BETA SER (Beta Serpentis). Unique among constellations, Serpens (the eponymous Serpent) is divided
in two parts, Serpens Caput (the Head) on the west side of snake-
enwrapped Ophiuchus, Serpens Cauda (the
Tail) on the east side. While Bayer gave the luminary, Unukalhai (the "serpent's neck") the
Alpha designation, he seems to have lettered most of the rest of
the plethora of rather similarly-dim stars by position. Which
leaves fourth magnitude (3.67) Beta Ser (falling in brightness
behind Eta, Mu, and Xi Ser) within the
northern extreme of Serpens' X-shaped head. The star's relative
obscurity, however, belies its significance as a curious multiple
with as many as five components. In the nineteenth century, Smythe
and Chambers saw it as "A delicate double star on the Serpent's
under-jaw...both of a pale blue tint, and there is a distant
telescopic star also blue..." The early observers, however, were
fooled again by dimness and by contrast effects. Beta Ser A, the
main member, is a common class A (A2) subgiant, which would make it
as close to pure white as possible. From a well-determined
distance of 155 light years (plus or minus just 2) and a
temperature of 8549 Kelvin (which places nearly all the radiation
in the visual spectrum), we
find a luminosity of 61 times that of the Sun and a radius of 3.6 solar. Madly spinning
with an equatorial velocity of at least 189 kilometers per second,
Beta Ser rotates in under 23 hours. Theory shows that the star is
indeed a subgiant, one that has given up (or is about to give up)
core hydrogen fusion, the mass falling between 2.4 and 2.5 Suns
depending on the exact state of evolution. Separated from Beta Ser
by 31 seconds of arc lies 10th magnitude (9.7) Beta Ser B, which is
keeping pace with much brighter Beta A as it moves through space
and clearly belongs to it. Hardly blue, the small companion is an
orange class K3 hydrogen-fusing dwarf (showing how easily the eye
can be deceived). With a true separation of at least 1500
Astronomical Units and an estimated mass of roughly three-fourths
that of the Sun, Beta-B takes at least 31,000 years to make an
orbit around Beta-A. From its brightness, Smythe and Chambers'
"distant telescopic star" (200 seconds of arc from A), 11th
magnitude Beta Ser C, which also keeps pace and seems to belong,
must be another orange mid-class K dwarf. With an orbital radius
of at least 9500 AU, it would take more than half a million years
to make a full circuit around the inner AB pair. Going vastly
farther afield, nearly half a degree away, we find eighth magnitude
Beta-D, which is ALSO keeping good pace. If a real companion, it
would orbit the inner trio in a minimum-million-year path that is
at least 80,000 AU wide. Bright enough to have a name of its own,
Beta Ser D is better known as HD 140665 (from the Henry Draper spectroscopic
catalogue, Beta Ser A also HD 141003). Beta-D's distance of
151 light years, which given an error of 10 l-y puts it at the same
distance as Beta-A, confirms the connection. A close look reveals
that it TOO is double! and that it consists of a sunlike star in
mutual orbit with yet another class K dwarf, the two separated by
at least 285 AU, giving a period of at least 3700 years. If all
this is correct, we thus have a highly unusual quintet with A and
B in orbit, then C going around AB, with fragilly-linked double D
going around ABC. It's almost hard to see how such an arrangement
could last over the system's 600 million year lifetime (but then
look at much older Alpha and Proxima
Centauri). From Beta Ser B, A might shine with the light of up
to 15 full Moons, while from double D it would appear more like a
few times Venus's brightness. From A, double D would look like a
Jupiter orbiting a star like Vega separated
by perhaps up to several minutes of arc, presenting a fine sight
were there anyone there to see it, the existence of any "debris
disk" not clear.
Written by Jim Kaler 5/13/11. Return to STARS.