EPSILON SCO (Epsilon Scorpii). Though bright, second magnitude
(2.29), in most other constellations Epsilon Sco would have a
prominent place and probably a proper name. Here, in one of the
grandest patterns of the sky, Scorpius, it
has neither. It's just one of many of the Scorpion's bright stars
and currently ranks just fifth after Antares (Alpha), Dschubba (Delta), Shaula (Lambda), and Girtab (Theta), consistent with its Greek letter designation. (Dschubba,
historically fainter than Epsilon, is in a brightened state.)
Scorpius is loaded with relatively distant, luminous, hot class B
stars, many of which are associated with one another. At a
distance of only 65 light years, Epsilon, a class K (K2.5) giant,
departs from the local norm and just happens to be in the general
line of sight to the rest of the figure. Epsilon's nature rather
eludes analysis. Even the temperature is not all that well
defined. Averaging all values gives 4400 Kelvin, a luminosity
(accounting for infrared radiation) of 72 times that of the Sun, a radius of 15 solar (not all that large),
an uncertain mass of 1.25 times solar, and an age rather similar to
that of our Sun. Direct measure of angular radius gives a
reasonably consistent value of 13 solar. The evolutionary status of
the star is also uncertain. Not a stable helium-fusing "clump"
giant (so called because on a graph of luminosity vs. temperature
there are so many of them), it could be brightening as red giant with a dead helium core,
dimming after just firing up its internal helium to fuse to carbon
and oxygen, or brightening for the second time with a dead C-O
core. Though the first option is the most likely, an apparent
slight variability of about 10 percent (with no defined period)
suggests the last option. We really do not know. A lower, and
perhaps better, temperature of 4250 Kelvin gives just one solar
mass. More detailed analysis with other evolutionary calculations
yields 1.5 solar masses. Whatever the case, the star gives at
least some idea of what will someday happen to the Sun. The metal
abundance is around 70 percent solar. A low projected rotation
speed of 1.7 kilometers per second leads to a rotation period that
could be as long as 1.3 years. There is also some evidence for
mass loss through an accelerated wind. Epsilon's most outstanding
characteristic is a high speed of 63 kilometers per second relative
to the Sun, three to four times greater than that of the local
stars of the Galaxy's disk, showing that not only is Epsilon not a
part of the general Scorpius gang, but that it is also something of
a visitor from an older thickened Galactic disk that provides a
transition to the outer, ancient halo.