ALPHARD (Alpha Hydrae). Not all that well known among bright stars, but surprisingly prominent,
Alphard both dominates and lies at the heart of the relatively dim
constellation Hydra, the Water Serpent
(the longest constellation of the sky),
hence clearly deserves to be Bayer's Alpha star. Right in the middle of
the range of second magnitude (1.98), Alphard is made more noticeable
by lying within a fairly blank region of sky to the southwest of
brighter Regulus. Appropriately,
Alphard's Arabic name means "the solitary one." A dark sky shows the
star glowing a pale orange color, indicative of a common class K (K2)
giant star, though one on the
bright side. An accurate distance of 180 light years (with an
uncertainty of just 2) and an assessment of infrared radiation from its 4260
Kelvin surface allows us to calculate a luminosity 946 times that of
the Sun, which with temperature implies a
radius of 56.6 times solar, a quarter of an Astronomical Unit, 60
percent the size of Mercury's orbit. The measured angular diameter
and distance give a radius of 57.8 times that of the Sun, just two
percent greater, showing the parameters of our lonely star to be
remarkably accurate. The theory of stellar structure and evolution
suggests that Alphard is quietly fusing helium into carbon and oxygen
in its deep core, and that it is a "clump star," one of very similar
other properties, but with a fairly high mass of around 4.5 times that
of the Sun. It probably started life as a hot star in the middle of
class B, by itself implying a one-time fairly fast rotation speed. But
when stars evolve as giants they spin more slowly, a consequence of
the conservation of angular momentum (the same reason a skater spins
faster as she brings in her arms). Current measures suggest that
Alphard has a projected equatorial rotation velocity of 3.3 kilometers
per second, meaning that the rotation period could be as long as 2.4
years (as opposed to the near-monthly solar rotation period). 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 close 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 shoveled 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. After sloughing off its outer
layers, which will briefly make a surrounding planetary nebula, it too will become a white dwarf,
with a mass of around 0.8 solar masses. The key is in the word "mild,"
as most such are probably are not real barium stars at all. In any
case, there is no evidence for a tiny close companion. There are,
however, two distant faint neighbors. Alphard C, now 4.3 minutes of
arc away, is moving too fast to be joined, and is clearly a line of
sight coincidence. But ninth magnitude (9.7) magnitude Alphard B, at
282 seconds separation, is more or less tracking bright Alphard A. If
really tied together, "B" is probably a low mass K0 dwarf (which as
a result of contrast effects in the nineteenth century was considered
to be "pale green") at least 15,700 AU away that takes at least 870,000
years to make a circuit. Don't wait up.
Written byJim Kaler 4/3/98; rewritten 6/16/15.
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