GAMMA SER (Gamma Serpentis). Part of the "X" structure that makes
the head of Serpens (the Serpent),
at the top of the western portion of constellation (Serpens Caput),
lies otherwise unnamed Gamma Serpentis. At mid-fourth magnitude
(3.85), it hardly qualifies as the third brightest star in the
constellation, exceeded in brightness not just by Alpha (Unukalhai) and Beta (as expected) but
also by those with Greek letters Delta, Epsilon, and even Mu (though
not by much). What makes the star special is its proximity and its
sunlike properties. Only 36 light years away, this class F (F6)
dwarf sports a temperature of 6280 Kelvin, just 500 Kelvin more
than that of the Sun, and shines with a
luminosity of just 2.9 times solar. From these parameters and the
theory of stellar structure, we obtain a mass only 25 percent
greater than our home star. Even that little bit, however, cuts
Gamma Ser's potential lifetime in half. At an age of about 3
billion years (as compared with the Sun's 4.5 billion year age), it
has another 2 billion years left to it before its core hydrogen
fuel runs out and it begins to die. (The star has been called a
subgiant, as if it were near the end of its days, but its
temperature and luminosity clearly define it as a dwarf.) Like
many stars hotter than solar, Gamma Ser rotates more quickly than
the Sun, its rotation speed of at least 8 kilometers per second,
coupled with a radius 1.4 times solar, giving it a rotation period
of less than 9 days. (Only limits are known, since the tilt of the
rotation axis cannot be determined). The star ought to display
some magnetic activity, but none has been found; nor has a dusty
disk that might indicate a planetary system. Hotter, slowly
rotating stars (such as Alioth and Acubens) present abundance anomalies
caused by the separation of chemical elements. Gamma Ser, with
otherwise rather normal abundances (a metal content about 2/3
solar), seems on the fringe of such peculiar stars with a weirdly
low aluminum abundance and some heavy element anomalies as well.
The star has a reputation of being somewhat variable. If so, from
its temperature and luminosity it may be an example of a "Gamma Doradus" star. Gamma Ser is
often listed with two 11th magnitude companions ("B" and "C,"
located about 3 minutes of arc away. Alas, they are merely line of
sight coincidences, which leaves Gamma Ser all alone, rather like
the Sun itself. Thanks to Jose Rodriguez, who suggested this
star.