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.