9 Sagittarii
From Wikipedia, the free encyclopedia
  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 03m 52.44501s[1] |
| Declination | −24° 21′ 38.6323″[1] |
| Apparent magnitude (V) | 5.97[2] |
| Characteristics | |
| Spectral type | O4V((f))z[3] (O3.5V((f+)) + O5–5.5V((f))[4]) |
| U−B color index | −0.89[2] |
| B−V color index | 0.00[2] |
| Astrometry | |
| Radial velocity (Rv) | 10.70[5] km/s |
| Proper motion (μ) | RA: +0.539[1] mas/yr Dec.: −2.019[1] mas/yr |
| Parallax (π) | 0.8001±0.0728 mas[1] |
| Distance | 4,100 ± 400 ly (1,200 ± 100 pc) |
| Absolute magnitude (MV) | −6.20[4] |
| Orbit[6] | |
| Period (P) | 3,261±69 d |
| Semi-major axis (a) | 4,100 R☉[7] |
| Eccentricity (e) | 0.648±0.009 |
| Inclination (i) | 86.5±0.5° |
| Semi-amplitude (K1) (primary) | 36+4 −1 km/s |
| Semi-amplitude (K2) (secondary) | 49±3 km/s |
| Details[6] | |
| 9 Sgr A | |
| Mass | 32.1±16.0 M☉ |
| Radius | 10.8±1.0 R☉ |
| Luminosity | 479,000 L☉ |
| Surface gravity (log g) | 3.87±0.20 cgs |
| Temperature | 46,000±1,000 K |
| Rotational velocity (v sin i) | 102+8 −12 km/s |
| 9 Sgr B | |
| Mass | 18.9±10.1 M☉ |
| Radius | 8.9±1.2 R☉ |
| Luminosity | 224,000 L☉ |
| Surface gravity (log g) | 3.87±0.20 cgs |
| Temperature | 42,000±1,000 K |
| Rotational velocity (v sin i) | 67+6 −13 km/s |
| Age | ~1 Myr |
| Other designations | |
| 9 Sgr, HR 6736, HIP 88469, HD 164794, CD−24°13814, 2MASS J18035245-2421386, WDS J18039-2422 | |
| Database references | |
| SIMBAD | data |
9 Sagittarii (9 Sgr) is a massive binary star in the constellation Sagittarius. It has an apparent magnitude of 5.97. Both components are highly luminous O-type main-sequence stars.
_in_the_32_inch_Schulman_telescope_on_Mt._Lemmon,_AZ.jpg)
9 Sgr is a naked eye star lying in the direction of the Lagoon Nebula (Messier 8), very close to the central condensation of the Hourglass Nebula around Herschel 36. It was not catalogued by Bayer with a Greek letter, but was listed by Flamsteed as number 9 in the constellation Sagittarius. It lies close to the open cluster NGC 6530. 9 Sgr is the main source of ionisation for much of the visible nebulosity in the region, although the young O star Herschel 36 ionises the dense Hourglass Nebula region.[8]
9 Sgr itself is surrounded by an ionised HII region about 30 light years across including the reflection nebulae NGC 6523 and NGC 6533. This ionised region lies in front of a denser molecular cloud.[8]
The distances to 9 Sgr, M8, and NGC 6530 are uncertain, but generally estimated to be between 1,200 and 1,800 parsecs. Recent studies derive a distance around 1,250 pc for the M8 region.[8] Erosion of the front of the molecular cloud apparently caused by 9 Sgr suggests that it lies in front of the cloud, but studies of 9 Sgr as a binary star give a distance of 1,790 pc.[4][9] A 2021 study of the binary system derives a distance of 1,310±60 pc. On this basis, it is thought to be a member of the NGC 6530 open cluster.[6]
