Epsilon Ursae Majoris - Alioth

Epsilon Ursae Majoris is the brightest star in Ursa Major, and at magnitude 1.76 is the 31st-brightest star in the night sky. It is the third star from the end of the Big Dipper's handle. Its proper name, Alioth, comes from a corruption of the Arabic "alyat-fat" for "black horse".

It is known as the Fifth Star of the Northern Dipper or the Star of Jade Sighting-tube in Chinese. Historically, the star was listed as one of the 57 navigational stars frequently used in maritime celestial navigation.

Properties

A white class A0 V star with a temperature of 9400 K, Alioth shines from a distance of 81 light years, with a luminosity 108 times the Sun's. From this, it has a diameter four times solar, and a mass close to triple the Sun's. Large and luminous for its class, Alioth is probably nearing the end of its main sequence, hydrogen-fusing lifetime.

Of greater significance, Alioth is the brightest of the "peculiar" A (Ap) stars, and is also classed as an α CVn star (after the prototype, Alpha Canum Venaticorum or Cor Caroli). In these stars, a variety of chemical elements are either depleted or enhanced, and appear to change regularly as the star rotates. Such chemically peculiar behavior in class A and B stars generally comes from diffusion in a thin stellar atmosphere: some elements fall downward within the star's gravitational field, while others are lofted upward by radiation.

The magnetic field then concentrates some elements into distinct regions that swing in and out of sight as the star spins. In the case of Alioth, the rotational and magnetic axes are at almost 90 degrees to one another; and the abundance of oxygen is 100,000 times greater near the magnetic equator than near the magnetic poles. Heavier elements, such as chromium and europium, also display strong variations. Though visually the brightest of the peculiar A stars, Alioth is also noted for having one of the weakest magnetic fields among its class: only about 100 times the strength of the Earth's, and 15 times weaker than Cor Caroli's.

A recent study suggests the 5.1-day variation may be due to a substellar object containing around 14.7 Jupiter masses in eccentric orbit (e=0.5) at an average separation of 0.055 AU from Alioth.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]