Cepheus

Cepheus's own article rightly centres Delta Cephei, the variable star that gave its name to an entire class of distance-measuring stars. A different star in the same king's figure earned its own name for the opposite reason: not for blinking on a schedule, but for its colour.

Mu Cephei is a red supergiant so deeply coloured that William Herschel, cataloguing the sky in the late eighteenth century, nicknamed it the Garnet Star. It is among the largest and most luminous stars known: were it placed at the centre of the solar system in the Sun's position, its outer atmosphere would extend out past the orbit of Jupiter, swallowing every inner planet and several outer ones along the way. Its intense red colour comes from a surface temperature far cooler than the Sun's, cool enough that the star radiates chiefly at wavelengths the eye reads as deep red rather than white or yellow.

Where Delta Cephei matters because its brightness changes on a precise, reliable clock, useful for measuring distance, Mu Cephei matters for almost the opposite reason: it barely needs measuring to be remarkable, since its sheer size and colour are obvious to anyone who points a telescope at it, no instruments or calculation required beyond noticing how red it is.

Two stars in one small constellation, one useful because it is regular, the other startling because it is extreme.

Cepheus is a northern constellation, one of Ptolemy's forty-eight, representing the mythical king of Aethiopia, husband of Cassiopeia and father of Andromeda, and so one of the royal figures of the great Perseus myth set together in the sky. Its five main stars form a shape like a child's drawing of a house, near the pole. It is astronomically important for its variable star Delta Cephei, the prototype of the Cepheid variables, whose regular pulsations gave astronomers a means of measuring cosmic distances. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign. These are the features of Cepheus as the classical and modern traditions describe it, distinct from any astrological reading; the king stands beside his queen Cassiopeia in the circumpolar sky, both set among the stars in the aftermath of their story.

Cepheus borders Camelopardalis, the early seventeenth-century giraffe introduced to cover the large, faint stretch of circumpolar sky between the king, the two Bears and Cassiopeia that Ptolemy's original forty-eight had left entirely unmarked. Delta Cephei's discovery as a regular pulsating variable, and the subsequent identification of the period-luminosity relation by Henrietta Swan Leavitt in the early twentieth century, made Cepheid variables the essential first rung of the cosmic distance ladder, the method by which astronomers first measured distances to other galaxies.