Sigma Orionis
 Location of σ Ori (circled) |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Orion |
| Right ascension | 05h 38m 42.0s |
| Declination | −2° 36′ 00″ |
| Apparent magnitude (V) |
A: 4.07 B: 5.27 C: 8.79 D: 6.62 E: 6.66 (6.61 - 6.77) |
| Characteristics | |
| AB | |
| Spectral type | O9.5V + B0.5V |
| U−B color index | −1.02 |
| B−V color index | −0.31 |
| C | |
| Spectral type | A2 V |
| U−B color index | −0.25 |
| B−V color index | −0.02 |
| D | |
| Spectral type | B2 V |
| U−B color index | −0.87 |
| B−V color index | −0.17 |
| E | |
| Spectral type | B2 Vpe |
| U−B color index | −0.84 |
| B−V color index | −0.09 |
| Variable type | SX Ari |
| Astrometry | |
| Radial velocity (Rv) | −29.45 ± 0.45 km/s |
| Parallax (π) |
AB: 3.04 ± 8.92mas D: 6.38 ± 0.90mas |
| Distance | 387.51 ± 1.32pc |
| Absolute magnitude (MV) | −3.49 (Aa) −2.90 (Ab) −2.79 (B) |
| Orbit | |
| Primary | Aa |
| Companion | Ab |
| Period (P) | 143.2002 ± 0.0024 days |
| Semi-major axis (a) | 0.0042860" (~360 R☉) |
| Eccentricity (e) | 0.77896 ± 0.00043 |
| Inclination (i) | ~56.378 ± 0.085° |
|
Semi-amplitude (K1) (primary) |
72.03 ± 0.25 km/s |
|
Semi-amplitude (K2) (secondary) |
95.53 ± 0.22 km/s |
| Orbit | |
| Primary | A |
| Companion | B |
| Period (P) | 159.896 ± 0.005 yr |
| Semi-major axis (a) | 0.2629 ± 0.0022″ |
| Eccentricity (e) | 0.024 ± 0.005 |
| Inclination (i) | 172.1 ± 4.6° |
| Details | |
| σ Ori Aa | |
| Mass | 18 M☉ |
| Radius | 5.6 R☉ |
| Luminosity | 41,700 L☉ |
| Surface gravity (log g) | 4.20 cgs |
| Temperature | 35,000 K |
| Rotational velocity (v sin i) | 135 km/s |
| Age | 0.3 Myr |
| σ Ori Ab | |
| Mass | 13 M☉ |
| Radius | 4.8 R☉ |
| Luminosity | 18,600 L☉ |
| Surface gravity (log g) | 4.20 cgs |
| Temperature | 31,000 K |
| Rotational velocity (v sin i) | 35 km/s |
| Age | 0.9 Myr |
| σ Ori B | |
| Mass | 14 M☉ |
| Radius | 5.0 R☉ |
| Luminosity | 15,800 L☉ |
| Surface gravity (log g) | 4.15 cgs |
| Temperature | 29,000 K |
| Rotational velocity (v sin i) | 250 km/s |
| Age | 1.9 Myr |
| Details | |
| C | |
| Mass | 2.7 M☉ |
| Details | |
| D | |
| Mass | 6.8 M☉ |
| Surface gravity (log g) | 4.3 cgs |
| Temperature | 21,500 K |
| Rotational velocity (v sin i) | 180 km/s |
| Details | |
| E | |
| Mass | 8.30 M☉ |
| Radius | 3.77 R☉ |
| Surface gravity (log g) | 3.95 cgs |
| Temperature | 22,500 K |
| Rotation | 1.190847 days |
| Other designations | |
| AB: HD 37468, HR 1931, HIP 26549, SAO 132406, BD−02°1326, 2MASS J05384476-0236001, Mayrit AB | |
| C: 2MASS J05384411-0236062, Mayrit 11238 | |
| D: HIP 26551, 2MASS J05384561-0235588, Mayrit 13084 | |
| E: V1030 Orionis, HR 1932, HD 37479, BD−02°1327, 2MASS J05384719-0235405, Mayrit 41062 | |
| Database references | |
| SIMBAD | σ Ori |
| σ Ori C | |
| σ Ori D | |
| σ Ori E | |
| σ Ori Cluster | |
Sigma Orionis or Sigma Ori (σ Orionis, σ Ori) is a multiple star system in the constellation Orion, consisting of the brightest members of a young open cluster. It is found at the eastern end of the belt, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The total brightness of the component stars is magnitude 3.80.
σ Orionis is a naked eye star at the eastern end of Orion's Belt, and has been known since antiquity, but it was not included in Ptolemy's Almagest. It was referred to by Al Sufi, but not formally listed in his catalogue. In more modern times, it was measured by Tycho Brahe and included in his catalogue. In Kepler's extension it is described as "Quae ultimam baltei praecedit ad austr." (preceding the outermost of the belt, to the south). It was then recorded by Johann Bayer in his Uranometria as a single star with the Greek letter σ (sigma). He described it as "in enſe, prima" (in the sword, first). It was also given the Flamsteed designation 48.
In 1776, Christian Mayer described σ Ori as a triple star, having seen components AB and E, and suspected another between the two. Component D was confirmed by FGW Struve who also added a fourth (C), published in 1876. In 1892 Sherburne Wesley Burnham reported that σ Ori A was itself a very close double, although a number of later observers failed to confirm it. In the second half of the twentieth century, the orbit of σ Ori A/B was solved and at the time was one of the most massive binaries known.
σ Ori A was discovered to have a variable radial velocity in 1904, considered to indicate a single-lined spectroscopic binary. The spectral lines of the secondary were elusive and often not seen at all, possibly because they are broadened by rapid rotation. There was confusion over whether the reported spectroscopic binary status actually referred to the known visual companion B. Finally in 2011, it was confirmed that the system is triple, with an inner spectroscopic pair and a wider visual companion. The inner pair was resolved interferometrically in 2013.
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