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| Observation data EpochJ2000 EquinoxJ2000 | |
|---|---|
| Constellation | Orion |
| Right ascension | 05h 24m 28.61672s[1] |
| Declination | −02° 23′ 49.7311″[1] |
| Apparent magnitude (V) | 3.42[2](4.50 + 5.90 + 5.65 + 4.95)[3] |
| Characteristics | |
| Spectral type | Aa: B1 V Ab: B3 V Ac: B3 V B: B2 V[3] |
| U−Bcolor index | −0.90[2] |
| B−Vcolor index | −0.17[2] |
| Variable type | Algol[4] |
| Astrometry | |
| Radial velocity (Rv) | +19.8[5] km/s |
| Proper motion (μ) | RA: −0.71[1]mas/yr Dec.: −3.46[1]mas/yr |
| Parallax (π) | 3.34±1.07 mas[1] |
| Distance | approx. 1,000 ly (approx. 300 pc) |
| Orbit | |
| Primary | Aa |
| Companion | Ab |
| Period (P) | 7.98763(22)[6] days |
| Eccentricity (e) | 0.0095±0.0010[6] |
| Inclination (i) | 87.62±0.42[6]° |
| Argument of periastron (ω) (primary) | 164±18[6]° |
| Semi-amplitude (K1) (primary) | 145.5±0.03[7] km/s |
| Semi-amplitude (K2) (secondary) | 150±3[7] km/s |
| Orbit[3] | |
| Companion | Ac |
| Period (P) | 9.442±0.012yr |
| Semi-major axis (a) | 0.0441±0.0015″ |
| Eccentricity (e) | 0.45±0.02 |
| Inclination (i) | 102.8±1.8° |
| Details | |
| η Ori Aa | |
| Mass | 10.87±0.44[6] M☉ |
| Radius | 6.477±0.073[6] R☉ |
| Surface gravity (log g) | 3.851±0.010[6] cgs |
| Temperature | 26,600[7] K |
| Rotational velocity (v sin i) | 20[7] km/s |
| η Ori Ab | |
| Mass | 10.54±0.22[6] M☉ |
| Radius | 4.79±0.10[6] R☉ |
| Surface gravity (log g) | 4.100±0.016[6] cgs |
| Temperature | 25,950[6] K |
| Rotational velocity (v sin i) | 130[7] km/s |
| η Ori Ac | |
| Mass | 6.78[8] M☉ |
| η Ori B | |
| Mass | 8.7[8] M☉ |
| Other designations | |
| η Ori,28 Orionis,BD−02°1235,HD 35411,HIP 25281,HR 1788,SAO 132071[9] | |
| Database references | |
| SIMBAD | data |
Eta Orionis orAlgjebbah,[citation needed]Latinized fromη Orionis, is amultiple star in theconstellationOrion. It lies a little to the west ofOrion's Belt betweenDelta Orionis andRigel, being closer to Delta Orionis than to Rigel. It lies at a distance of around 1,000light-years fromEarth and is part of theOrion OB1 association.
Eta Orionis is listed in multiple star catalogues as having two companions: a bright component B less than 2″ away; and a faint component C nearly 2′ away.[10] The two are estimated to orbit every 1,800 years.[8]
The primary star, Eta Orionis A, is itself aspectroscopic triple star, known from multiplespectral lines with varyingradial velocities.[7] The most distant component Ac, has been resolved usingspeckle interferometry, at a separation of about 0.04″. It orbits the other two in 9.4 years.[3] The two closest stars, Aa and Ab, are separated by only about a tenth of anastronomical unit and orbit in just under eight days.[7]
The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion.[11] It is assigned to the oldest and closest part of the association, known as OB1a.[12]
Eta Orionis drops in brightness every four days from a combinedapparent magnitude of 3.31 to about magnitude 3.6. This is due to eclipses between the two closest components, Aa and Ab.[4] The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep, respectively.[14]
It has also been suggested that component Ab is intrinsically variable with a period of 0.3 days and a very small amplitude. This star has unusual variable spectral lines and lies with theβ Cephei variable instability strip.[7] However, it is now thought that the variable component is either B and Ac, possibly due to an unseen companion or rotational modulation. The actual period is 0.432 days and the 0.3-day period was analias.[14]
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