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 45 days for Xi-2, but with
a huge error. Neither has a companion, at least one we can see,
Written by Jim Kaler 9/20/17. Return to STARS.