Delta Cephei — Variable Star in Cepheus

Delta Cephei is the prototype Cepheid variable — a yellow F5–G3 supergiant pulsating between magnitude 3.48 and 4.37 with a remarkably steady period of 5 days, 8 hours, 47 minutes, 32 seconds. About 890 light-years away, it is also a wide visual binary: a magnitude 7.5 blue companion sits 41″ from the supergiant, an easy split in any small telescope.

The variability is easy to track by eye with a few weeks of patient comparison against Zeta Cep (mag 3.4) and Epsilon Cep (mag 4.2), which lie nearby in the same field. The blue + yellow colour pair at 41″ is a beautiful sight in a 60mm refractor. Best observed in the late autumn and winter, high in Cepheus.

John Goodricke discovered the variability in 1784 — the same young deaf-mute astronomer who had identified the eclipsing nature of Algol two years earlier. In 1908 Henrietta Swan Leavitt, working at Harvard, found that Cepheids in the Magellanic Clouds obey a strict period–luminosity relation. That discovery, calibrated on Delta Cep itself, became the cosmic distance ladder rung that took us to Andromeda and beyond.