Appendix A: Binocular Double Star Program Observing List
Read across the column headings from left to right. Stars are listed by the Constellation in which they are located, their Designation, their celestial coordinates of RA (Right Ascension) and Dec. (Declination), the Magnitudes of the stars within the system with the primary (brightest) star noted first, the Separation of companion stars in seconds of arc (“), and the Position Angle (PA) of companion stars in degrees (°) relative to the primary.
Most primary stars on the double star list are identified by their traditional Greek letter and/or by their number designation and will be found on many star charts as well as night sky software programs. Star charts label the brighter stars in constellations with Greek letters and proper names where they exist (Sirius, Arcturus, Rigel, etc). Somewhat dimmer stars are identified only by their numbers, which customarily progress higher from west to east within a constellation. Stars on the list that are fainter still, which have no assigned Greek letters or numbers, are referenced by the symbols or abbreviations commonly used in double star catalogs to designate their discoverers. (For information on the Greek alphabet and double star abbreviations, see the end of this appendix.)
The majority of primaries on the list have magnitudes of six or brighter and should be readily found on charts displaying stars to that value. These stars should also be visible to the naked eye under dark skies. Star charts represent stars as disks of varying sizes, the brighter the star the larger the disk. Double stars are shown as disks bisected with horizontal lines, so even if the star bears no label on the chart, it should be identifiable based on that symbol and its chart location according to its RA and Dec. coordinates on the list. Wide star pairs may appear on charts as individual disks, separate or overlapping, without lines.
The Bright Star Atlas 2000.0 by Wil Tirion and Brian Skiff (Willmann-Bell, 2006) is an affordable, general-purpose atlas of the heavens that also lists hundreds of double stars to a magnitude of 6.5. An excellent, spiral bound book highly recommended for double star enthusiasts is The Cambridge Double Star Atlas by James Mullaney and Wil Tirion, (Cambridge University Press, 2009). This book also plots Messier and other deep sky objects. James Mullaney’s Double and Multiple Stars and How to Observe Them, (Springer, 2005) and Observing and Measuring Visual Double Stars, by Bob Argyle, (Springer, 2004) are also valuable resources on the subject.
Once a pair’s primary is located, companions in the grouping will usually be the next brightest and closest stars in the field. If necessary, the position angles of the group’s members will assist you in determining which stars you are seeing. A position angle denotes the relationship of the companion stars to the primary star, in degrees, as seen from earth. As you observe, take note of your position relative to north. When you look through your binocular, place the primary star in the center and draw an imaginary circle of 360° around it running counter-clockwise with north representing 0°, east 90°, south 180° and west 270°. Visualize this circle fastened around the periphery of your binocular field. Use this method and reference the position angle from the lists to identify the other star(s) in the grouping.
Many systems are, in fact, multiple groupings of three or more stars, even though only the brighter members are noted on the list. Fainter companions that do not appear on the list may be detected with larger binoculars, by observers with excellent vision, or under ideal seeing conditions. Be sure to make a note of these sightings on your log.
While not specified on the list, these double stars may be gravitationally bound to one another (binary), share the same common proper motion (CPM) through space, or simply be chance optical alignments. However, all can be pleasing sights in a binocular.
If you enjoy observing the double stars on this list and would like to know more about observing double stars as well as other deep sky objects, you’ll find lots of useful information at The Webb Deep-Sky Society site.
My thanks to fellow Minnesota Astronomical Society members John Marchetti, Dave Tosteson, and Dave Venne for their valuable assistance in compiling the following observing list.