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Alioth, Epsilon Ursae Majoris, is the brightest star in the constellation 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, as such, a member of the Ursa Major moving cluster.

According to Hipparcos, Alioth is 81 light years (25 parsecs) from Earth. Its spectral type is A0p; the "p" stands for peculiar, as the spectrum of its light is quite odd, of a kind characteristic of an Alpha2 Canum Venaticorum-class variable. Alioth, as a representative of this type, is believed to look the way it does because of two interacting processes: first, the star's strong magnetic field separating different elements salting the star's hydrogen fuel, then a rotation axis at an angle to the magnetic axis 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. In the case of Alioth, the rotational and magnetic axes are at almost 90 degrees to one another; in the map of Alioth linked below, note how the darker (more dense) regions of chromium form a band at right angles to the equator.

Chromium distribution on Alioth

Of its type, Alioth has a relatively weak magnetic field, but it is still fifteen times stronger than that of the Earth.

The origin of its name is unclear, but one theory derives it from the Arabic al-Jawn -- "black horse". This is believed to be a result of its misidentification with the nearby Mizar's dim companion Alcor, the pair of which were a horse and rider in Arab mythology; Alcor's name has a similar etymology.