OBSERVER’S CHALLENGE* – December, 2022
by Glenn Chaple
iota Cassiopeiae Triple Star (AB Magnitudes 4.6+6.9, Separation 2.9”, Position Angle 230o; AC Mags 4.6+9.1, Sep 6.7”, PA 117o)
Backyard astronomers who favor gossamer deep sky targets like galaxies and nebulae eagerly await the dark, ultra-clear nights that bring these “faint fuzzies” to light. The same can’t be said for the double, triple, and multiple star aficionado, as such evenings are also marred by poor seeing conditions which render the splitting of close stellar partnerships all but impossible.
Such is the case with this month’s Observer’s Challenge, the triple star iota Cassiopeiae. The separations of its three members aren’t the problem. Components A and B are about 3 arc-seconds apart, while a little under 7 arc-seconds separate A and C - separations well within reach of a common 60mm (2.4-inch) refractor. The difficulty lies in the magnitude differences between these stars. The main component, Iota Cassiopeia A (magnitude 4.6), is 8 times brighter than B (magnitude 6.9) and over 60 times brighter than C (magnitude 9.1). You’ll need steady seeing and a reasonably high magnification to bring all three to light.
Locating iota Cassiopeia is no problem at all. A 5th magnitude star to the unaided eye, it’s found by tracing an imaginary line from delta through epsilon – both part of the Cassiopeia “W” – and extending it an equal distance beyond.
My first observation of iota Cassiopeiae was on the evening of October 18, 1971. Encouraged by the fact that I had already split several reasonably close, unequal pairs with my 3-inch f/10 reflector, I decided to give it a try. Despite its faintness, the C component was glimpsed at 60X, but B remained elusive, even at higher magnifications.
It was the Observer’s Challenge that brought me back to iota Cassiopeiae early this past November. On successive evenings, first with the trusty 3-inch and then with a 4.5-inch f/8 reflector, I looked for the B component without success. Skies were clear but slightly turbulent on both occasions. On the third evening, there was the slight haze that often comes with nights of good seeing. Taking no chances, I pulled out a 6-inch f/8 reflector. The one-degree field of my 43X “search” eyepiece showed iota and two 8th magnitude stars to its east – an attractive sight. A switch to 133X did the trick – all three of iota’s component stars were visible. I boosted the magnification to 200X and made an eyepiece sketch. The C component seemed slightly reddish to me, an impression later borne out when I learned that it’s an extremely close binary pair comprised of a K spectral class star and a K or M class companion.
Not only is iota Cassiopeiae C a tight binary pair, but so is iota Cassiopeiae A. Each of these sub-arc-second duos was discovered through the magic of modern-day adaptive optics. This remarkable system lies some 140 light years away.
*The purpose of the Observer’s Challenge is to encourage the pursuit of visual observing. It is open to anyone who is interested. If you’d like to contribute notes, drawings, or photographs, we’d be happy to include them in our monthly summary. Submit your observing notes, sketches, and/or images to Roger Ivester (email@example.com). To find out more about the Observer’s Challenge, log on to rogerivester.com/category/observers-challenge-reports-complete.
iota Cassiopeiae Finder Chart