By what astronomical characteristic did Hubble precisely calculate the distance to nebulae like Andromeda?

Answer

The period of Cepheid variable stars' light variation.

Hubble identified Cepheid variable stars within nebulae; because the period of these stars' light variation is directly related to their true brightness, measuring their apparent brightness allowed for precise distance calculation.

By what astronomical characteristic did Hubble precisely calculate the distance to nebulae like Andromeda?
discoveryAstronomerSocietyimpactcosmology