XI SER (Xi Serpentis). In spite of its lowly standing among the
letters of the Greek alphabet, Xi
Serpentis (no proper name) is among the brighter stars of the
constellation, and lies just to the east of where the tail of the
Serpent (Serpens Cauda, the
constellation in two parts) joins the classic figure of Ophiuchus at Sabik (Eta Ophiuchi). Fourth magnitude (at
3.54, just short of third magnitude), Xi Ser is a class F (F0)
either subgiant or giant (depending on which classifier you wish to
believe) that lies 105 light years away, and shines with the light
of 31 Suns from its 7100 Kelvin surface.
Application of evolutionary theory gives a mass of twice solar, an
age near 1.1 billion years, and a confirmation that the star is
shutting down its core hydrogen fusion if it has not done so
already. Slowly rotating with an equatorial velocity of at least
32 km/sec, the star, 3.7 times the radius of the Sun, rotates in
less than 5.8 days. The details of the classification are not
certain either, one source calling it a "
Delta Scuti" type variable
(rather like Caph, Beta Cassiopeiae),
another calling it "p" for "peculiar," which commonly implies a
magnetic field and odd chemical abundances. However, there are no
clear measures of variation, nor of a magnetic field. Moreover,
except for a low carbon abundance, the chemical composition is more
or less solar. The star thus remains cryptic. From the spectrum,
however, we do know that it has a close companion with an orbital
period of just 2.2923 days that must orbit at a mere 0.05
Astronomical Units, or just 4 million miles from the bright star's
surface. Nothing else about it is known, the star thus keeping its
secrets to itself.