Rho Orionis (Struve 654): An Orange-Yellow Double in Orion

The double star Rho Orionis, also known as Struve 654, lies in the winter constellation Orion. The pair shines down from 350 light years away.

This double star was first observed and recorded in 1825 by astronomer Friedrich Georg Wilhelm Struve. It consists of two stars that are not physically tied to each other. The primary star, an orange-yellow giant with a spectral type of KRII, is 25 times larger than our sun. Nearby is the dimmer companion at magnitude +8.5 that sits only 6.5 arcseconds away. When viewed through a telescope with a low-power eyepiece, the binary appears as a single star. Employing a higher power will help separate the two stars nicely.

My Observations

I’ve been in a comatose state following Thanksgiving dinner earlier, so much so that I’ve lacked the energy to set up my scope for stargazing. However, I am grateful that I made the effort. Tonight’s skies are so clear, though a near-full moon fills up the sky with light.

It’s good the skies are so transparent as I’ve landed on a few tight doubles, including Struve 742 a few minutes ago. On landing on Rho Orionis, I first noticed the bright orange color of the primary star. Nestled fairly close is the secondary star that appears off-white (perhaps with a tinge of blue). Apparently, there is a third star (11th magnitude) companion, but I don’t see that in the sky filled with moonlight.

Key Stats

Constellation	Orion
Best Viewing	Winter
Visual Magnitude	+ 4.6 | +8.5
Separation	6.7″
Position Angle	64°
Distance	350 ly
My Viewing Grade	B
Designations	Rho Ori, STF 654, SAO 112528, 17 Orionis

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.

05133+0252 STF 654 ab (Rho Ori). WDS 05133+0252 STF 654 AB (Rho Ori). (n.d.). https://www.stelledoppie.it/index2.php?iddoppia=18355