NU OPH (Nu Ophiuchi). There are several oddities (perhaps better
said, several points of interest) about this modestly bright third
magnitude (3.34) star in southeastern Ophiuchus (the Serpent Bearer). First, practically on
the border with Serpens Cauda (the eastern portion of divided
Serpens), because of jogs in constellation
boundaries, the star lies right on the ragged line that makes the
outline of the celestial Serpent. But no, it is really within the
confines of Ophiuchus. Second, while sometimes called "Sinistra"
(meaning "on the left side"), the proper name does not appear in
any classical listings. It probably derives from Chinese lore and
seems more astrological in nature, so we stick here with the good
old Greek letter name of Nu Oph. Third,
while listed as belonging to the Scorpius-Centaurus association of massive stars, it doesn't.
Fourth, the star has been said to have a seventh magnitude binary
companion a couple seconds of
arc away. But Nu Oph is not listed as double in either Burnham's
exhaustive "Celestial Handbook" nor in the authoritative Washington
Double Star Catalogue, so it is clearly "single" (though with
wonderfully important exceptions as told below). Lying 151 light
years away (give or take two), Nu Oph then at first appears as just
one more class K (K0 at that) giant. But even here there are
points of interest. With a temperature of 4825 Kelvin and shining
with the radiance of 107 Suns, theory shows
Nu Oph to be fairly massive, carrying the "weight" of three Suns
(and no less than 2.7), the 400-million-year-old star beginning
life as a class B8 dwarf now fusing its internal helium into carbon
and oxygen. While a "CN-weak" star (with a lower-than-normal
cyanogen abundance, implying low nitrogen or carbon), it is
actually metal-rich, with an iron content (relative to hydrogen) a
third greater than found in the Sun. A radius of 14 times solar
coupled with a projected equatorial rotation speed of 3 kilometers
per second yield a rotation period that could be as long as 234
days. There is also a suggestion of far infrared variability. Now
to the "exceptions." As the final "oddity," while "single," Nu Oph
is attended by a pair of tiny bodies that could at first be thought
of as "planets." But they are too massive,
and are both really "brown
dwarfs," failed stars that are below the 0.075 solar mass limit
required to run full fusion, yet are above the 13 Jupiter-mass
limit that allows fusion of deuterium (a heavy form of hydrogen).
From the Extrasolar Planets Encyclopedia, Nu
Oph b, discovered in 2004, carries a mass of at least 22 Jupiters
in a 536-day (1.47 year) orbit that averages 1.8 Astronomical Units
from Nu proper. Much farther out, at 5.9 AU in an 8.7-year orbit,
is Nu Oph c. Found in 2010, it holds at least 25 times the mass of
our largest planet (the two respectively having masses of 0.021 and
0.029 solar masses). While called "brown dwarfs" are they really
supermassive planets that formed from dust and gas accumulation in
an early disk around just-born star, or are they true "stellar"
companions that were formed by
direct condensation from interstellar matter (as was the star)?
And therein lies a specific case of one of the great mysteries of
modern astronomy. Not bad for an "ordinary" class K giant (once
again showing that at some level, all stars are unique). (Thanks
to Paolo Colona, who suggested this star.)
Written by Jim Kaler 8/12/11. Return to STARS.