Delta Cephei

Delta Cephei is a well-known variable star located in the constellation Cepheus. Its brightness fluctuates regularly over about 5.4 days, causing it to appear brighter and dimmer from Earth. These brightness changes are caused by rhythmic expansions and contractions of the star's outer layers, making it a "pulsating variable star." Delta Cephei is important because it serves as a standard candle for measuring distances in space; by knowing its intrinsic brightness and observing its apparent brightness, astronomers can determine how far away it is. This helps calibrate cosmic distance scales and improves our understanding of the universe.