With increasing distance fainter stars, and therefore those of lower luminosity, older populations, become less observable. This means that with increasing distance we actually probe different parts of a Color-Magnitude diagram (CMD), which mainly consists of the brightest and more evolved massive stars. This is best shown in this figure by Annibali & Tosi (2022, “Chemical and stellar properties of star-forming dwarf galaxies” – which is a nice review for the dwarf galaxies).
Fig. 1 The effect of distance on observational CMDs. Absolute magnitude vs intrinsic colour of Star Forming Dwarfs at increasing distances are plotted from panel a to d. The horizontal lines show the magnitude level of the evolutionary phases used as old age indicators (see text for details): RGB tip in dotted cyan, Horizontal Branch in dashed blue, 12 Gyr main-sequence turnoff in solid magenta. The lookback time corresponds to the age of the oldest stars identified in the CMD. Galaxy name, distance and lookback time are labelled in each panel. The dwarfs plotted in panels a and b are within the Local Group, and their oldest turnoff is visible. Farther out, at 3.4 Mpc, HST allows to (barely) reach the Horizontal Branch, hence stars 10 Gyr old (panel c); at 18 Mpc, the oldest stars resolvable with accuracy are on the RGB, with age ≥1-2 Gyr (panel d). The photometric uncertainty increases with increasing distance: for instance, at the RGB tip it ranges from 0.01 mag in panel a to 0.20 mag in panel d.
As of today, with the photometric depth reachable with the most powerful existing instruments, the Horizontal Branch can be detected in galaxies closer than 3-4 Mpc, the Red Clump in galaxies closer than 5-6 Mpc, the RGB in galaxies up to 18-20 Mpc.
