Sigma Ophiuchi
Star in the constellation Ophiuchus
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Sigma Ophiuchi, Latinized from σ Ophiuchi, is a single,[11] orange-hued star in the equatorial constellation Ophiuchus. Its apparent visual magnitude is 4.31,[2] which is bright enough to be faintly visible to the naked eye. The annual parallax shift of 3.62 mas as seen from Earth provides a distance estimate of roughly 900 light years. It is moving closer to the Sun with a radial velocity of −28 km/s.[5]
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ophiuchus |
| Right ascension | 17h 26m 30.8806s[1] |
| Declination | +04° 08′ 25.2876″[1] |
| Apparent magnitude (V) | 4.31[2] (4.25 - 4.51[3]) |
| Characteristics | |
| Spectral type | K2 III[4] |
| U−B color index | +1.57[2] |
| B−V color index | +1.51[2] |
| Variable type | suspected[3] |
| Astrometry | |
| Radial velocity (Rv) | −27.81[5] km/s |
| Proper motion (μ) | RA: +1.459[6] mas/yr Dec.: +6.760[6] mas/yr |
| Parallax (π) | 3.7578±0.1559 mas[6] |
| Distance | 870 ± 40 ly (270 ± 10 pc) |
| Absolute magnitude (MV) | −2.85[7] |
| Details | |
| Mass | 5–9[8] M☉ |
| Radius | 100±30[8] R☉ |
| Luminosity | 2630±894[8] L☉ |
| Surface gravity (log g) | 1.3±0.5[8] cgs |
| Temperature | 4,129±566[8] K |
| Metallicity [Fe/H] | −0.07[9] dex |
| Rotational velocity (v sin i) | 4.2[10] km/s |
| Other designations | |
| σ Oph, 49 Ophiuchi, NSV 8664, BD+04°3422, FK5 1459, GC 23621, HD 157999, HIP 85355, HR 6498, SAO 122387, 2MASS J17263087+0408252 | |
| Database references | |
| SIMBAD | data |
This is an evolved giant star of type K with a stellar classification of K2 III.[4] Wittkowski et al. (2017) consider it to have a luminosity class of II-III, suggesting it is in a transitional zone between giants and supergiant stars.[8] It has around 5 to 9 times the mass of the Sun and one hundred times the Sun's radius. The star is radiating 2,600 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,130 K.[8]