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.