Spring sky

Springtime Double Stars in Boӧtes

The Herdsman boasts several rewarding pairs for small telescopes.

High in the spring evening sky is Arcturus, shining brightly from Boötes, the herdsman. This whole region is popular with double-star enthusiasts. Starting in constellation’s far north we encounter Iota (ι) and Kappa (κ), a pair of doubles you might think belong in Ursa Major. Indeed, they’re just a few degrees from Alkaid, the star that marks the end of the Big Dipper’s handle. In a wide field eyepiece you can squeeze both Iota and Kappa Boötis into a single view. Each is a nice double. Kappa is the tighter pair (13.5 arc seconds apart), while Iota’s components are separated by a generous 38.7 arc seconds. Kappa’s stars also display a striking colour contrast — the 4.5-magnitude primary is golden, while the 6.6-magnitude secondary has a blue-green hue.

Bootes double-star chart
Use this chart to locate the double stars (labelled in yellow) featured in this article. (Click on the image to display a larger version.)

Moving southward, we see Epsilon (ε) Boötis (also known as Izar). It’s a close double, and among the toughest to split because it’s so bright. Izar’s components are only 2.9 arc seconds apart, and with the brightness of the 2.6-magnitude primary flooding the vicinity, you’ll need a night of very steady seeing and top-notch transparency to see the 4.8-magnitude secondary.

Now we move to Pi (π) Boötis, which I’ve dubbed “the lonely pair” because there seem to be no other stars in the field of view. The double’s 4.9- and 5.8-magnitude white components are 5.5 arc seconds apart. By comparison, nearby Xi (ξ) Boötis is in a relatively crowded field of stars. Xi’s 4.8-magnitude primary is golden while the 7.0-magnitude secondary, blue. They’re 6.3 arc seconds apart.

For the grand finale of this brief Boötes tour, we again head north to the delightfully named Alkalarops. (Sounds like something you might uncover in your garden and want to eradicate, doesn’t it?) Alkalarops is actually an old Arabic word that means “the herdsman’s crook or staff.” Also designated Mu (μ) Boötis, this one is a triple star. The main pair can be split easily in binoculars — they’re 107 arc seconds apart and shine at magnitudes 4.3 and 7.1. But look closely at these two components and you’ll see that the fainter one, Mu², is a double in its own right, sporting a 7.6-magnitude partner. You’ll need high-power and steady seeing for this pair, as they’re separated by just 2.2 arc seconds.

David A. Rodger is an editor, a writer, broadcaster and an amateur astronomer living in North Vancouver, British Columbia. From 1967 to 1980, he served as the first director of Vancouver’s H.R. MacMillan Planetarium.