Those are stars. Red giants, specifically. In a dwarf spheroidal galaxy called Leo II. I know that the diagram doesn't make much sense to most people, unless they happen to be astronomers versed in photometry, but trust me. I'm so happy I'm just about dancing around like an idiot. Because after an unreasonable amount of work, that diagram is starting to look sort of like what would be reasonable to expect. I might finish my master's thesis yet.
Love and astronomy,