Cepheids are a type of very luminous variable stars. There is a strong direct relationship between a Cepheid's luminosity and its pulsation period. This makes Cepheids important standard candles for the galactic and extragalactic distance scales.
Cepheid variables are divided into several subclasses which exhibit clearly different masses, ages, and evolutionary histories:
- Classical Cepheids
- Type II Cepheids
- Anomalous Cepheids
- Dwarf Cepheids
The first cepheid known was Delta Cephei in the constellation Cepheus, found by John Goodricke in 1784. Delta Cephei is of great importance because its distance is extremely well known, thanks in part to it being in a star cluster, and the precise Hubble Space Telescope/Hipparcos parallaxes.
Cepheids are one of two ways in which the rate of expansion of the Universe can be measured.