Nu Bootis

NU BOO (Nu-1 and Nu-2 Bootis), a two-for one special; and actually three! A few naked-eye double stars dot the sky, the classic example being Mizar and Alcor, which lie 12 minutes of arc apart. Epsilon Lyrae is another, Epsilon-1 and -2 much closer but still splittable. Most, though, are just line-of-sight coincidences, as Nu-1 and Nu-2 Bootis can attest, the stars lying to the north of the classic figure of Bootes (the Herdsman) and to the north of Corona Borealis. Just 10 minutes of arc apart, they look like a faint version of Mizar and Alcor. Nu-1 (the western one), however, is 838 light years away (give or take 43), while Nu-2 is 388 distant, the two stars actually 450 light years away from each other.

Looking like a naked-eye double star, separated by just 10 minutes of arc, Nu Bootis (Nu-1 on the right, Nu-2 on the left) fools the observer. In reality, Nu-1 is more than twice as far away. By complete coincidence, the two are almost exactly the same brightness as viewed from Earth. Yet there the similarity ends, as Nu-1 is an orange class K giant, while Nu-2 is a white class A dwarf, the color contrast obvious. Oddly, Nu-2 is a REAL double, whose components (too close together to be seen here) are REAL twins.

Remarkably, however, and quite by another coincidence, they are of almost exactly the same apparent brightness, both fifth magnitude, more exactly each 5.02. There the similarity ends, as Nu-1 is a K (K5) orange giant, while Nu-2 is a white class A hydrogen-fusing dwarf, the pair making a fine color contrast in binoculars or a small telescope. Nu-2 then remarkably enhances the coincidence by being a REAL double whose stars are ALSO identical, both magnitude-5.8 A5 dwarfs in tight orbit less than a tenth of a second of arc apart.

Take the simpler (but perhaps more impressive) one, Nu-1, first. Though traditionally given as K5, it's also been classed as an M0 supergiant, the temperature hovering around 3900 Kelvin. A hefty luminosity of 1650 Suns leads to a radius 88 times solar, making it just a bit bigger than the orbit of Mercury. A projected rotation velocity of 4 km/s gives it a rotation period that might be as long as 3 years. Brightness and temperature plus theory give an impressive mass of 5 Suns and show it to be a quiet helium-to-carbon burner about 100 million years old. Chemical analyses are conflicted, some studies showing a 50 percent enhancement of metals over solar, another showing less than solar.

Nu-2, however, cannot be shoved aside, as it has its own fine style. Its twin A-stars orbit each other on nearly circular paths every 8.48 years 7.2 Astronomical Units apart. Kepler's Laws then give them a combined mass of 5.1 times that of the Sun.

The identical components of Nu-2 Bootis actually orbit each other about a point almost exactly between them every 8.48 years at a constant separation of 7.2 Astronomical Units. They are graphed here, however, as if one (set at the cross) is being orbited by the other. Given their similarity, it hardly matters which one orbits which. The orbit's apparent elliptical shape is the result of a 65-degree tilt to the plane of the sky. The scale is in seconds of arc. Given their angular separation of under a tenth of a second of arc, they are very difficult to see as individuals, as evidenced by the enormous scatter in the measuements. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

With an estimated temperature of 8300 Kelvin, each shines with the light of 54 Suns, which leads to radii of 3.6 solar and masses of 2.4 times solar. The combined mass of 4.8 solar found this way is satisfyingly close to that found from the orbit. The agreement is especially impressive given the difficulty of observing such a tight-knit pair. Nu-2 then goes one better by also being an off- and-on "shell star" whose high rotation speed of 200 km/s leads to a variable, more-or-less edge-on circulating disk. The problem here of course is that we can't tell which star the disk belongs to. Given their nearly identical natures, perhaps it's both.

Written by Jim Kaler 7/23/10. Return to STARS.