CURSA (Beta Eridani). Orion is so important, he is not only accompanied by two hunting dogs, but his rest is assured by two footstools, one forward, the other to the rear. The latter is made from a quartet of stars in his prey, Lepus the Hare, of which Arneb (Alpha Leporis) is at the upper right hand corner. The "foremost footstool" consists of a four-star box just up and to the right of Rigel, which marks Orion's left foot. Of these, Cursa, just to the northwest of Rigel, is notably the brightest, and as a result took on the name of the whole "footstool," "Cursa" from an Arabic phrase meaning "the foremost footstool of the Central One," the ancient Arabic name for our Orion. This same group of four stars was once also called "the ostrich's nest." Cursa begins the River Eridanus, the celestial depiction of the Greek's "Ocean Stream," which ends in the great southern star Achernar. Achernar easily won Bayer's Alpha designation, and from its position as head of the River and its rank as second brightest star (bright third, 2.79), Cursa received Beta. Only 89 light years away, Cursa shines with a soft white light from a surface with a temperature of 8360 Kelvin. A giant star of class A (A3) about three times the solar diameter, it radiates 45 solar luminosities into space. Containing two to 2.5 times the mass of the Sun, Cursa is near or even at its termination as a "main sequence" hydrogen-fusing star. Having just reached its giant status, the star will next rapidly expand and cool at its surface to become a much larger orange giant before it brightens and begins the fusion of its core helium. Cursa is commonly considered to be a part of the "Ursa Major Moving Group," a set of stars spattered all over the sky (that includes Sirius) whose core is the " Ursa Major Cluster," which contains of 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 status, suggesting that Cursa really does not belong. The star's most notable claim is its inclusion in a set of very rare stars that seem to exhibit huge flashes. In 1985 it 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 Enif and Mu Cephei, are suspected of producing such flashes. Reasoning from the Sun and its flares, the flashes may be produced by magnetic activity, but no one knows, as they are so very difficult to study. As a result they are among the great mysteries of stellar astronomy.
Written by Jim Kaler. Return to STARS.