Theta Cephei
Star in the constellation Cepheus
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Theta Cephei is a white-hued binary star system in the northern constellation of Cepheus. Its name is a Bayer designation that is Latinized from θ Cephei, and abbreviated Theta Cep or θ Cep. The system is visible to the naked eye as a point of light with an apparent visual magnitude of 4.22.[3] Based on parallax measurements, it is located at a distance of approximately 139 light-years (43 pc) from the Earth.
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cepheus[1] |
| Right ascension | 20h 29m 34.86518s[2] |
| Declination | +62° 59′ 38.6216″[2] |
| Apparent magnitude (V) | 4.22[3] |
| Characteristics | |
| Spectral type | A7 III[4] or kA7hF1mF2[5][6] |
| U−B color index | +0.16[3] |
| B−V color index | +0.20[3] |
| Astrometry | |
| Radial velocity (Rv) | −6.8[7] km/s |
| Proper motion (μ) | RA: 34.798 mas/yr[2] Dec.: −13.904 mas/yr[2] |
| Parallax (π) | 23.481±0.6894 mas[2] |
| Distance | 139 ± 4 ly (43 ± 1 pc) |
| Absolute magnitude (MV) | +1.12[1] |
| Orbit[8] | |
| Period (P) | 914.3±7.0 d |
| Eccentricity (e) | 0.377±0.006 |
| Periastron epoch (T) | 2456991.3±8.5 JD |
| Argument of periastron (ω) (primary) | 48.8±1.1° |
| Semi-amplitude (K1) (primary) | 7.72±0.06 km/s |
| Details | |
| primary | |
| Mass | 2.75[9] M☉ |
| Radius | 3.09+0.36 −0.25[10] R☉ |
| Luminosity | 46.5[9] L☉ |
| Surface gravity (log g) | 3.74[4] cgs |
| Temperature | 7,375+307 −404[10] K |
| Metallicity [Fe/H] | +0.27±0.04[1] dex |
| Rotational velocity (v sin i) | 52.0[4] km/s |
| Age | 55[9] Myr |
| companion | |
| Mass | >0.62[8] M☉ |
| Radius | 0.654+0.044 −0.102[8] R☉ |
| Temperature | 4,070+340 −130[8] K |
| Other designations | |
| θ Cep, 2 Cep, BD+62°1821, FK5 767, HD 195725, HIP 101093, HR 7850, SAO 18897[11] | |
| Database references | |
| SIMBAD | data |
This is a spectroscopic binary star system with an orbital period of 2.5 years;[8] the eccentricity was long thought to be low, at 0.03,[12] but calculations published in 2020 have put it at a much higher 0.377.[8] They are separated by 4.16 AU.[13]
The primary component has a Stellar classification of A7III,[4] displaying as a metallic-line Am star.[5] At an estimated age of 55 million years,[9] it is spinning with a projected rotational velocity of 52 km/s.[4] The star has 2.75[9] times the mass of the Sun and three times the Sun's radius.[10] It is radiating 46.5[9] times the luminosity of the Sun from its photosphere at an effective temperature of 7,375 K.[10] The companion star is calculated to be about 400 times fainter than the primary. It is completely invisible in the spectrum, but is estimated to be a K7 main sequence star.[9]
Shared with η Cep, this star system has the title Al Kidr.[14] In Chinese, 天鈎 (Tiān Gōu), meaning Celestial Hook, refers to an asterism consisting of 4 Cephei, HD 194298, η Cephei, α Cephei, ξ Cephei, 26 Cephei, ι Cephei and ο Cephei.[15] Consequently, the Chinese name for θ Cephei itself is 天鈎三 (Tiān Gōu sān, English: the Third Star of Celestial Hook.).[16]