ALMACH (Gamma Andromedae). You take your new telescope to the back
yard perhaps wondering what to examine. When finished with the
Moon and the bright planets you turn to the stars, first perhaps to
the grand Orion Nebula, next maybe to
the magnificent Andromeda Galaxy.
Then it is time for double
stars. The sky abounds with them, northern winter's Castor, springtime's Mizar and Alcor,
summer's Albireo (the seasons reversed for
the southern hemisphere), dozens of others easily found. Among the
best of all, however, is the last star of the string of bright
beauties that helps make the constellation Andromeda, second magnitude (2.16) Almach, Andromeda's
Gamma star. The Arabic name, which has nothing to do with its
constellation of residence, refers a kind of middle-eastern wild
cat. Through the telescope the star is extraordinarily lovely,
even a small instrument showing a superb pair separated by a good
10 seconds of arc, the brighter one golden yellow the other blue.
Star colors are usually subtle, rather washed out. But put two
contrasting stars close together and they play against each other,
the colors becoming far more vivid to the eye. Admiral Smythe, who
wrote the definitive nineteenth century book on celestial sights,
refers to them as "orange and emerald green." The second magnitude
(2.26) brighter component, called "Gamma-1," is a class K (K3)
bright giant with a temperature around 4500 Kelvin and a star now
in the act of dying. From its distance of 355 light years, we find
a luminosity about 2000 times that of the Sun and a radius 80 solar, big enough to take
the star to the orbit of Venus. More remarkable, the fainter blue-
green component, Gamma-2, is ALSO double, though the duplicity is
far more difficult to see. Fifth (5.1) and sixth (6.3) magnitude
white hydrogen-fusing dwarfs (with respective temperatures of about
12,000 and 10,000 Kelvin) orbit each other with a period of 63.7
years separated on average by but 0.3 seconds of arc, which
translates to 33 Astronomical units. A fairly high eccentricity
takes them as close together as 13 AU and as far apart as 52 AU.
Yet again the system splits, as the brighter of these two is ALSO
double, though detectable only with the spectrograph, the components very
close and orbiting every 2.7 days, Gamma-2 thus triple. The orbit
gives a combined mass for the three stars of 8.7 times that of the
Sun, which is consistent with advanced B8 and A0 dwarfs joined with
a lower mass class A7 (or so) dwarf star about which nothing is
known. The naked-eye star we know as Almach is thus quadruple,
making it a feast for both the mind and for the eye. One or two
other more distant stars might belong to the system as well.