CHI AND PSI SCO (Chi and Psi Scoprpii), a two-for-one special. We could even have a three-for-one, all in northern Scorpius, if we include Phi Sco, giving us a consecutive run of Phi, Chi, and Psi. Unfortunately, Phi Sco is one of Bayer's "lost stars." It does not exist. It's been confused with both 48 Librae and 49 Librae, which are just over the border of Scorpius with Libra, but modern boundaries place the latter two firmly in Libra, not in Scorpius. We are then left with fifth magnitude Chi (5.22) and Psi (4.94), which with Xi Sco (the brightest of the trio) occupy the most northerly part of the narrow panhandle of Scorpius that juts northward between Libra and Ophiuchus. The only Flamsteed stars farther north are faint-fifth magnitude (5.43) 16 Sco (a class A3 dwarf 252 light years away) and a solar clone, 18 Scorpii. Other than position, Chi and Psi two have nothing to do with each other. Chi is a relatively ordinary class K (K3) giant 378 light years away (give or take 15), while Psi is class A (A3) subgiant at a distance of 156 light years (plus or minus 10). Chi Sco is a common helium-fusing giant with a temperature of 5270 Kelvin. Adding in some infrared radiation, the star is seen to shine with the light of 249 Suns, which leads to a radius of 28.9 solar. Chi Sco is a calibrator for the interferometric measures of stellar angular diameters, from which we derive a very supportive 27.1 solar radii, showing that the parameters are all pretty close to the mark. Theory then gives a mass around 1.6 times that of the Sun (such stars with a range of masses having similar properties, so precision is difficult). The iron content is around 3/4 that of the Sun.

Psi Sco, with a temperature of 8590 Kelvin, radiates at a rate of 19.2 times that of the Sun, which in turn gives as radius of 2.0 solar radii. Other values of mass from the literature average 2.05 Suns. Theory shows it to be a hydrogen-fusing dwarf rather than a subgiant. A rather slow equatorial rotation velocity of at least 36 kilometers per second yields a rotation period of less than 2.0 days. Yet there seems to be no serious separation of elements of the kind we see in so many class A stars, so the rotation speed is probably quite a lot higher (rotation mixing the elements) and the rotation pole tipped more or less towards us. Psi Sco is roughly halfway through its hydrogen-fusing dwarf lifetime of a billion years, while Chi is just over two billion years old. As seen from Chi, Psi would be a bit fainter, sixth magnitude, Chi from Psi a bit brighter, fourth magnitude. From each, the Sun would shrink to invisibility within the distant stars of Taurus, tenth magnitude from Chi, eighth from Psi.
Written byJim Kaler 8/12/16. Return to STARS.