XI AQR (Xi Aquarii). Aquarius (the Water Bearer) sprawls all over the place south of the celestial equator to the west of the Vernal Equinox in Pisces next door. The only really identifiable structure is the famed "Water Jar," a "Y"-shaped asterism near the constellation's northern border. Coming off the Water Jar to the west and southwest are the "lucky stars," Sadalmelik and Sadalsuud (Alpha and Beta Aquarii), oddly both third magnitude class G supergiants. Winding around to the southeast of Sadalsuud, near the border with Capricornus we find lonely fifth magnitude (but at 4.69 almost fourth) Xi Aquarii, a class A (A7) dwarf that sadly, but not surprisingly, lacks a proper name. What makes it jump out is an old note of a "massive companion" that was later rather unfortunately found to be not there. Nevertheless, the star is still double, though more modestly. The two are very close together, at the limit of direct visual observation. The duplicity is known both through lunar occultation (in which the Moon glides in front of the star) and interferometry, which allows an orbit to be constructed. The former observation gives a difference of two magnitudes between the pair, the latter three. Choosing an average of 2.5, Xi Aqr A then has a visual magnitude of 4.79, Xi-B of 7.29, which would make it an F6 dwarf. A distance of 179 light years (give or take under 3) and respective temperatures of 7720 (measured) and 6500 (estimated) Kelvin (which shows nearly all of the radiation to be in the optical domain of the spectrum) give respective luminosities of 28.1 and 2.9 times that of the Sun and radii of 3.0 and 1.4 times solar. An equatorial rotation velocity of 162 kilometers per second applied to brighter Xi-A yields a rotation period of under 0.92 days The orbit is nearly in the line of sight, tilted by just 5 degrees, so if the pair is in any kind of rotational-orbital synchrony, the calculated rotation period is close to the true value. Luminosity, temperature, and theory show Xi Aqr A to carry a mass of 2.1 Suns and that the star is maybe three-fourths of the way through its billion year dwarf lifetime. Xi-B then comes in at 1.3 Suns. But now return to the orbit.

Xi Aqr Xi Aquarii B goes around the brighter member of the pair with an orbital period of 25.5 years at an average calculated separation of 15.3 Astronomical Units. In reality the two orbit a common center of mass between them. Their closeness is seen from the scale on the axes. The orbit is quite eccentric and is presented nearly edge on, so the true orbital ellipse is highly distorted. The sum of the masses from Kepler's laws is significantly larger than that derived from luminosity, temperature, and theory. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

The duo is separated on average by a mere 0.28 seconds of arc, which with distance translates to a physical mean separation of 15.3 Astronomical Units, the two going around each other every 25.5 years. A high eccentricity carries them between about 20 and 6 AU apart. They were last closest together toward the end of 1995, and farthest apart in mid-2008 (physically, not on as seen on the sky). Kepler's laws, however, give a total mass to the system of 5.6 Suns, notably larger than the 3.4 Suns derived from evolutionary theory. The two values can be reconciled by squeezing the size of the orbit (mean separation) down to 13.0 AU, not all that much difference given the difficulty of the observations, more of which will eventually being things into line.

Written byJim Kaler 10/03/14. Return to STARS.