Theta Cephei
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Cepheus |
| Right ascension | 20h 29m 34.86518s |
| Declination | +62° 59′ 38.6216″ |
| Apparent magnitude (V) | 4.22 |
| Characteristics | |
| Spectral type | A7 III or kA7hF1mF2 |
| U−B color index | +0.16 |
| B−V color index | +0.20 |
| Astrometry | |
| Radial velocity (Rv) | −6.8 km/s |
| Proper motion (μ) | RA: 52.416 mas/yr Dec.: −2.544 mas/yr |
| Parallax (π) | 25.7611±0.5808 mas |
| Distance | 127 ± 3 ly (38.8 ± 0.9 pc) |
| Absolute magnitude (MV) | +1.12 |
| Orbit | |
| 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 M☉ |
| Radius | 3.09+0.36 −0.25 R☉ |
| Luminosity | 46.5 L☉ |
| Surface gravity (log g) | 3.74 cgs |
| Temperature | 7,375+307 −404 K |
| Metallicity [Fe/H] | +0.27±0.04 dex |
| Rotational velocity (v sin i) | 52.0 km/s |
| Age | 55 Myr |
| companion | |
| Mass | >0.62 M☉ |
| Radius | 0.654+0.044 −0.102 R☉ |
| Temperature | 4,070+340 −130 K |
| Other designations | |
| θ Cep, 2 Cep, BD+62°1821, FK5 767, HD 195725, HIP 101093, HR 7850, SAO 18897 | |
| Database references | |
| SIMBAD | data |
Theta Cephei, Latinized from θ Cephei, is a stellar class A7, fourth-magnitude star in the constellation Cepheus. This displays at a metallic-line Am star. It is a white-hued, single-lined spectroscopic binary system, located about 127 light-years from Earth. The pair have an orbital period of 2.5 years; the eccentricity was long thought to be low, at 0.03, but recent calculations have put it at a much higher 0.377. They are separated by 4.16 AU. 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.
Shared with η Cep, this star system has the title Al Kidr. 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. Consequently, the Chinese name for θ Cephei itself is 天鈎三 (Tiān Gōu sān, English: the Third Star of Celestial Hook.).