STARS OF THE WEEK: XI-1 and XI-2 SGR (Xi-1 and Xi-2 Sagittarii). There are not a lot of letters (Greek or Roman) that our astronomical ancestors had available for which to name the stars. One popular way of extending names was to add numbers, as in "Pi-1 Pegasi" and "Pi-2 Peg," where the numbers appear as superscripts. Don't get them confused with lettered binary stars in which for example Sirius B orbits Sirius A. The stars with superscripted numbers are unrelated to each other except for accidental alignment. Brightness has nothing to do with it, as"1" is commonly brighter than "2." Such numbers run east to west. (Though not always. A glaring exception to the rule is Pi-1 through Pi-6 Orionis, where the numbers go from south to north). Eastern Sagittarius has a goodly collection of such pairs that include Rho-1 and 2, Nu-1 and 2, Chi-1 and 2, and Xi-1 and 2. The Greek and Roman letters are joined to a set of 88 formal constellations that had flexible and diverse boundaries that were not fixed in place until 1930. There are far more celestial figures than these, however, informal "asterisms" that were never adopted as true constellations. Among them are some of the most beloved figures of the sky such as the Big and Little Dippers, the Summer and Winter Triangles, etc. Sagittarius, the Archer of the Zodiac, has a rather striking number of them. We can readily identify an obvious old-fashioned "Teapot" and an upside-down (and presumably empty) "Little Milk Dipper" with its handle stuck into the Milky Way. Lesser known is the "Teaspoon" that you need to measure and stir the whole mess. The Teaspoon's stars are arguable, but consist of at least those of the above numbered pairs, including Rho, Xi, and if you wish, Nu, the whole figure dominated by second magnitude Pi Sgr. Rather down the line are fifth magnitude (5.08) Xi-1 and fourth magnitude (3.51) Xi-2 (Xi-1 dimmed a half a magnitude by interstellar dust). Both are evolving giants, Xi-1 532 (give or take 74) light years away, Xi-2 369 (plus/minus 23) light years. Xi-1 is an interesting class A (A0) 6.5 solar mass "bright giant" with a temperature of 10,100 Kelvin that seems to be evolving towards eventual helium fusion in its core. Xi-2 is a more ordinary five solar mass K1 giant with a 6000 Kelvin surface temperature that has already arrived in such a state. Xi-1 and 2 have radii of 21 and 38 times that of the Sun. Projected rotation speeds are only 12 and 6 kilometers per second, yielding rotation periods under 90 and 300 days. Lunar occultations give a radius of Suns for Xi-2, but with a huge error. Neither has a companion, at least one we can see,

Written by Jim Kaler 10/06/17. Return to STARS.