Alpha Librae
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Libra |
| α1 Lib (α Lib B) | |
| Right ascension | 14h 50m 41.18097s |
| Declination | −15° 59′ 50.0482″ |
| Apparent magnitude (V) | +5.153 |
| α2 Lib (α Lib A) | |
| Right ascension | 14h 50m 52.71309s |
| Declination | −16° 02′ 30.3955″ |
| Apparent magnitude (V) | +2.741 |
| Characteristics | |
| α1 Lib | |
| Spectral type | F3 V + M0–1 V |
| U−B color index | −0.02 |
| B−V color index | +0.39 |
| α2 Lib | |
| Evolutionary stage | Main sequence or blue straggler (α2 A) |
| Spectral type | kA2hA5mA4 IV-V |
| U−B color index | +0.10 |
| B−V color index | +0.15 |
| Astrometry | |
| α1 Lib | |
| Radial velocity (Rv) | −23.8±0.3 km/s |
| Proper motion (μ) | RA: −136.27 mas/yr Dec.: −59.04 mas/yr |
| Parallax (π) | 43.52±0.43 mas |
| Distance | 74.9 ± 0.7 ly (23.0 ± 0.2 pc) |
| Absolute magnitude (MV) | +3.35 |
| α2 Lib | |
| Proper motion (μ) | RA: −105.68 mas/yr Dec.: −68.40 mas/yr |
| Parallax (π) | 43.03±0.19 mas |
| Distance | 75.8 ± 0.3 ly (23.2 ± 0.1 pc) |
| Absolute magnitude (MV) | +0.88 |
| Orbit | |
| Primary | α1 Lib A |
| Name | α1 Lib B |
| Period (P) | 16.229±0.043 yr |
| Semi-major axis (a) | 0.346±0.006″ |
| Eccentricity (e) | 0.244±0.008 |
| Inclination (i) | 54.4±1.5° |
| Longitude of the node (Ω) | 122.7±1.4° |
| Periastron epoch (T) | 2021.95±0.12 |
| Argument of periastron (ω) (secondary) | 184.1±3.3° |
| Semi-amplitude (K1) (primary) | 3.83±0.12 km/s |
| Orbit | |
| Primary | α2 Lib A |
| Name | α2 Lib B |
| Period (P) | 70.34±0.07 days |
| Semi-major axis (a) | 0.52 au |
| Eccentricity (e) | 0.41±0.02 |
| Inclination (i) | 71±3° |
| Longitude of the node (Ω) | 333±17° |
| Periastron epoch (T) | JD 2455294.3±0.5 |
| Argument of periastron (ω) (secondary) | 48±7° |
| Semi-amplitude (K2) (secondary) | 43.1±0.5 km/s |
| Details | |
| α1 Lib A | |
| Mass | 1.95 M☉ |
| Radius | 2.21 R☉ |
| Surface gravity (log g) | 3.91 cgs |
| Temperature | 8,200 K |
| Age | 700 Myr |
| α1 Lib B | |
| Mass | 1.79 M☉ |
| Radius | 1.92 R☉ |
| Temperature | 7,930 K |
| Age | 700 Myr |
| α2 Lib A | |
| Mass | 1.36±0.02 M☉ |
| Radius | 1.50 R☉ |
| Luminosity | 3.83 L☉ |
| Surface gravity (log g) | 4.23 cgs |
| Temperature | 6,707±56 K |
| Metallicity [Fe/H] | −0.03 dex |
| Rotational velocity (v sin i) | 5.95 km/s |
| Age | 1.01±0.24 Gyr |
| α2 Lib B | |
| Mass | 0.5–0.6 M☉ |
| Other designations | |
| Zubenelgenubi, Kiffa Australis, Lanx Australis, α Lib, WDS J14509-1603 | |
| α Lib B: α1 Lib, 8 Librae, BD−15°3965, FK5 1387, HD 130819, HIP 72603, HR 5530, SAO 158836 | |
| α Lib A: α2 Lib, 9 Librae, BD−15°3966, FK5 548, HD 130841, HIP 72622, HR 5531, SAO 158840 | |
| Database references | |
| SIMBAD | α1 Lib |
| α2 Lib | |
Alpha Librae (α Librae, abbreviated Alpha Lib, α Lib) is a quadruple star system and, despite its 'alpha' designation, it is the second-brightest star in the constellation of Libra. The two components are designated α1 Librae and α2 Librae. The system bore the traditional name of Zubenelgenubi /zuːˌbɛnɛldʒɪˈnuːbi/, though the International Astronomical Union now regards that name as only applying to α2 Librae.
Alpha2 Librae is 0.33 degrees north of the ecliptic so it can be occulted by the Moon and (very rarely) by planets. It was occulted by Venus on October 25, 1947; the next occultation by a planet will be by Mercury on 10 November 2052. Both components are eclipsed (occulted) by the sun from about 7–9 November. Thus the star can be viewed the whole night, crossing the sky, in early May.