Phecda
Phecda /ˈfɛkdə/, also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. Based upon parallax measurements with the Hipparcos astrometry satellite, it is located at distance of around 83.2 light-years (25.5 parsecs) from the Sun.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 11h 53m 49.84732s |
| Declination | +53° 41′ 41.1350″ |
| Apparent magnitude (V) | 2.438 |
| Characteristics | |
| Spectral type | A0 Ve + K2 V |
| U−B color index | +0.008 |
| B−V color index | –0.013 |
| Astrometry | |
| Radial velocity (Rv) | −12.6 km/s |
| Proper motion (μ) | RA: +107.68 mas/yr Dec.: +11.01 mas/yr |
| Parallax (π) | 39.21 ± 0.40 mas |
| Distance | 83.2 ± 0.8 ly (25.5 ± 0.3 pc) |
| Absolute magnitude (MV) | 0.4 |
| Orbit | |
| Period (P) | 20.5 ± 1 yr |
| Semi-major axis (a) | 0.460″ |
| Eccentricity (e) | 0.3 ± 0.3 |
| Inclination (i) | 51 ± 15° |
| Longitude of the node (Ω) | 6 ± 61° |
| Periastron epoch (T) | B 1984.0 ± 2.0 |
| Argument of periastron (ω) (secondary) | 185 ± 37° |
| Details | |
| γ UMa A | |
| Mass | 2.94 M☉ |
| Radius | 3.04 ± 0.08 R☉ |
| Luminosity | 65.255 L☉ |
| Surface gravity (log g) | 3.79 cgs |
| Temperature | 9,355 K |
| Rotational velocity (v sin i) | 178 km/s |
| Age | 0.3 Gyr |
| γ UMa B | |
| Mass | 0.79 M☉ |
| Luminosity | 0.397 L☉ |
| Temperature | 4,780 K |
| Other designations | |
| Database references | |
| SIMBAD | data |
It is more familiar to most observers in the northern hemisphere as the lower-left star forming the bowl of the Big Dipper, together with Alpha Ursae Majoris (Dubhe, upper-right), Beta Ursae Majoris (Merak, lower-right) and Delta Ursae Majoris (Megrez, upper-left). Along with four other stars in this well-known asterism, Phecda forms a loose association of stars known as the Ursa Major moving group. Like the other stars in the group, it is a main sequence star, as the Sun is, although somewhat hotter, brighter and larger.
Phecda is located in relatively close physical proximity to the prominent Mizar–Alcor star system. The two are separated by an estimated distance of 8.55 ly (2.62 pc); much closer than the two are from the Sun. The star Beta Ursae Majoris is separated from Gamma Ursae Majoris by 11.0 ly (3.4 pc).