Regular variable stars are stars that exhibit predictable changes in brightness over time. These variations occur in a consistent cycle, allowing astronomers to classify them based on their periodicity. Common types of regular variable stars include Cepheid variables and RR Lyrae stars, which are essential for measuring cosmic distances. The brightness changes in regular variable stars are often caused by pulsations in their outer layers. As these stars expand and contract, they emit varying amounts of light. This behavior makes them valuable tools for understanding stellar evolution and the scale of the universe.