Epsilon Ursae Majoris
 Alioth in Ursa Major (circled). |
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| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Ursa Major |
| Right ascension | 12h 54m 01.74959s |
| Declination | +55° 57′ 35.3627″ |
| Apparent magnitude (V) | 1.77 |
| Characteristics | |
| Spectral type | A1III-IVp kB9 |
| U−B color index | +0.02 |
| B−V color index | -0.02 |
| Variable type | α2-CVn |
| Astrometry | |
| Radial velocity (Rv) | -9.3 km/s |
| Proper motion (μ) |
RA: +111.91 mas/yr Dec.: -8.24 mas/yr |
| Parallax (π) | 39.51 ± 0.20mas |
| Distance | 82.6 ± 0.4 ly (25.3 ± 0.1 pc) |
| Absolute magnitude (MV) | –0.2 |
| Details | |
| Mass | 2.91 M☉ |
| Radius | 4.2 ± 0.2 R☉ |
| Luminosity | 108 L☉ |
| Surface gravity (log g) | 3.5 cgs |
| Temperature | 10,800 K |
| Metallicity [Fe/H] | +0.00 dex |
| Rotational velocity (v sin i) | 33 km/s |
| Other designations | |
| Database references | |
| SIMBAD | data |
Epsilon Ursae Majoris (ε Ursae Majoris, abbreviated Epsilon UMa, ε UMa), also named Alioth, is (despite being designated 'epsilon') the brightest star in the constellation of Ursa Major, and at magnitude 1.76 is the thirty-first-brightest star in the sky.
It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper (Plough) closest to the bowl. It is also a member of the large and diffuse Ursa Major moving group. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.
According to Hipparcos, Alioth is 81 light-years (25 parsecs) from the Sun. Its spectral type is A1p; the "p" stands for peculiar, as the spectrum of its light is characteristic of an Alpha2 Canum Venaticorum variable. Alioth, as a representative of this type, may harbor two interacting processes. First, the star's strong magnetic field separating different elements in the star's hydrogen 'fuel'. In addition, a rotation axis at an angle to the magnetic axis may be spinning different bands of magnetically sorted elements into the line of sight between Alioth and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Alioth to have very strange spectral lines that fluctuate over a period of 5.1 days. The kB9 suffix to the spectral type indicates that the calcium K line is present and representative of a B9 spectral type even though the rest of the spectrum indicates A1.
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