NU SER (Nu Serpentis). Good Nu's or bad Nu's? Neither, just bit
of confusing Nu's, since fourth magnitude (4.33) Nu Serpentis is
just 10 degrees west southwest of Nu
Ophiuchi. The entanglement of Serpens (the Serpent) and Ophiuchus (the Serpent Bearer), and the resulting
irregular constellation boundaries that
make Nu Oph look like it belongs as much to Serpens as it does to
Ophiuchus, could lead to confusion between the two stars. Not that
it's actually ever happened. In any case the two are very
different, as Nu Oph is a K0 giant 151 light years away, while Nu
Ser is a class A (A2) dwarf
(though it's been accused of being A1 or even A0) 203 light years
(give or take just 3) away. With a temperature of 9120 Kelvin
(about right for the class), needed to account for some ultraviolet light, Nu Serpentis
shines brightly with a luminosity 61 times that of the Sun, which in turn gives a radius of 3.1 Suns.
Nu Ser's minimum equatorial rotation speed of 119 kilometers per
second yields a rotation period under 1.3 days and provides enough
stirring to prevent separation of elements so as to keep the
chemical composition normal. The theory of stellar structure and
evolution reveal a star with a mass 2.5 times that of the Sun and
show that, with an age of around 450 million years, it is through
about 80 percent of its hydrogen fusing dwarf lifetime, far less
than that of the Sun (higher mass stars living much shorter
lifetimes as a result of higher rates of internal nuclear fusion).
About 45 seconds of arc away from Nu Serpentis proper is a ninth
magnitude "companion" first
observed in 1821 by John Herschel. The pair epitomizes the
charming colors seen in double stars by older astronomers. From
the nineteenth century, Smythe and Chambers call the brighter "pale
sea-green," the fainter "lilac." While the colors are the result
of contrast effects, that makes them no less lovely to look at.
Sadly the duplicity is not real either. Over the past two
centuries, the stars have moved much too quickly relative to each
other (5 seconds of arc) for them to be caused by their being in
mutual orbit. The pairing is just another line of sight
coincidence. As a bit of a curiosity, Nu Ser lies 1.6 degrees
almost exactly due east of a small "planetary
nebula" (a glowing cloud of gas ejected from a dying star)
called NGC 6309, which is just across the
border in Ophiuchus. If you point a telescope with no clock drive
(which would follow the stars as they appear to move across the
sky) at Nu Serpentis and wait just short of seven minutes, the
nebula will drift into the field of view four minutes of arc to the
south of center.
Written by Jim Kaler 7/19/13. Return to STARS.