Seeing Double Double

Epsilon Lyrae, the "Double Double"

We all of course know of the iconic Double Double (Epsilon Lyrae) next door to brilliant Vega. Many of us use this star as a test of our instrument's resolving power, or of the steadiness of a night's air. But have you ever taken a look at the other double double, over in Boötes? I am speaking of Iota and Kappa Boötis, or Asellus Secundus and Asellus Tertius (a.k.a., 2nd and 3rd Donkey Colts). Not a true "double double", since Iota Boötis is a mere 95 light years away, whilst Kappa Boötis stands off at more respectable 151 light years. A chance line of sight alignment is what allows us to see them side by side.

Iota and Kappa Boötis 

As sketched by someone calling himself "asteroid7" on Cloudy Nights

Both both components of this charming mashup are themselves doubles - and easily splittable ones at that. The A and B components of Iota Boötis are separated by 38.8 seconds of arc and 1130 AU of actual distance. (A single orbit of the two stars about a common center takes a whopping 24,000 years!) Together, they have an apparent magnitude of 4.75 making them a naked eye star. Meanwhile, the two components of more distant Kappa Boötis (known as Kappa-1 and Kappa-2) are separated by a tighter 13.5 seconds of arc and a mere 640 AU of actual distance. They orbit each other once each 8700 years, and are themselves each double stars (making Kappa Boötis a true "double double", but observable as such only spectrographically). The combined apparent magnitude of the entire Kappa Boötis system is 4.53, remarkably close to that of Iota Boötis.

As usual with double stars, different observers report different colors, so I won't "poison the well" here, and allow you to come away with your own impressions.

Iota and Kappa Boötis are the two stars at the top right of this rather simplified star chart