Beta Eridani - Cursa, Dhalim

Cursa, or Beta Eridani, is the second brightest star in Eridanus, at magnitude 2.79. Cursa begins the River Eridanus, the ancient Greek celestial depiction of the "Ocean Stream," which ends at the great southern star Achernar. Cursa lies just to the northwest of Rigel, which marks Orion's left foot. It is the brightest of a four-star box that makes up Orion's western "footstool". The name Cursa comes from an Arabic phrase meaning "the foremost footstool" of Orion. This same group of four stars was also once called "the ostrich's nest." Beta Eridani also has the traditional name Dhalim.

Physical Properties

Only 89 light years away, Cursa shines with a soft white light, from a surface with a temperature of 8360 K. A giant star of class A3 about three times the Sun's diameter, it radiates 45 solar luminosities into space. Containing 2 to 2.5 times the Sun's mass, Cursa is near the end of its main sequence hydrogen-fusing stage. Having just reached giant status, the star's surface will rapidly expand and cool to become a much larger orange giant, then brighten and begin to fuse helium in its core.

Cursa is thought to be a part of the Ursa Major Moving Group, a set of stars spattered all across the sky that includes Sirius, and whose core contains the five middle stars of the Big Dipper. The Group is thought to be about 300 million years old, actually too young for Cursa's apparent age, which suggests that Cursa may not actually belong. Beta Eridani may also be a member of the proposed Sirius Supercluster, along with other scattered stars such as Sirius itself, β Aur, α CrB, β Crt, and β Ser.

Variability

The star's most notable claim is its inclusion in a set of very rare stars that seem to exhibit huge flashes. In 1985, β Eri was observed to brighten by a phenomenal three magnitudes - a factor of 15 - for a period of over two hours. About two dozen stars, including ε Peg (Enif) and μ Cep, are also suspected of producing such flashes. The flashes may be produced by magnetic activity, as are the Sun's solar flares. But no one knows, as they are very difficult to study, and remain among the great mysteries of stellar astronomy.

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