A pulsating variable star is a type of star that changes its brightness over time due to periodic expansions and contractions. This behavior is caused by internal processes, such as changes in temperature and pressure, which affect the star's outer layers. These stars can be classified into different types, including Cepheid variables and RR Lyrae stars, based on their pulsation characteristics. The pulsation period of these stars can range from a few hours to several days. Astronomers study pulsating variable stars to understand stellar evolution and to measure cosmic distances, as their brightness variations can be used as reliable indicators of distance in the universe.