MS stars, either because they are crowded together or are real binaries. Tails of reddened stars depart from the most prominent features, namely the RC and AGB bump, along the reddening vector, reflecting spreads in the internal extinction that often exceed half a magnitude in the V band. We compare our photometry to ground-based photometry of the same region from LGGS (Massey et al. 2006 ) and PAndAS (McConnachie et al. 2018 ) in Figure 19 . Compared to ground-based imaging, our catalog extends 3–5 magnitudes deeper, has three orders of magnitude more stars (22 million versus ∼70,000 in both LGGS and PAndAS), and has qualitatively more and tighter features. This comparison clearly demonstrates the improvement in available photometry and the power of HST to resolve stars in the M33 disk. Figure 19. Comparison of optical CMDs of M33 with photometry from LGGS (left, VI ), PAndAS (center, gi ), and this work (right, F475W/F814W GST). Although the filter systems are not identical, we use a common color and magnitude range on all axes to illustrate the difference in depth that can be achieved with HST. The LGGS and PAndAS catalogs have been culled to cover approximately the same area as the HST survey, but have not been culled on any photometric quality metrics. The median color and magnitude uncertainties in 1 mag bins are shown on the right side of each panel (black lines). 3.2. Star Clusters The PHATTER data are able to resolve individual stars within star clusters in M33, allowing detailed age and metallicity studies. We show a few examples of young stellar clusters of a range of masses in Figure 20 , all of which were identified by citizen scientists as part of the Local Group Cluster Search on Zooniverse (Johnson 2019 ; Wainer et al. 2020 ). 19 19 https://www.zooniverse.org/projects/lcjohnso/local-group-cluster-search The lower panels of this figure show zoomed-in F475W images of the clusters, and the upper panels show optical CMDs of the regions within the clusters’ radii. In all cases, the blue plume of main-sequence stars is visible, showing the young population of cluster stars. Moreover, the sequence on the CMD is more populated in the more massive, larger clusters, showing the quality of the resolved photometry. While
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