Figure 11-25The Period–Luminosity Relationship The period of a Cepheid variable is directly related to its average luminosity: The more luminous the Cepheid, the longer its period and the slower its pulsations. Type I Cepheids (δ Cephei stars) are brighter, more massive, and more metal-rich stars than Type II Cepheids. The greater brightness of the Type I Cepheids is a result of their higher mass.