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. ... This discovery allows one to know the true luminosity of a Cepheid by simply observing its pulsation period.

How do Cepheid variables measure distance?

Through observations of Cepheid variables, astronomers have determined the distances to other galaxies. They compare the Cepheid variable's apparent brightness with its intrinsic brightness. The difference between observed and actual brightness yields the distance.

What are Cepheid variables and why are they important?

Cepheids are extremely important to astronomers because their periodic changes in brightness can be used to figure out how far away they are. Because Cepheids are intrinsically bright stars, they can be seen far away, and astronomers can use them to determine the distance to very distant galaxies.