Star of the Month - October 2017

Pretty much everyone in HAL knows that I have long been a huge fan of double stars. Indeed, the very first deep sky object that I recall ever seeing through a telescope was the venerable double star Albireo in Cygnus. My friend Craig Covault (of Aviation Week and Space Technology fame) had just been given one of the cheaper varieties of “department store” telescopes (a 3-inch refractor, if I remember correctly) as a birthday present by his wife Nancy. Her heart was in the right place, not knowing any better. And as a matter of fact, neither did we. In blissful ignorance of the instrument’s grave shortcomings, we set it up as soon as it got dark in the front yard of their Vienna, Virginia home. (This was in 1981, so the light pollution there was nowhere near as bad as today.) Craig knew enough to look for Albireo (I had never heard of it), and focused in on the star. When it came my turn to look through the eyepiece, I literally gasped. I was totally unprepared – not only for the wonder of the sight, but for the sheer beauty of it. The electric blue and warm gold of the star’s two components were like nothing I had ever expected; I could scarcely accept their reality. I was hooked for life.


To this day, I make a point of including a fair amount of double star viewing nearly every time I go out to observe. A night at the eyepiece just doesn’t seem complete without them. I have far too many favorites to list, but they include Albireo (still!), Iota Cancri, Rigel, the Double-Double in Lyra (of course), 18 Geminorum, Struve 2398, and many, many others too numerous to list… including this month's recommendation - Gamma Andromedae (a.k.a., Almaak, or even Almach).


Almaak is ridiculously easy to find in the night sky. Start with the star Alpheratz at the upper left corner of the Great Square of Pegasus. The constellation of Andromeda extends outward from that point, looking (to my eyes, at least) like a somewhat bent cone (see illustration).

Almaak is the last star to the left on the lower leg of the cone. Its 2.26 magnitude makes it easily visible even from my extremely light polluted location of downtown Baltimore (from which I am lucky to make out a mere dozen stars in the sky on a given night). A naked eye viewing may reveal little of its magic, but even a modest sized telescope (say, a 4 inch refractor) will present to the observer a true spectacle. What you will see is an easily splittable double star of amazing color. A bright golden yellow primary attended by... get this... a GREEN companion star. Yes, yes. I know there are no such things as green stars, but bear with me here. It turns out that γ² Andromedae (the designation of the companion star) is itself a triple star, composed of a primary, orbited by two smaller suns that themselves orbit each other. So there are four stars in all making up what our unaided eye sees as one. If this seems too complicated, perhaps this schematic (not to scale) will help.

You may note that none of the stars that make up γ2 Andromedae are green. But even in the largest telescopes, they appear as a single point of light. And somehow their combined colors of blue, yellow, and orange combine in a way that the human eye perceives a lovely emerald hue.

There are other (apparently) green stars out there, but this is the most striking example.