Gamma Delphini: You’ll find this lovely tinted gem marking the “nose” of the little diamond-shaped constellation Delphinus, the Dolphin. A 3- or 4-inch telescope at 50× shows a neat close pairing of stars that differ slightly in brightness. Observers have noted various hues, with most seeing a deep yellow primary and pale emerald green companion. Viewed in a small telescope at a power of at least 150×, those pinpoints of light become spurious diffraction disks that make the colors more obvious. Astronomers have found little indication of orbital motion, but the stars are drifting through space together as a system.

Enif (Epsilon Pegasi): Here’s another “nose,” this time in Pegasus, the flying celestial Horse. While not a striking double in itself, I’ve included Enif here as one of the few deep-sky objects that “does something” while you watch it. A faint bluish violet companion attends the yellowish and much brighter primary at a wide distance. Gently tapping or shaking your telescope tube causes the dimmer star to appear to swing back and forth like a pendulum. This phenomenon was first noticed by the great 19th-century observer John Herschel, who correctly deduced that light from the fainter star takes longer to stimulate the retina, so its motion appears to lag behind that of the brighter star. While you can see this effect in a 4-inch scope at 50×, it becomes ever more striking as aperture increases. Skygazers have nicknamed Enif the "Penulum Star."

Delta Cephei: At the upper-right corner of the house-shaped constellation Cepheus, poised on its pointy roof high in the northern sky during autumn, lies an object with a dual personality. It’s an attractive, widely separated double star that might remind you of Albireo. While its contrasting pale orange and pale blue tints are a fine sight in a 3-inch scope at 30×, you'll find that they are no match for Albireo’s vivid hues. But Delta is not only a double star, it’s a pulsating sun as well — the prototype of the Cepheid variables routinely used by astronomers to measure distances to nearby galaxies. Every 5.4 days Delta’s primary star cycles in brightness by nearly a full magnitude (a factor of 2½ times) — obvious even to the unaided eye.

Almach (Gamma Andromedae):This radiant duo sits at the end of the curved line of stars marking Andromeda, in the northeastern sky in fall evenings. Its magnificent topaz and aquamarine tints are a joy to behold in any telescope. The bright primary and fainter secondary lie snugly together, but even a 2-inch aperture at 60x separates them. Many observers feel that Almach’s colors are more intense than those of rival Albireo — and I agree! Since both doubles can be observed at the same time in the autumn sky, why not compare them yourself? While no definite orbital motion has been detected, Almach’s stars are moving through space together.

As these fine examples attest, you can dabble in double stars using even the smallest telescopes. Take it from me: until you’ve tried “double-stargazing,” you’re simply missing out on some of the brightest and most colorful wonders the heavens have to offer.
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James Mullaney has logged more than 20,000 hours of stargazing time and is the author of Celestial Harvest: 300-Plus Showpieces of the Heavens for Telescope Viewing and Contemplation (Dover, 2002).

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