Eta Ursae Majoris
 Alkaid in the Big Dipper of Ursa Major. |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Ursa Major |
| Right ascension | 13h 47m 32.43776s |
| Declination | +49° 18′ 47.7602″ |
| Apparent magnitude (V) | +1.86 |
| Characteristics | |
| Spectral type | B3 V |
| U−B color index | –0.68 |
| B−V color index | –0.19 |
| Astrometry | |
| Radial velocity (Rv) | –10.9 km/s |
| Proper motion (μ) |
RA: –121.17 mas/yr Dec.: –14.91 mas/yr |
| Parallax (π) | 31.38 ± 0.24mas |
| Distance | 103.9 ± 0.8 ly (31.9 ± 0.2 pc) |
| Details | |
| Mass | 6.1 ± 0.1 M☉ |
| Radius | 3.4 R☉ |
| Luminosity | 1,350 L☉ |
| Surface gravity (log g) | 4.27 cgs |
| Temperature | 16,823 ± 177 K |
| Rotational velocity (v sin i) | 150 km/s |
| Age | 10.0 ± 2.6 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Coordinates: 
13h 47m 32.4s, +49° 18′ 47.8″
Eta Ursae Majoris (η Ursae Majoris, abbreviated Eta UMa, η UMa), also named Alkaid, is a star in the constellation of Ursa Major. It is the most eastern (leftmost) star in the Big Dipper (Plough) asterism. However, unlike most stars of the Big Dipper, it is not a member of the Ursa Major moving group. With an apparent visual magnitude of +1.84, it is the third-brightest star in the constellation and one of the brightest stars in the night sky.
Eta Ursae Majoris is a 10-million-year-oldB-type main sequence star with a stellar classification of B3 V. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It has six times the mass; 3.4 times the radius, and is radiating around 1,350 times as much luminosity as the Sun, the latter from its outer atmosphere at an effective temperature of about 16,823 K, giving it the blue-white hue of a B-type star. This star is an X-ray emitter with a luminosity of 9.3 × 1028 erg s−1.
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