40 Eridani
40 Eridani is a triple star system in the constellation of Eridanus, abbreviated 40 Eri. It has the Bayer designation Omicron2 Eridani, which is Latinized from ο2 Eridani and abbreviated Omicron2 Eri or ο2 Eri. Based on parallax measurements taken by the Gaia mission, it is about 16.3 light-years from the Sun.
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Eridanus |
| 40 Eridani A | |
| Right ascension | 04h 15m 16.31962s |
| Declination | −07° 39′ 10.3308″ |
| Apparent magnitude (V) | 4.43 |
| 40 Eridani B | |
| Right ascension | 04h 15m 21.79572s |
| Declination | −07° 39′ 29.2040″ |
| Apparent magnitude (V) | 9.52 |
| 40 Eridani C | |
| Right ascension | 04h 15m 21.53600s |
| Declination | −07° 39′ 20.6946″ |
| Apparent magnitude (V) | 11.17 |
| Characteristics | |
| 40 Eridani A | |
| Spectral type | K0.5V |
| U−B color index | +0.45 |
| B−V color index | +0.82 |
| 40 Eridani B | |
| Spectral type | DA4 |
| U−B color index | +0.45 |
| B−V color index | +0.03 |
| 40 Eridani C | |
| Spectral type | M4.5eV |
| U−B color index | +0.83 |
| B−V color index | +1.67 |
| Variable type | Flare star |
| Astrometry | |
| 40 Eridani A | |
| Radial velocity (Rv) | −42.47±0.12 km/s |
| Proper motion (μ) | RA: −2,240.085 mas/yr Dec.: −3,421.809 mas/yr |
| Parallax (π) | 199.6080 ± 0.1208 mas |
| Distance | 16.340 ± 0.010 ly (5.010 ± 0.003 pc) |
| Absolute magnitude (MV) | 5.93 |
| 40 Eridani B | |
| Radial velocity (Rv) | −21 km/s |
| Proper motion (μ) | RA: −2,236.169 mas/yr Dec.: −3,338.955 mas/yr |
| Parallax (π) | 199.6911 ± 0.0512 mas |
| Distance | 16.333 ± 0.004 ly (5.008 ± 0.001 pc) |
| 40 Eridani C | |
| Radial velocity (Rv) | −44.06±0.20 km/s |
| Proper motion (μ) | RA: −2,247.183 mas/yr Dec.: −3,409.824 mas/yr |
| Parallax (π) | 199.4516 ± 0.0692 mas |
| Distance | 16.353 ± 0.006 ly (5.014 ± 0.002 pc) |
| Orbit | |
| Primary | 40 Eridani A |
| Companion | 40 Eridani BC |
| Period (P) | ~8,000 yr |
| Semi-major axis (a) | ~400 AU |
| Orbit | |
| Primary | 40 Eridani B |
| Companion | 40 Eridani C |
| Period (P) | 230.30±0.68 yr |
| Semi-major axis (a) | 6.930±0.050" (~35 AU) |
| Eccentricity (e) | 0.4294±0.0027 |
| Inclination (i) | 107.56±0.29° |
| Longitude of the node (Ω) | 151.44±0.12° |
| Periastron epoch (T) | 1847.7±1.1 |
| Argument of periastron (ω) (secondary) | 318.4±1.1° |
| Details | |
| 40 Eridani A | |
| Mass | 0.78±0.08 M☉ |
| Radius | 0.812±0.017 R☉ |
| Luminosity | 0.457±0.002 L☉ |
| Surface gravity (log g) | 4.45±0.19 cgs |
| Temperature | 5072±53 K |
| Metallicity [Fe/H] | −0.42±0.04 dex |
| Rotation | ~37–43 days |
| Rotational velocity (v sin i) | 1.23 ± 0.28 km/s |
| Age | 6.9±4.7 Gyr |
| 40 Eridani B | |
| Mass | 0.573±0.018 M☉ |
| Radius | 0.014 R☉ |
| Luminosity | 0.013 L☉ |
| Temperature | 16,500 K |
| 40 Eridani C | |
| Mass | 0.2036±0.0064 M☉ |
| Radius | 0.31 R☉ |
| Luminosity | 0.008 L☉ |
| Temperature | 3,100 K |
| Age | 5.6 Gyr 1.8 Gyr |
| Other designations | |
| A: Keid, BD−07° 780, HD 26965, HIP 19849, HR 1325, SAO 131063, LHS 23, LTT 1907 | |
| B: BD−07° 781, HD 26976, SAO 131065, G 160-060, LHS 24, LTT 1908 | |
| C: DY Eri, BD−07°781 C, LHS 25, LTT 1909 | |
| Database references | |
| SIMBAD | A |
| B | |
| C | |
The primary star of the system, designated 40 Eridani A and named Keid, is easily visible to the naked eye. It is orbited by a binary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by William Herschel.: p73 It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.
In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a white dwarf, the first discovered. Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than Van Maanen's Star, the nearest solitary white dwarf, and unlike the companions of Procyon and Sirius it is not swamped in the glare of a much brighter primary.