At the southeast corner of the house-shaped constellation Cepheus the King, there is an intriguing variable star called Delta Cephei. With clocklike precison, this rather faint star doubles in brightness and fades again every 5.36 days. You can see it change over a period of days.
The entire cycle is visible to the eye alone in a dark-enough sky. This star and others like it have secured a place as important standard candles for establishing the scale of the galaxy and universe.
Delta Cephei itself looms large in the history of astronomy. An entire class of supergiant stars – called Cepheid variables – is named in this star’s honor.
Like Delta Cephei, Cepheid variable stars dependably change their brightnesses over regular intervals. The time period can range from about one to 100 days, depending on the star’s luminosity or intrinsic brightness. Astronomers have learned that – the longer the cycle – the greater the intrinsic brightness of the star. This knowledge is a powerful tool in astronomy.