During spring and summer nights, the double star Iota Boötis shines down from the northern constellation of Boötes. The two stars are separated by approximately 39 arcseconds of space, making the pair visible to those with smaller telescopes—and maybe even binoculars. Additionally, this duo is located nearly 95 lightyears away.
The primary star, Iota Boötis A, is a main-sequence star with a stellar classification of A, shining down from the heavens at a magnitude of +4.75. Being visible to the naked eye, it can be observed from darker areas without light pollution. On the other hand, Iota Boötis B, which is situated 38.6 arcseconds away, is a cooler star with a stellar classification of K0V and appears much dimmer at magnitude +7.4.
Much like my observation of nearby Kappa Boötis (Asellus Tertius) and Struve 1835, I find it interesting that several online observers, including myself, report Iota Boötis A as yellow in color when one would expect a hotter temperature blue star due to the star’s “A” stellar classification. In a similar manner, I see the companion Iota Boötis B looking blue (or lilac in particular), while it’s actually a cooler “K” star. This all goes to show how our eyes can play tricks on us with double stars, which is a science in itself!
I should point out that this double actually has a third dim companion that’s tough to see. John Nanson provides a nice overview of this elusive magnitude +12 star on his Star Splitters site.
Historically speaking, the astronomer William Herschel first recorded this double on September 27, 1779.1
My Observations
| Date | July 15, 2023 |
| Time | 11:00 P.M. |
| Location | Seattle, WA |
| Magnification | 169x |
| Scope | Meade 8″ SCT |
| Eyepiece | 12mm |
| Seeing | Average |
| Transparency | Above Average |
The primary star sparkles mostly white, but appears to have a yellow tinge to it. The companion nearby is a dim white or lilac (blueish perhaps?) in color. I don’t see any other close stars in the field as I look for the 12th-magnitude third companion. My suburban backyard skies hide this elusive star behind a veil of light-washed skies.
Key Stats
| Constellation | Boötes |
| Best Viewing | Spring |
| Visual Magnitude | +4.7, +7.4 |
| Separation | 38.9″ |
| Position Angle | 33° |
| Spectral Class | Primary=A, Sec=K0V |
| Absolute Visual Magnitude | +2.43 |
| Distance from Earth | 94 ly |
| Milky Way Location | Orion Spur |
| My Viewing Grade | B |
| Designations | Struve (STF) 26, SAO 29071, HIP 69713, H V 9 |
Sources and Notes
The banner photo of Iota Boötis is by David Ritter – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=99109200
1William Herschel’s double star catalog. (n.d.-a). http://www.handprint.com/ASTRO/Herschel_All.html
Sketch by Wayne McGraw
The Garden Astronomer 