EPS HYA (Epsilon Hydrae). One of the more obscure
constellations of the sky, Hydra (the
Water Serpent) is known mostly for its bright Alpha star, Alphard, which lies to the south and a bit
west of Regulus in Leo. South of Cancer
lies the Water Serpent's head, made of a tiny distorted circlet
whose stars are (from Arabic) collectively called (from Allen) "Min
al Az'al, belonging to the uninhabited spot." None of the stars,
including Bayer's Epsilon, has a proper name to itself, though
Epsilon is singled out as the most northerly of them (while beat
out in brightness by Zeta just to the east of it). Appearing
undistinguished to the eye, "Eps Hya" is a remarkable multiple star
with at least five members. The principal star is a fourth
magnitude yellow-white class G (G5) giant that has a very close
(0.2 seconds of arc away) white class A (A5) subgiant fifth
magnitude (4.7) companion that together make the third magnitude
(3.38) naked eye star. They orbit each other every 15.05 years at
an average separation of 10.5 Astronomical Units, about the
distance of Saturn from the Sun, on an eccentric path that takes
them from 17.4 AU to as close as 3.6 AU (that point passed in
1991). The orbit gives the two a total mass of 5.1 times that of
the Sun. The G star, at 4900 Kelvin, shines with a light of 57
solar, the A star (at 7700 Kelvin) 17 solar, which gives it a mass
of double the Sun (and showing that it is
really a hydrogen-fusing dwarf). The G star therefore contains 3
solar masses, which makes it a "transition star" with a dead helium
core that is preparing to become a brighter giant. The G star is
somewhat magnetically active, and has been detected as an X-ray
source. Separated from this close double by 4.5 seconds of arc is
a class F (F5) dwarf that takes 900 years to orbit the inner pair
at a distance of about 190 AU. At 6600 Kelvin, it shines 2.5 times
as brightly as does the Sun, but carries just over a solar mass. It too
is double, its components even closer, just 0.09 AU in a period of
a mere 9.9 days. Farther out (19 seconds of arc away) is a dim
10th magnitude star that orbits the inner quartet. From its
brightness, it must be a class M dwarf with about a third of a
solar mass. Physically some 800 AU away, it must take close to
10,000 years to make a full circuit. In a loose way, the system is
similar in structure to Mizar and Alcor in Ursa Major,
with an inner double-double and an outlier, though there is some
indication that Epsilon Hydrae may have yet another member. From
the closer pair, the brighter pair would have a separation of some
three degrees, the G star shining with the light of 500 full Moons.
From the brighter pair, the closer pair would be just resolvable
with the human eye.