Build up an H-R Diagram for nearby stars with parallax distances. Practical limit few 100,000 pc. Works best for star clusters. Luminosity classes have some spread. H-R diagram location depends on composition .
Faint spectra give poor classifications.
Stars Stars whose brightness varies with a characteristic, periodic pattern. Period of their brightness variations. Period-Luminosity Relation exists for certain classes of periodic variable stars. Measuring the Period gives the Luminosity. Crucial for measuring distances to galaxies.