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, 71 light years as opposed to Beta's 24.4. 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), is the closest reasonably bright star to the Small Magellanic Cloud, a small nearby naked-eye irregular galaxy 200,000 light years away, and was the south pole star around 2900 BC. With a temperature of 7140 Kelvin, the star radiates most of its light in the visual spectrum, shining with the light of 26 Suns, that and temperature yielding a radius of 3.3 solar. From the theory of stellar structure and evolution, the star's mass lies between 1.9 and 2.0 solar (depending on its exact state of ageing). Starting life around class A0-A2 about a billion years ago, Alpha Hyi is cooling and nearing the end of its hydrogen fusing life, 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. Rotating with a fast minimum equatorial velocity of 155 kilometers per second, the rotation period must be under 26 hours (as opposed to 25 days for the Sun). Some chromospheric activity has been taken as acoustically (rather than magentically) induced. Alpha Hyi's most outstanding characteristic is probably its 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 a mere 12 percent. The star is not far enough along its evolutionary path to have altered its surface chemistry, so it must have been born that way, once again showing that no two stars are quite alike.
Written by Jim Kaler. Return to STARS.