ETA SER (Eta Serpentis). All constellations are unique, but Serpens (the Serpent), in violation of the
rules of English, is even more so. It is the only one that comes
in two separate parts. To the west are the snake's foreparts and
head (Serpens Caput), to the east the
tail (Serpens Cauda). The two are widely separated by Ophiuchus,
the (no surprise) Serpent Bearer, who holds the coiled beast.
Bayer on the other hand, took them as one figure, Serpens Caput
holding Alpha (Unukalhai, the
luminary) through Epsilon, Serpens Caput
beginning its lettering with fifth magnitude Zeta. Given that
third magnitude (3.26) Eta Serpentis is the brightest over on this
eastern side and that Theta Ser (Alya) is
on top, Bayer rather clearly ordered at least some of his stars by
position rather than brightness (seen in a lot of other constellations as well). Eta, one of the
many class K (K2) orange giants that flock the sky, lies
at a precisely-known distance of 60.5 (give or take just 0.2) light
years. With a well-determined temperature of 4890 Kelvin, Eta Ser
radiates at a rather paltry rate of just 19 times that of the Sun (including its infrared radiation), from which
we derive a radius of 6.1 times solar. Interferometric measures of
angular diameter give a satisfyingly close value of 5.9. The
rotation, at least the projected spin velocity, is slow, just 0.44
kilometers per second, yielding a rotation period of as much as 1.9
years. Of fairly low mass, about 1.5 Suns, Eta seems most likely
to be brightening as a red giant with a dead helium core. By the
time it fires its helium to fuse to carbon and oxygen, it will have
become some 25 times brighter. With an age of about 2.8 billion
years, it gave up hydrogen fusion 150 million years ago. With
roughly half the metals of the Sun (compared with hydrogen), and a
bit rich in the carbon-nitrogen cyanogen molecule, the star is
speeding across the line of sight at nearly a second of arc per
year, its velocity relative to the Sun a rather high 79 kilometers
per second. Eta Ser thus appears to be visiting from another part
of the Galaxy. A seeming double with a dim 11th magnitude
companion, it isn't. Eta Serpentis B has moved a whopping 140
seconds of arc over a period of 175 years, so it is just a distant
star that lies in the line of sight. As such, it makes a fine
marker with which to view the fast motion of nearby Eta Ser A.
Written by Jim Kaler 7/06/12. Return to STARS.