What is the difference between a Type I and Type II Cepheid variable star?

Cepheid variable stars are important astronomical objects used to measure the distance to remote galaxies. They are pulsating stars that vary in brightness with a well-defined period. There are two types of Cepheid variables: Type I and Type II, which have different physical properties and are located in different regions of galaxies.

Type I Cepheids are also known as Delta Cephei stars, and are relatively young stars with masses between 3 and 8 times that of the sun. They are located in the disk of galaxies, where young stars are formed. Type I Cepheids have short periods of less than 10 days and exhibit a regular, sinusoidal variation in their brightness.

Type II Cepheids, on the other hand, are older stars with masses between 0.5 and 2 times that of the sun. They are located in the halo of galaxies, where old stars are found. Type II Cepheids have longer periods of 1 to 50 days and exhibit an irregular variation in their brightness. They are further divided into two subclasses: Type IIa and Type IIb, which have different properties and are believed to have different evolutionary histories.