Getting to Know Them
A double star is exactly what it sounds like: two tiny beacons of light positioned close together in your scope’s eyepiece. Some of these pairings are merely chance alignments of unrelated objects, known as optical doubles, that lie at very different distances from us.
But usually there’s a true physical connection. In most cases the stars are whirling in orbits around each other, typically taking anywhere from a few decades to many centuries to complete a full twirl. These are visual binaries. (Some doubles are so close together that their orbital periods are measured in minutes! These require large observatory telescopes and special instruments to be revealed.)
In yet other systems, the stars are so widely separated that they are likely not orbiting one another at all. But they’re still related, drifting through space together as what astronomers call common-proper-motion pairs. And groupings of three or more suns are technically called multiple stars, though usually you’ll see the catchall term “double star” used to describe all these circumstances.
You can find good examples of double and multiple stars in any given part of the sky at any hour of night during the year. This is a real plus if your overhead view is limited by trees, houses, or other obstructions, as my observing site is. Also, because they’re often bright and easy to spot, these stars can be viewed on nights you find useless for looking at nebulas and galaxies due to haze, moonlight, or light pollution. And despite their profusion, no two doubles look exactly alike! They display a seemingly endless combination of brightnesses, separations, and configurations.
But it’s the perceived colors of these objects that I find especially fascinating. For me, double stars are the tinted jewels of the sky, and many lovely combinations are found among the brighter pairs. I used the adjective “perceived” because in most cases the color is illusory, a contrast effect between stars of different brightness. Yet sometimes the hues are real. True color differences between stars result mainly from differences in temperature, with red ones being relatively cool and blue ones quite hot.
A trick for enhancing color perception is to slightly defocus the view through a low-power eyepiece as the stars smear out into roundblobs, their colors become more apparent. Even pairs that exhibit no contrasting hues (which often happens when they’re of similar brightness) or no apparent color at all can still be thrilling sights. In the eyepiece they typically appear like radiant diamonds against the velvet blackness of space.
The sampling in this article describes eight of the most fascinating double and multiple stars visible in small telescopes on early evenings in September and October. In each case the dominant (primary) star is bright enough to be seen with the unaided eye from reasonably dark skies. And while every pairing can be resolved, or “split,” in a small telescope, the view typically becomes more exciting (brighter images and crisper separation) in telescopes with larger apertures.
Generally, you’ll get the best visual impact if you use the lowest magnification that just nicely separates the two stars typically 25× to 50× for wide pairings. For close-together doubles, you’ll need powers of 100× or more and a steady atmosphere (often encountered on hazy nights) to separate the components well.