ALPHARD (Alpha Hydrae). Not well known, but surprisingly prominent, Alphard
dominates the dim constellation
Hydra, the Water Serpent, and is
hence also known as Alpha Hydrae. The star, right in the middle of
the range of second magnitude (1.98), is made more noticeable by
lying within a fairly blank region of sky to the southwest of
brighter Regulus.
Appropriately, its Arabic name means "the
solitary one." A dark sky, or better, binoculars, will show the
star glowing a pale orange color, indicative of a giant star, one
of the most common kinds that inhabit the naked-eye sky. Its
distance of 175 light years allows us to calculate a true
luminosity that to the eye would be 400 times that of the Sun.
With a temperature of 4000 degrees Kelvin, the term "giant" is apt,
as the star is some 40 times larger than the
Sun. Placed at the
solar position, Alphard would extend halfway to the orbit of
Mercury. As such the star is in league with several others that
include both Arcturus and
Aldebaran, which are more prominent
because they are closer to us even though Alphard is actually a bit
more luminous (making it a so-called "bright giant."). Alphard
also distinguishes itself by being a mild form of "barium star," in
which barium and other elements that are formed by the slow capture
of neutrons are enhanced. Barium stars are thought to be doubles.
When Alphard was young, it had a more massive companion that died
first, and in the process contaminated it with the by-products of
nuclear fusion that had been shovelled to the top. The companion
has now died as a dim white dwarf, and the star that had been
contaminated is now itself dying, presenting us with evidence for
what once happened.