RANA (Delta Eridani). In the middle of the western flow of Eridanus, the River, just to the east of Epsilon Eridani, lies the star Rana. Star names are sometimes a confusing mess.
"Rana" seems to have been erroneously transferred in the 20th
century from the Latinized version of an alternative name for Deneb Kaitos, "Rana Secunda," which
means (from old Arabic culture) the "second frog" (Fomalhaut being the "first frog").
Better to call the star by its Greek
letter name, Delta Eridani, though this barely fourth magnitude
(3.54) star is actually the fifth brightest in the celestial
stream. The star itself has a bit of confusion about it too, as it
is listed as an "RS Canum Venaticorum" variable like Lambda Andromedae (in which a pair of
stars orbit closely, making one of them more magnetically active
than normal), which it almost certainly is not. It is
active however. Rana is a class K (K0) subgiant, a star that
has recently given up its core hydrogen fusion. It is quite close
to us, only 30 light years away, and shines but with a luminosity
of 3.3 times that of the Sun. Factoring in its coolish temperature
of 5075 Kelvin, its radius is 2.3 times solar, and from the theory
or stellar evolution its mass is just 20 percent greater than that
of the Sun. Having started life as an F8
dwarf (just a bit hotter and brighter than the Sun, in accord with
its slightly greater mass) 7.5 or so billion years ago, the
cessation of core hydrogen fusion has caused it to expand and cool.
It is now just preparing to make its run to much higher luminosity
and radius as a true giant star. Magnetic activity, associated
with sunspots (rather, starspots), derives from a dynamo produced
by stellar rotation. In spite of the fact that Rana has slowed
with age to a projected rotation period of 116 days (though in fact
probably less), it does exhibit some sort of activity, from whence
cometh the "RS Can Ven" label. It does not, however, exhibit any
evidence for a companion. The advanced age of the star is
consistent with its lithium content. Lithium is a rare chemical
element that is easily destroyed by nuclear reactions in a high
temperature environment. As gases of the cooler stars circulate up
and down by convection, their lithium contents therefore steadily
decline. While Delta Eri has an overall metal abundance 50 percent
greater than that of the Sun, its lithium content is only half
solar.