Cepheid variables are named after
Delta Cephei, the first star of this type to be discovered. It was discovered in 1784 by
John Goodricke, a
deaf and mute English astronomer. It has a period of 5.4 days, less than any
variable star known at the time.
Cepheids, along with RR Lyrae variables, occupy an area on the H-R Diagram known as the instability strip. This lies in the top right, between the main sequence and the long period variables. This means that Cepheids have a surface temperature around that of the Sun but are 1,000-100,000 times more luminous.
The variation in a Cepheid's brightness is caused by the star's oscillating outer envelope. The rate at which this oscillates is determined by the star's metal content and average brightness. As the metal content can be determined through spectroscopy, it is possible to calculate the star's true (not observed) brightness. By comparing this to the observed brightness, the distance to the star can be calculated quite accurately.
For Type I (metal rich) Cepheids, the true luminosity (as a multiple of that of the Sun), is approximately 800 times its period (in days). For Type II (metal poor) Cepheids, the luminosity/period factor is roughly 200.