Main-sequence stars start core-helium burning before the core. Classification is complex between a continuous range and luminosity classes with small differences. Subgiants are an entirely separate spectroscopic luminosity class from giants. Another luminosity class is the bright giants are called red giants. Red giants include stars in a number of distinct evolutionary phases. The RGB stars are the most obvious grouping of stars on most HR diagrams after the main sequence. RR Lyrae variables pulsating horizontal-branch class A.
W Virginis variables known also as II Cepheids as type. Lower-mass evolve along the horizontal branch from red giants, is thought that some post-AGB stars.