Struve 2404 is a tight attractive double star in the summer constellation of Aquila, which lies on the celestial equator. The Russian astronomer Otto Wilhelm von Struve first observed this double star in 1822.
This pair features a magnitude +6.9 primary star with a beautiful warm orange hue. The orange-red giant star has a stellar classification of K5III and is nearly 60 times the diameter of our sun! The secondary star lies closely by at 3.6 arcseconds away and shines at a magnitude of +7.8. Other observers will mostly find, as I did, that a Barlow 2x lens will help in splitting the two stars. Astronomers are uncertain if Struve 2404 is a true binary star orbiting a common center of gravity.
My Observations
| Date | September 15, 2023 |
| Time | 8:57 P.M. |
| Location | Seattle, WA |
| Magnification | 169x |
| Scope | Meade 8″ SCT |
| Eyepiece | 12mm |
| Seeing | Above Average |
| Transparency | Average |
This is a tight pair of stars, so I combined my 2x Barlow with the 12mm eyepiece. The primary is a warm orange star with a dim secondary. I’m struggling to decide what color this dimmer star is. According to other observers, this secondary star can appear yellow in color, and I tend to agree. What’s interesting is that I first thought it was white with blue tint. I’ll have to return to this pair and take more time studying the colors.
Key Stats
| Constellation | Aquila |
| Best Viewing | Summer |
| Visual Magnitude | +6.9 | +7.8 |
| Separation | 3.6″ |
| Position Angle | 180° |
| Spectral Class | A=K5III | B=K3 |
| Distance from Earth | 1259 ly |
| Milky Way Location | Orion Spur |
| My Viewing Grade | B+ |
| Designations | STF 2404, HD 174569, SAO 104170, HR 7099 |
Sources and Notes
Banner illustration was created by the European Southern Observatory (ESO) and is under the Creative Commons Attribution 4.0 International License. Per ESO agreement: Here is the original image on the ESO website.
Sketch by Wayne McGraw
The Garden Astronomer 