Peculiarities
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 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.
With Alioth, the rotational and magnetic axes are at almost 90 degrees to one another. Darker (denser) regions of chromium form a band at right angles to the equator.
A recent study suggests Alioth's 5.1-day variation may be due to a substellar object of about 14.7 Jupiter masses in an eccentric orbit (e=0.5) with an average separation of 0.055 astronomical units.
Alioth has a relatively weak magnetic field, 15 times weaker than α CVn, but it is still 100 times stronger than that of the Earth.
Read more about this topic: Epsilon Ursae Majoris