BETA SEX (Beta Sextantis). Drop a line due south of the Sickle of Leo to find a faint triangle of stars. Set into a bend in the stream that makes Hydra (the Water Serpent), to the northeast of Hydra's Alphard, it represents Sextans, the Sextant, one of the sky's navigational instruments. Alpha and Beta Sextantis almost exactly define the celestial equator. There is also some similarity between them. Both are fifth magnitude (Alpha 4.49, Beta to the east 5.09). Both are white. Alpha is a class A giant, though Beta, a class B (B6) dwarf that with a temperature of 14,300 Kelvin, is notably hotter and tends more toward the blue. Most stars of Beta's kind, which are recently born, lie along the plane of the Milky Way. It's therefore rather unusual to see such a star so far away from the Milky circle, something Beta Sextantis shares with nearby Regulus, which comes in just a bit cooler. At a distance of 406 light years (give or take 18), though, our Beta is much the farther. From that and temperature (to account for its ultraviolet radiation), we find a luminosity 316 times that of the Sun and a radius of 2.9 solar. A modest (for such a star) projected equatorial rotation speed of 93 kilometers per second gives a rotation period of under 1.6 days. The theory of stellar structure and evolution then yields a mass of 4 times solar and indeed shows that the star is rather young, roughly 70 million years old, not that far along its core-hydrogen- fusing lifetime of 165 million years. After ridding itself of its overlying envelope, the eventual evolved core will die as a rather massive white dwarf of close to 0.8 solar masses. Three other items recommend the star. The simplicity of its spectrum makes it ideal for probing the intervening interstellar gases. More interesting, Beta Sextantis sometimes has a "p" (originally for "peculiar," but now meaning "magnetic") attached to its class, the star rich in helium with a magnetic field that wobbles with a 15 day period, in that sense making it similar to Alpha-2 Canum Venaticorum, the prototype of such stars. None of this, though, seems to have been confirmed, the variation far too long compared with the rotation period. The third relates back to position on the sky. Beta Sextantis is just over half a degree to the south of the heavenly equator. Precession, the 26,000-year wobble in the Earth's rotational axis, is dragging the local equator to the north, so the star appears to be drifting even further south. At the rate it is going, it crossed over the equator (from north of it to south) around the year 1875.
Written by Jim Kaler 4/06/12. Return to STARS.