Galaxies abound in the deep sky of northern autumn and imagers and visual observers with big telescopes and dark sky have plenty of choice targets. But for us urban visual observers with smaller scopes, galaxies, despite shining with the combined light of several hundred million stars, usually look a little underwhelming. So what’s left to see? Double stars, of course! Often overlooked for flashier sights, double (and multiple) stars offer a great challenge, contrasting colours, and understated but still gasp-worthy beauty that doesn’t take long to appreciate. Here are five superb double stars for northern autumn observers that will make you a fan of these under appreciated objects, listed approximately in order of easiest to hardest to observe.
Albireo. Arguably the prettiest double star in the heavens, Beta Cygni lies, depending on how you look at it, at the base of the large Northern Cross asterism or at the beak of the constellation Cygnus, the Swan. With a separation of 35”, you can split the star in 10×50 binoculars but it looks better with a little more magnification. The 3rd-magnitude primary shines a golden yellow while the 5th-magnitude primary has an unmistakable blue-white color. Lower magnifications, say less than 50x, show the colours nicely. Try defocusing your scope slightly to see the colors better. This one’s an old favourite – I’ll look at it any and every night I’m out with the scope. Astronomers are still trying to figure out if these two stars form a true gravitationally bound double star of if they’re just in a chance alignment. Both lie approximately 400 light years away.
Almaak. Also known as Gamma Andromedae, this lovely double star has resembles Albireo with a golden-orange primary and blue-white secondary component. And like Albireo, this pair is about 400 light years away. The pair are closer, however, about 9.4” apart, and appear to form a true binary system that revolves around a common centre of mass every 5,000 years. The pair make a very fine sight in a small telescope at 50x-100x or so with the primary shining at magnitude 2.3 and the secondary at 4.8.
Eta Cassiopeiae (Achird). The components have a larger separation than Almaak, about 13”, but they require a little more care to spot them because of their brightness differential. The primary, a G-type main sequence star very much like our Sun, shines at magnitude 3.5 while the yellow-orange K-type secondary is four full magnitudes fainter. This pair has an orbital period of 480 years and lies at a distance of just 19 light years. Our Sun would look very much like the primary of Eta Cass if it were the same distance away. (Fun fact – for an observer in the vicinity of Alpha Centauri, the nearest star system to Earth, the Sun would be a bright star located in the constellation Cassiopeia).
Sigma Cassiopeiae. Now we get to a more challenging double. Located at the top (the west end) of Cassiopeia near the open star cluster NGC 7789, Sigma has components of magnitude 5.0 and 7.2 separated by 3.1”. It splits nicely at about 130x or more with both components appearing whitish-blue. The pair lies about 1,500 light years away.
Struve 7. OK, this is a tough one. This pair has components of magnitude 8.0 and 8.5 split by just 1.3”. I’ve just managed to split this double in moments of good seeing with a 102mm f/11 ED refractor at 172x. There’s not much color here – the pleasure comes from resolving this tight and obscure pair. More aperture will help split this star – if you have steady sky. The pair sits at the vertex of a squat triangle formed with Alpha Cass and Beta Cass and lies about 2º east southeast of the lovely open star cluster NGC 7789 and not far from Sigma Cass. This pair is about 2,000 light years away. It may appear on your star map or planetarium app as HD709.
Give these stars a try yourself. They, like other double stars, are perfect for taking in during a short observing session after dinner on a cool autumn night.
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