Struve 649: A White-Blue Primary with a Dim Blue Companion in Eridanus

Struve 649 is a double star featuring a fairly bright primary white-blue colored star with a dimmer blue companion nearby. This quaint pair lies in the constellation Eridanus, the river, best viewed in winter. With the wealth of double stars and deep-sky objects in popular constellations in winter, it may be easy to pass over the southern celestial hemisphere constellation Eridanus. But you should for sure pay the river a visit as it has several nice objects, including Struve 649, which astronomer William Herschel first recorded on October 27, 1781.¹

The primary star in Struve 649 is a variable star lingering around the 5th magnitude. This blue-white star has a stellar classification of B8V. At a friendly distance of 21.3 arcseconds away, the secondary star is more blue in color and is much dimmer at a magnitude of +9. This nice pair makes their home 681 light years away from us. Notably, these two stars are not physically related, so they don’t orbit a common center of mass.

My Observations

My fingers are tightening up a bit as I sketch this double. This late November night has brought on some beautiful skies along with a freezing night. Come on, fingers, don’t fail me now! While this double is not really remarkable, I do like how the bright white-blue primary shines beside a fairly dim blue companion. These two stars are easily split as well.

Key Stats

Constellation	Eridanus
Best Viewing	Winter
Visual Magnitude	+5.8 | +9
Separation	21.3″
Position Angle	69.3°
Spectral Class	B8V
Milky Way Location	Orion Spur
My Viewing Grade	B-
Designations	STF 649, SAO 131806, HD 33224, HR 1671

Sources and Notes

Banner adapted from an illustration created by Pablo Budassi. The illustration is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. Full details here.

¹ William Herschel’s double star catalog. (n.d.). https://www.handprint.com/ASTRO/Herschel_All.html

Sketch by Wayne McGraw