An approximate relationship between the mean spectral type of a pulsating variable and its period. Generally, the later the spectral type, the longer the period. The larger a star, the cooler its surface temperature and the longer it will take to pulsate. Period–spectrum relations are found in Cepheid variables and Mira stars. In Mira stars a period–spectrum–luminosity relation can be established.
Subjects: Astronomy and Astrophysics.