Eta Cephei
 Location of η Cephei (circled) |
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| Observation data Epoch J2000.0 Equinox J2000.0 |
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|---|---|
| Constellation | Cepheus |
| Right ascension | 20h 45m 17.37555s |
| Declination | +61° 50′ 19.6167″ |
| Apparent magnitude (V) | 3.426 |
| Characteristics | |
| Spectral type | K0 IV |
| U−B color index | +0.613 |
| B−V color index | +0.918 |
| Astrometry | |
| Radial velocity (Rv) | –87.55 ± 0.11 km/s |
| Proper motion (μ) |
RA: +86.50 mas/yr Dec.: +818.02 mas/yr |
| Parallax (π) | 70.10 ± 0.11mas |
| Distance | 46.53 ± 0.07 ly (14.27 ± 0.02 pc) |
| Absolute magnitude (MV) | 2.631 |
| Details | |
| Mass | 1.6 M☉ |
| Radius | 4.12 ± 0.07 R☉ |
| Luminosity | 9.7 ± 0.5 L☉ |
| Surface gravity (log g) | 3.41 cgs |
| Temperature | 4,950 K |
| Metallicity [Fe/H] | –0.32 dex |
| Rotational velocity (v sin i) | 6.79 km/s |
| Age | 2.5 ± 0.3 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
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.
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