At the southeast corner of the house-shaped constellation Cepheus the King, there is an intriguing variable star called Delta Cephei. With clock-like precison, this rather faint star doubles in brightness every 5.36 days. But that’s only the beginning of the story. The two stars lodging by Delta Cephei – Epsilon Cephei and Zeta Cephei – match the low and high ends of Delta Cephei’s brightness scale. At its faintest, Delta Cephei is as dim as the fainter star, Epsilon Cephei. At its brightest, Delta Cephei matches the brightness of the brighter star, Zeta Cephei.

Delta Cephei looms large in the history of astronomy. In fact, a unique class of supergiant stars – called Cepheid variables – is named in this star’s honor. Cepheids vary in brightness over regular periods of time. Depending on the star’s luminosity (intrinsic brightness), the time period can range from about 1 to 100 days. The longer the period, the greater the luminosity. Because these variable Cepheid stars can be seen up to a distance of 20 million light-years, they are used to measure the distances to galaxies outside our Milky Way.

Delta Cephei is circumpolar – always above the horizon – in the northern half of the United States. Even so, this star is much easier to see when it’s high in the northern sky on autumn and winter evenings. You can find Cepheus by way of the Big Dipper. First, use the Big Dipper “pointer stars” to locate Polaris, the North Star. Then jump beyond the Polaris by a fist-width to land on Cepheus. You’ll see King Cepheus close his wife, Cassiopeia the Queen, her signature W or M-shaped figurine of stars making her the flashier of the two constellations. They’re high in your northern sky on November and December evenings.

The astronomer Henrietta Leavitt discovered Cepheid variables in 1912. In 1923, the astronomer Edwin Hubble used Cepheids to determine that the so-called Andromeda nebula is actually a giant galaxy lying beyond the confines of our Milky Way.