Cepheid

Cepheid variables are a type of pulsating star whose brightness varies regularly over time. This cycle of brightening and dimming occurs because the star expands and contracts periodically. The key feature of Cepheids is a well-established relationship between their pulsation period (how long their brightness cycle lasts) and their intrinsic brightness (actual luminosity). By measuring how long it takes a Cepheid to complete a cycle, astronomers can determine its true brightness and then compare it to how bright it appears from Earth, enabling accurate distance measurements across the universe. Cepheids are essential tools for understanding cosmic scale and expansion.