Cor Caroli (Alpha Canum Venaticorum) (Σ 1692) (H IV 17)
HIP: 63121 SAO: 63257
RA: 12h 56m Dec: +38° 19′
Magnitudes: 2.9, 5.5
Position Angle: 228° (WDS 2011)
Distance: 82 Light Years
Spectral Type: A0, F0
Named after King Charles II of England, Cor Caroli, or “Charle’s Heart,” is a splendid sight in a 60m f16.7 refractor at 40x — a bright yellow primary accompanied by a small, white, dot-like secondary with a tinge of blue to it! I also had a 152mm f8 refractor out on this night, which is quite a bit more aperture than necessary for this pair, but I noticed that the primary appeared more white than yellow in it at 76x. The difference in visual size is matched by their luminosity – the primary is eighty-three more times luminous than our sun, while the secondary is only five times brighter.
Σ 1723 Σ I 24 (15/17 Canum Venaticorum)
HIP: 64104 SAO: 63362 HIP: 64246 SAO: 63380
RA: 13h 08m Dec: +38° 44′ RA: 13h 10m Dec: +38° 30′
MG: 7.3, 8.6 Sep: 7″ PA: 8° MG: 6.0, 6.3 Sep: 278″ PA: 277°
Distance: 668 Light Years Distance: 1144 LY (15 CnV), 202 LY (17 CnV)
Spectral Type: K0 Spectral Type: F0, B9
My long-distance observing companion, Greg, mentioned a few days ago that if you let Cor Caroli drift out of view, another double would come into view in two minutes. Sure enough, Σ I 24, also known as 15/17 Canum Venaticorum, came sliding in from the right side of the eyepiece. Actually, as I noticed later, with Cor Caroli placed over toward the left side of the view, Σ I 24 can be seen off to the right in both the 60mm scope and the larger 152mm refractor as well. This is a wide pair, easily split, but the first thing you notice is that in contrast to Cor Caroli, the two stars are quite a bit dimmer. Haas describes them as “pearly white” and “sapphire white,” but due to the dimness, I would leave it at white. Note the distances of these two stars — 15 CnV is 1144 light years away from us and 17 CnV is 202 light years — which means this is an optical double, as opposed to a pair of stars that are gravitationally linked.
Just to the north of this pair, which would be between them and Cor Caroli, is a very challenging double, Σ1723, that forms the second leg of a triangle with 15/17 CNV. Despite the seven arc second separation, they are very difficult to split because of the lower magnitudes in comparison to the other stars discussed so far. With averted vision, I could just catch sight of the faint 8.6 magnitude companion at 76x in a 127mm refractor. I tried going up in magnification, but didn’t do any better. This one may well be worth pulling out the C9.25 one of these nights.
25 Canum Venaticorum (Σ 1768) HIP: 66458 SAO: 63648
RA: 13h 37.5m Dec: +36° 18′
MG: 5.0, 6.9, 8.6 Sep: 1.8″, 218″ PA: 99°, 141°
Distance: 192 LY
Spectral Type: F0
A bit less challenging, but still not easy unless you are fortunate enough to have some very good seeing, is 25 Canum Venaticorum, lying a bit to the east of Cor Caroli. Seeing was a little on the unsteady side the first night I looked at this one. I was able to get a clean split in a 102mm refractor with an 8mm Radian (110x), and also with a 127mm refractor in a 9mm UO ortho (131x). The second night out I had to move up to a 6mm Plössl (197x) in the 127mm scope to separate them. Still, the battle was worth the effort – the secondary is just a small white spot right up against the much larger fifth magnitude primary. Haas describes it as “lemon-white,” but the only lemon I could find was the missing 8.6 magnitude “C” component. Either the position angle (PA) is wrong or I just couldn’t see it. Next time out, this one warrants a look with either the 152mm refractor or the C9.25.
These observations were made on June 5th and June 11th, 2010, with the refractors of 60mm, 102mm, 127mm, and 152mm aperture.