Final answer:
Main sequence stars differ in age, size, and temperature but not significantly in composition. Temperature, rather than composition, mainly influences the spectra and classification of stars, which is illustrated on the H-R diagram.
Step-by-step explanation:
Main sequence stars may differ in various attributes, but the one aspect they do not differ in is composition. The question asks about what main sequence stars do not typically vary in, which includes age, size, temperature, and composition. Based on astrophysical studies, we understand that differences in the spectra of stars are principally due to differences in temperature, not composition. The spectral classes of stars, from hottest to coolest, are O, B, A, F, G, K, M, L, T, and Y, indicating variations in temperature, not composition. Additionally, the Hertzsprung-Russell (H-R) diagram shows that stars of different masses have different evolutionary tracks, but those tracks, for stars of similar composition, illustrate how mass, not composition, primarily determines a star's evolution. Therefore, while main sequence stars do differ in age, size, and temperature, their composition remains relatively similar, especially within the same spectral class.