Rho Herculis: A Close Pair that Teases Different Colors

Rho Herculis (ρ Herculis) is a double star in the constellation Hercules, which is best viewed in summer.

According to Jim Kaler, Rho Herculis may be a multiple-star system with up to four stars. In the telescope, however, you’ll see a fairly close system made up of two stars called Rho Herculis A and B, which are separated by four arcseconds in the sky. You’ll have to experiment with several medium to higher-power pieces to separate the stars clearly.

Also, Rho A and B are similar in brightness and type. Most observers report both stars as being white, which they truly are. However, since they are close, observers may see them as something other than white. For example, Jim Kaler writes that “Admiral Smythe’s 19th century “A Cycle of Celestial Objects” refers to the brighter as ‘bluish-white,’ the fainter as ‘pale emerald.’ There seem to be no actual temperature measures, so we adopt 10,500 and 10,000 for “A” and “B” from their classes.” ¹

I fall in line with Admiral Smythe’s position as I didn’t see both as white. Read my observation below.

My Observations

I begin this evening turning back toward this area in Hercules. I notice the time—it’s only 15 minutes after 10, and I’m already able to view my first double.

Rho is a close double, and while it’s not a terribly remarkable set of stars, the pair are vibrant and almost jump out at you. The rest of the eyepiece field is pretty dark, so they stand out well. What’s interesting is that I don’t see them as similarly white in color. Instead, the primary and slightly larger Rho A appears off-white, while Rho B appears a pale blue-green. I stared for around 30 seconds, and during that time, I even thought Rho B looked faint yellow! Rho B can play games with your eyes—so watch out!

Key Stats

Constellation	Hercules
Best Viewing	Summer
Visual Magnitude	+4.6 | +5.4
Absolute Visual Magnitude	-.01
Distance from Earth	520 ly
Separation	4.1″ | 648 AU
Orbital Period	4600+ years
Milky Way Location	Orion Spur
My Viewing Grade	B

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 Rho Herculis, http://stars.astro.illinois.edu/sow/rhoher.html.

AKA: ρ Her, Σ 2161, HIP 85112, SAO 66000, STF 2161, HR 6484