BETA HYI (Beta Hydri). Toward the bright end of third magnitude
(2.80) and the closest bright star to the
South Celestial Pole,
the Beta star in
modern (and deep southern hemisphere) Hydrus (the Water Snake) just barely beats
out
Alpha (magnitude 2.86) as the constellation's luminary. By
rather odd coincidence, both are relatively lower mass stars.
Alpha is a class F (F0) dwarf under two solar masses, while class
G (G2) Beta is close to being a clone of the Sun, differing only slightly in mass, though as
a subgiant near or at the end of its hydrogen fusing life.
Also by odd coincidence, both were once pretty good southern pole
stars (the result of
precession of the Earth's rotation axis), Beta
around 150 BC (when it was just under two degrees off the South
Celestial Pole), Alpha around 2900 BC (when it was a bit over two
degrees away). With a
temperature of 5750 Kelvin, Beta Hydri is just 30 degrees cooler
than the solar surface. The temperature and a precisely-known
distance of 24.4 light years yield a luminosity of 3.7 times that
of the Sun and a radius 1.9 times solar. Luminosity and
temperature coupled with the theory of stellar structure and
evolution give a mass of only about 1.2 times that of the Sun,
while another more detailed study leads to 1.1 solar. A measured
projected rotation speed of 3.3 kilometers per second gives a
maximum rotation period of 29 days, quite similar to the 25-day
period of our own star (though since the axial tilt is not known,
the number could be much less). Whatever the details, we are
looking at a star that is not much different from the one around
which we revolve. Beta Hydri began its life some 5 to 6 billion
years ago as a dwarf at the cooler end of class F, around class F8.
As it used up its internal hydrogen, it brightened (in part as a
result of core contraction and heating) and expanded to the star we
see today. At the end of its core-hydrogen-fusing life, Beta is
about to make its grand transition to becoming a red giant. By the time the core
is hot enough to re-stabilize the star as a helium-burner (helium
fusing to carbon and oxygen), it will have brightened to 1000 solar
luminosities, cooled to under 3400 Kelvin, and will have swollen to
a radius half the size of the Earth's orbit. Beta Hydri will then
lose its outer envelope, the nuclear-burning core dying as a white dwarf with a mass of
only about half that which the star carries today. A high velocity
relative to the Sun of 83 kilometers per second (five times greater
than normal) suggests that the star is a visitor from a different
part of the Galaxy.