Struve 2840: An Easy to See Blue and White Pair in Cepheus

The double star Struve 2840 is located in the northern sky within the constellation Cepheus. The constellation takes its name from Cepheus, a mythical king in Greek mythology, and it boasts several remarkable autumn celestial objects, including this pair first observed by William Herschel on September 27, 1782.

Backyard astronomers should find it pretty easy to separate Struve 2840’s two stars, unlike the more challenging nearby pair of Omicron Cephei. Stuve 2840 features a bright blue-white primary with a magnitude of +5.6 with a stellar classification of B6IV-V. At a reasonable 18 arcseconds away is the dimmer companion star with a magnitude of +6.4. The pair lies 675 light years away and is not physically related; they don’t orbit a common center of mass.

My Observations

This pair is easily distinguishable unlike nearby Omicron Cephei, which I observed a few minutes ago . On first glance, both stars appear white to me. As I let my eyes adjust, I detect a slight blue tinge in the brighter primary, while the secondary may have a yellow tinge to it. Overall, this is a welcoming pair to view.

Key Stats

Constellation	Cepheus
Best Viewing	Autumn
Visual Magnitude	+5.6, +6.4
Separation	18.1″
Position Angle (between AB & C)	197°
Spectral Class	Primary=B6IV-V
Distance from Earth	675 ly
Milky Way Location	Orion Spur
My Viewing Grade	B+
Designations	STF 2840, SAO 33819, HD 208063, HR 8357

Sources and Notes

The 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.

21520+5548 STF 2840 AB (NSV13909). WDS 21520+5548 STF 2840 AB (NSV13909). (n.d.). https://www.stelledoppie.it/index2.php?iddoppia=96955

Sketch by Wayne McGraw