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.
Written by Jim Kaler. Return to STARS.