A Cepheid variable is a type of star that pulsates in a regular cycle, causing its brightness to change over time. These stars expand and contract, leading to variations in luminosity that can be observed from Earth. The period of these pulsations is directly related to the star's intrinsic brightness, making them useful for measuring distances in space. Astronomers use Cepheid variables as standard candles to determine how far away galaxies are. By comparing the observed brightness of a Cepheid variable to its known intrinsic brightness, scientists can calculate the distance to the star and, consequently, the galaxy it resides in.