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.