Eta Cephei

Eta Cephei (η Cep, η Cephei) is a star in the northern circumpolar constellation of Cepheus. It shares the name Al Kidr with θ Cep, although the meaning of this name is unknown. This star, along with α Cep (Alderamin) and β Cep (Alfirk), were identified as Al Kawākib al Firḳ (الكوكب الفرق), meaning "the Stars of The Flock" by Ulug Beg. With an apparent visual magnitude of 3.4, this is a third magnitude star that, according to the Bortle Dark-Sky Scale, is readily visible to the naked eye. Parallax measurements put it at a distance of 46.53 light-years (14.27 parsecs) from Earth.

In Chinese, 天鈎 (Tiān Gōu), meaning Celestial Hook, refers to an asterism consisting of η Cephei, 4 Cephei, HD 194298, θ Cephei, α Cephei, ξ Cephei, 26 Cephei, ι Cephei and ο Cephei. Consequently, η Cephei itself is known as 天鈎四 (Tiān Gōu sì, English: the Fourth Star of Celestial Hook.).

Eta Cephei is a subgiant star with a stellar classification of K0 IV, which indicates it is exhausting the supply of hydrogen at its core and is in the process of evolving into a giant star. With 1.6 times the Sun's mass, at an age of 2.5 billion years it has reached a radius four times larger than the Sun and a luminosity ten times greater. It is radiating this energy from its outer atmosphere at an effective temperature of 4,950 K, giving it the orange-hued glow of a K-type star. Eta Cephei has a high proper motion across the celestial sphere and a large peculiar velocity of 112 km s–1.