KX LIB (KX Librae) = HR 5568 = GJ 570. And many other names, but these are the most used. Want to see a brown dwarf, a "substar," one that has so little mass (below about 7.5 percent of the Sun's) that it cannot get hot enough inside to support internal nuclear fusion? Well, you can't. GJ 570 D, as it is most commonly called, is not visible to the eye under any circumstances: it's too cool at the surface to radiate visible light, and thus can be "seen" only through its weak infrared radiation. You CAN, however, see the triple star system to which it belongs and at least know that you are in the neighborhood, KX Librae easily located in Libra (the Scales) not quite half way from Zubenhakrabi (Sigma Librae) to famed Zubenelgenubi (Alpha).

The system is notable in its own right. At a distance of 19.1 light years (give or take just 0.1!), HR 5568 = KX Lib is the 70th- nearest star system (which gives a sense of the number of stars that flock around us). At the bright end of sixth magnitude (5.74), the principle star, a class K (K4), hydrogen-fusing dwarf, is one of the coolest of its kind visible to the unaided eye, comparable to a few others such as the 61 Cygni pair. With a temperature of 4680 Kelvin, KX Lib A radiates only 20 percent of the energy produced by the Sun, which tells of a radius of 0.68 solar, and is consistent with a mass of around 0.7 Suns. A projected solar rotation velocity gives a rotation period under 17 days. There is some sense of variability (hence the "KX" variable star name), either from spots, flares, or both. The metal content is normal even though the star is moving fairly rapidly relative to the Sun, four times the average speed, suggesting an origin in a different part of the Galaxy.

Older tradition lists a family of six faint companions, KX-B through G, with separations from KX Lib proper ("A") that range from around 20 seconds of arc for "B" to 258 seconds for "G." Motions show that all but 8th magnitude (8.18) "B," a class M2.5 red dwarf, are line- of-sight coincidences. B then harbors a surprise, a 9.5-mag (from its brightness and temperature, an M4-4.5 dwarf) that is NOW called "C" and that hovers less than 0.2 seconds of arc from its somewhat brighter mate, the luminosities of both but a few percent solar. With a mean separation of 0.88 Astronomical Units, B and C orbit with a period of 308.88 days, a high eccentricity taking them between 0.2 and 1.5 AU apart. Kepler's Laws then give BC a combined mass of 0.95 Suns, later refined to 0.98, which is broken down to 0.59 and 0.39 Suns for B and C individually. The BC pair then orbit KX Lib A over a much- longer period of 2130 years at an average distance of 189 AU, a lower eccentricity taking them between 227 and 181 AU (the latter occurring in 1689). Kepler's Laws then yield a total KX-ABC mass of 1.49 Suns. Subtraction of KX-BC's mass from the grand total leaves half a solar mass for KX Lib A alone, which is too small. Long-term orbits, though, are subject to considerable uncertainty. The mass of "A" can be reconciled by stretching the orbit of the BC pair around A by just 4 percent to 197 AU.
KX Lib KX Lib
The scales of the two orbits are in seconds of arc. At left, KX Librae B (actually BC) orbits KX Librae A. A fitted orbital path leads to a period of 2130 years at an average separation (dependent on distance) of 189 Astronomical Units. They were closest together in the year 1689. A tilt of 72 degrees to the plane of the sky distorts the orbit and places "A" off the apparent focus of the ellipse. In reality, the two (BC and A) are in mutual orbit around a center of mass that lies between them. At right, we see KX Librae C going around KX-B in an orbit of just 0.8457 years, the two separated on average by just 0.88 AU but with a huge swing of 0.2 to 1.5 AU apart. A tilt of 71 degrees again distorts the orbit from what one would see were it face-on. Curiously, though the tilts are about the same, they are opposite each other and the directions of the orbits are reversed. (From the Sixth Catalog of Orbits of Visual Binary Stars, W. I. Hartkopf and B. D. Mason, U. S, Naval Observatory.)
While such triples with known orbits are unusual and important for the determination of individual stellar masses, the real intrigue of the system involves a much more distant companion discovered at the turn of the century (20th to 21st!) that lies 258 seconds of arc from KX Lib A. Parallel motion to its brighter mate strongly indicates that it is a real, orbiting member of the system. Visible only in the infrared, its spectrum shows it to be a class T (T7.5) brown dwarf with a temperature of but 810 Kelvin (537 C, 1000 F, not much hotter than the surface of Venus and the same as the T7.5 brown dwarf that inhabits the 54 Piscium system). Feeble indeed, it radiates at a rate of only 3 millionths that of the Sun, its mass estimated at 43 times that of Jupiter or four percent solar, making it a real brown dwarf. Its radius is actually 15 percent smaller than that of Jupiter, the result of its larger gravity. A minimum separation of 1500 AU from the inner triple gives an orbital period around the trio of at least 43,000 years. How it formed way out there is unknown, but then we have the same problem with the double brown dwarf that orbits Epsilon Indi and an even worse one with the the classic example, Proxima Centauri. Calculations do show, however, that GJ 570 D's distant orbit is stable.

Some astronomers once wondered if the Sun might not have a such a distant, optically-invisible companion ("Death Star") that might disrupt our surrounding comet cloud, leading to greater impact rates on Earth. But no. Such a body would gravitationally affect planets and spacecraft in obvious ways. Moreover, it would glow brightly in the infrared where it would by now have been "seen" by IR surveys. So we are safe, at least from that.
Written by Jim Kaler 7/01/11. Return to STARS.