  | |
| Observation data EpochJ2000.0 EquinoxJ2000.0 | |
|---|---|
| Constellation | Cepheus |
| Right ascension | 20h 45m 17.37517s[1] |
| Declination | +61° 50′ 19.6142″[1] |
| Apparent magnitude (V) | 3.426[2] |
| Characteristics | |
| Spectral type | K0 IV[3] |
| U−Bcolor index | +0.613[2] |
| B−Vcolor index | +0.918[2] |
| Astrometry | |
| Radial velocity (Rv) | −87.55±0.11[4] km/s |
| Proper motion (μ) | RA: +85.993mas/yr[1] Dec.: +818.451mas/yr[1] |
| Parallax (π) | 69.5976±0.1279 mas[1] |
| Distance | 46.86 ± 0.09 ly (14.37 ± 0.03 pc) |
| Absolute magnitude (MV) | 2.631[3] |
| Details | |
| Mass | 1.40±0.14[5] M☉ |
| Radius | 3.86±0.02[6] R☉ |
| Luminosity | 8.4±0[6] L☉ |
| Surface gravity (log g) | 3.30±0.06[5] cgs |
| Temperature | 5,000±9[6] K |
| Metallicity [Fe/H] | −0.22±0.02[5] dex |
| Rotational velocity (v sin i) | 6.79[7] km/s |
| Age | 2.5±0.3[8] Gyr |
| Other designations | |
| 3 Cephei,BD+61 2050,FK5 783,HD 198149,HIP 102422,HR 7957,SAO 19019[9] | |
| Database references | |
| SIMBAD | data |
Eta Cephei (η Cep, η Cephei) is astar in the northerncircumpolar constellation ofCepheus. With anapparent visual magnitude of 3.4,[2] this is a third magnitude star that, according to theBortle Dark-Sky Scale, is readily visible to the naked eye.Parallax measurements put it at a distance of 14.37parsecs (46.9light-years) from Earth.[1]
Eta Cephei, along withα Cephei (Alderamin) andβ Cep (Alfirk), were identified asAl Kawākib al Firḳ (الكوكب الفرق), meaning "the Stars of The Flock" byUlug Beg. Together withθ Cephei, it formAl Kidr.[10][11] Among its other designations includes the nameKabalfird.[12]
InChinese,天鈎 (Tiān Gōu), meaningCelestial Hook, refers to an asterism consisting of η Cephei,4 Cephei,HD 194298,θ Cephei,α Cephei,ξ Cephei,26 Cephei,ι Cephei andο Cephei.[13] Consequently, theChinese name for η Cephei itself is天鈎四 (Tiān Gōu sì, English:the Fourth Star of Celestial Hook.).[14]
Eta Cephei is asubgiant star with astellar classification of K0 IV,[3] which indicates it is exhausting the supply of hydrogen at its core and is in the process ofevolving into agiant star. With 1.4 times theSun's mass,[5] at an age of 2.5 billion years[8] it has reached a radius 3.86 times larger than the Sun and a luminosity eight times greater.[6] It is radiating this energy from itsouter atmosphere at aneffective temperature of 5,000 K,[6] giving it the orange-hued glow of aK-type star. Eta Cephei has a highproper motion across thecelestial sphere[9] and a largepeculiar velocity of112 km s−1.[8]
According to Nelson & Angel (1998),[15] Eta Cephei would show two significant periodicities of 164 days and 10 years respectively, hinting at the possible presence of one or more jovian planets in orbit around the subgiant. The authors have set an upper limit of 0.64Jupiter masses for the putative inner planet and 1.2Jupiter masses for the putative outer one. Also Campbell et al. (1988)[16] inferred the existence of planetary objects or evenbrown dwarfs less massive than 16.3Jupiter masses.
However, more recent studies have not yet confirmed the existence of any substellar companion around Eta Cephei.McDonald Observatory team has set limits to the presence of one or more planets[17] with masses between 0.13 and 2.4Jupiter masses and average separations spanning between 0.05 and 5.2AU.
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b(unconfirmed) | ≥0.64MJ | 0.638 | 163.57 | ?0.17 | — | — |
