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.