ALPHA SEX (Alpha Sextantis). In the pantheon of modern constellations, one navigational/positional instrument (Octans, the Octant) finds the way in the southern sky, while another, Quadrans (the Quadrant) seeks the stars in the northern, while yet another, Sextans (the Sextant) falls in the middle, straddling the celestial equator. Only two of these, however, remain, Quadrans long ago dropped from the official roll call. Neither is very bright. The luminary of Sextans, Alpha Sextantis, just barely escapes fifth magnitude (4.49) by a mere hundredth of a division. Physically there is not too much special about this class A (A0) giant. With a surface temperature of 9900 Kelvin, it shines from a distance of 285 light years with a luminosity 122 times that of the Sun, its mass 3 times solar. While spectroscopically a "giant" (with a radius only 3.8 times that of the Sun), this 300 million year old star is simply nearing the end of its hydrogen-fusing lifetime. It will within a short time-span of only 60 million years become a true and much brighter orange giant, its hydrogen core expired to helium. The lower limit to the equatorial rotation velocity is but 7 km/s. Since most stars of this class rotate more rapidly, and since low rotators tend to be chemically peculiar (which the star is not), Alpha Sextantis's rotation pole probably points towards Earth. The star's claim to any sort of greatness lies in its position. It is almost exactly south of Regulus in Leo, shifted by a mere 0.4 minutes of arc (0.007 of a degree) to the west. More interesting, it is one of the sky's informal "equator stars," currently lying less than a quarter of a degree south of the celestial equator. The action of the Moon and Sun on the Earth's equatorial bulge slowly causes the axis of the Earth to wobble over a 26,000 year period, which in turn causes changes in the pole stars and in stellar positions. In 1900, Alpha Sextantis was 7 minutes of arc NORTH of the equator, rather than south of it. It crossed over from one hemisphere to the other in December of 1923 (the celestial equator actually shifting, not the star).
Written by Jim Kaler. Return to STARS.