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Rho (ρ) Ophiuchi

Rho (ρ) Ophiuchi  (H II 19)        HIP: 80473    SAO: 184381
RA: 16h 25.6m   Dec: -23° 27′
Magnitudes  A: 5.07  B: 5.74  C: 7.29  D: 6.81  E: 8.42  F: 11.7
Separations  AB: 3.0″ AC: 149.2″ AD: 156.4″  CF: 4.8″ DE: 0.3″
Position Angles  AB: 334°  (WDS 2016)  AC: 0° (2000) AD: 252° (2000) CF: 206° (2000) DE: 286° (2000)
Distance: 361 Light Years (Simbad)
Spectral Type: B2, B2
Note: CF is KOU 63, DE is BU 1115

You’ll find Rho (ρ) located a bit north of Antares in Scorpius, and just across the border into Ophiuchus.   And, the first thing you’ll notice in your eyepiece is a pleasing asterism shaped like a reversed open “L,” with Rho (ρ) at the center of it — or, depending on how the asterism is  oriented in your eyepiece, you might see a triangle instead.

This is a sketch of Rho Ophiuchi provided by Steve McGaughey. West is near the top at about 11 o’clock, north at the right. The AB pair is shown in the two insets, the C component is the star lying due north from the AB pair, and the D component is seen in the sketch above and to the left of the AB pair.  Not shown here is the F component, which lies a mere 4.8″ to the southwest of C.  With magnitudes of 7.29 and 11.7, you’ll need averted vision to pry F out of the glare of C. Click to enlarge and get the full effect of the sketch!

But however you see it, you’ll find this is a fine, yellow-tinged double that requires a bit of magnification in a 60mm scope in order to split it.  Using a sixty millimeter f/16.7 (focal length of 1000mm) and a 25mm Plössl (40x), the primary and secondary were just touching.  Doubling the magnification to 80x with a 12.5mm Ortho, I had no problem separating them provided the seeing was relatively steady —- which it wasn’t on the first night out, but was much improved a couple of nights later.  In an 80mm f/15, I was able to get a clean split at 48x with the 25mm Plössl.

According to the information on Jim Kaler’s web site , these are LARGE stars!  The 5.1 magnitude primary has nine times the mass of our sun and is 4900 times more luminous.  The slightly dimmer secondary is not very far behind it with a mass eight times greater than the sun and is 2100 times more luminous.  Rho (ρ) is surrounded by dark clouds of dust which makes it appear two magnitudes fainter to us than what it would otherwise, and also lies in a reflection nebula illuminated by surrounding stars – none of which, unfortunately, is visible in a small refractor.


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