ALPHA HYI (Alpha Hydri). Barely losing out as luminary of Hydrus, the Water Snake, third magnitude (2.86) Alpha Hydri is just 0.06 of a magnitude fainter than Beta Hyi. While both are "middle class," Alpha, as a class F (F0) dwarf, is really the more luminous (Beta a class G2 subgiant). It just seems a bit fainter because it is farther away, 72 light years (give or take 1) as opposed to Beta's 24.3. Alpha Hydri goes by the English proper name "Head of Hydrus," though that is so obviously made up in modern times that we will forego it. The star Hydri served as the south pole star around 2900 BC, and was a lot better than the one we have today, Sigma Octantis, in Octans, the Octant. With a temperature of 7360 Kelvin, the star radiates mostly in the visual spectrum, shining with the light of 26.9 Suns, that and temperature yielding a radius of 3.35 times solar. From the theory of stellar structure and evolution, the star's mass is close to 2.0 times that of the Sun. Starting life around class A0-A2 about a billion years ago, Alpha Hyi is cooling and nearing the end of core hydrogen fusion, and will before long turn itself into a subgiant as it prepares to become a much larger red giant 40 times more luminous than it is today. Detailed examination of a star's spectrum gives a number of parameters, including the strength of gravity at the surface. (The higher the gravity, the more the compression of the gas, and the closer the atoms are to each other, which affects the way in which they produce their absorption lines.) The gravity in turn depends on mass and radius. Gravity measure with the above radius gives a mass of 1.9 solar, which satisfyingly agrees with that derived from theory. Above (more massive than) the "rotation break" at which stars suddenly begin to spin faster as a result of a lack of magnetic braking, Alpha Hyi spins with an equatorial speed of at least 135 kilometers per second, yielding a rotation period of under 30 hours (as opposed to 25 days for the Sun). Some chromospheric activity has been taken as acoustically, rather than magnetically, induced. Alpha Hyi's most outstanding characteristic may be a high metal content (to an astronomer, "metal" a code word for anything other than hydrogen or helium). Averaging not quite double that of the Sun (relative to hydrogen), the elevation depends strongly on chemical element, oxygen up by a factor of four, sulfur by 12 percent. Newer results give an iron content elevated by just 17 percent. If the star were in the northern hemisphere, we'd certainly know more about it. (11/04/05; 6/18/15)

Written byJim Kaler 10/10/14. Return to STARS.