<P> A Cepheid variable (/ ˈsɛfiːɪd, ˈsiːfiːɪd /) is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well - defined stable period and amplitude . </P> <P> A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances . This robust characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of variable stars in the Magellanic Clouds . This discovery allows one to know the true luminosity of a Cepheid by simply observing its pulsation period . This in turn allows one to determine the distance to the star, by comparing its known luminosity to its observed brightness . </P> <P> The term Cepheid originates from Delta Cephei in the constellation Cepheus, identified by John Goodricke in 1784, the first of its type to be so identified . </P>

What makes cepheid variable stars useful in determining star distances