noun

• A Cepheid belonging to a subcategory typified by the star δ Cephei (also called type I Cepheids), used to establish a period–luminosity relation for stars with a similar variability pattern, which subcategory comprises population I stars typically having a period of 5–10 days (as contrasted with type II Cepheids, which are population II stars with generally longer periods and lower luminosity).

  The Cepheid period–luminosity relation is used for determining extragalactic distances up to 15 megaparsecs (the Virgo Cluster).