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Cor Caroli and Royal Friends: Σ 1723, Σ I 24 (15/17 Canum Venaticorum), and 25 Canum Venaticorum

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
Separation:  19″
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.

Canes Venatici, showing the positions of the stars described here. (Stellarium screen image, click on the chart for a larger view)

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.

Image as seen in a refractor with a diagonal – west is left, north is up.

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.


6 Responses

  1. Ah – nice report, as usual, John – and as usual you give me some new stars to look for, though of course I’m familiar with Cor Caroli and consider it a “60mm jewell.” I always see the secondary as violet, or some subtle shade of purple.

    Hmmmm . . . “pearly white” and “sapphire white” – I need to go take another look under different circumstances, but my overwhelming impression last night was that these two were just dim with no color evident, even though I was using a 127mm refractor.

    Haas doesn’t tend to be overly poetic in her color descriptions, so I started wondering what size refractor she was using on this. Her book says “60mm.” but as I reread the introduction it seems to me the telescope she lists for a particular star may not always be the one she used – but more like the smallest she recommends – not sure. In any event, It is just beginning to sink in with me that color descriptions may be more dependent on the size instrument used than is usually noted. A good example of this is how the primary of Cor Caroli changed from yellow to white when you went from 60mm to 127mm.

    I’m sure you’ve noticed this before. So have I. But I can’t recall much of a discussion on this in various double star references. Color differences are usually simply attributed to different observers and different ways individuals perceive color. I’m sure that’s a factor – but it makes sense that the larger the objective, the more light you gather and the more light you gather, the easier it is to detect color because our eyes are simply not that sensitive to color. And, of course, looking directly at a star star is essential if you are going to see color – with averted vision were not using our color detectors.

  2. Using smaller scopes I repeatedly see the secondary as lilac – a subtle, sweet purple tint. I also wpnder about who this was named for. I know Kaler says Charles II and I usually trust him, but I believe it was named in honor of Charles I.

    Charles I was executed in the rebellion and the guy doing the naming of this star was the physician to Charles II. It makes sense, then, that he would flatter his king (Charles II) by naming the star for his king’s “martyred” father, Charles I.

    With three visitors over last night looking at it with an 85mm refractor under bright moonlight the opinions ran from no color – white – to blue for the secondary. (I did not prejudice them by giving my opinion and i had everyone look and make up their mind, before anyone spoke. Me? I saw it once more as lilac. Once I have that in my head, that’s what I see 😉

  3. Hmmm, I checked Burnham to see what information there was on the naming of Cor Caroli and he lists two possibilities.

    The first is a reference to Sir Charles Scarborough, the court physician to Charles II — but it states he named it after Charles II because the star “shone with special brilliance on the eve of the King’s return to London, May 29th, 1660.” Wonder if Sir Scarborough might have actually named it after himself?

    The second states the original name of the star was “Cor Caroli Regis Martyris,” in reference to Charles I. Burnham credits that to Deborah J. Warner of the National Museum of History and Technology.

    Of course, that information is rather dated, so it’s possible there may be a more up to date source out there. I’m still waiting for a copy of Admiral Smyth’s Bedford Catalog, which may have more info — not more up to date of course, but the Admiral may have had some insight to the situation.

    Personally, I think it was named by Galileo. I can see him turning his scope on it for the first time and exclaiming: “Holi Moli, what a Cor Caroli.”

    But then again I could be entirely wrong. I’ve been suffering from a severe case of photon deprivation lately.

    Meanwhile, I’ll go hide in the archives for a while and see what surfaces. 😉


    Hang on – the Bedford Catalog just arrived an hour after I posted this message — so I guess I can come out of the archives now.

    First, as far as colors, Smyth saw the primary as “flushed white” and the secondary as “pale lilac.” I’ve got to take another look at this since he and Greg are on the same page.

    As to the naming of the star, here’s what the good Admiral Smyth has to say on the subject:

    “But it came to pass that it was named Cor Caroli by Halley, at the suggestion of Sir C. Scarborough, after a worthless man’s heart. The popular story, or rather the vulgar one, runs, — how Scarborough, the court physician, gazed upon a star the very evening before the return of King Charles II to London, the which, as in duty bound, appeared more visible and refulgent than heretofore; so the said star, which Hevelius had already made the lucida of Chara’s collar, was thereupon extra-constellated with a sort of Valentine figure of a heart, with a royal crown upon it; and so the monarch, it would seem, by this extraction, remained heartless.” (The Bedford Catalolog: From a Cycle of Celestial Objects, by Admiral William Henry Smyth, p. 288: Willman-Bell, 1986)

    So there you have the mid-nineteenth century version of the tale of what took place. I suspect the real story may be lost and what we have is more myth than fact, more legend than literal.

    Whatever the case may be, beware of the act of “extra-constellating” — sounds like it might be downright painful.

  4. Hi Guys,

    Got a giggle out of all that stuff about “extra constellating” the sky and all that.

    I just got in from a charming evening with Tonya and the Hunting Dogs. Twilight is here again unfortunately, but I did manage to look at Cor Caroli. A beauty at 63x but I canny see purple in the secondary., more yellow to me.

    Got a good look at 25CVn too. It was very cleanly resolved in the 5-inch at 225x with lots of dark sky between the components.

    After staring at the hot coals of the carbon star ‘La Superba’ (Y CVn) further off to the west, I had a gander at 2CVn before I packed up.

    Wasn’t disappointed! The 9mm ortho( 125x) framed this colour contrast double just dandy. The 6th magnitude primary presented as deep orange to my eye while the 9th magnitude secondary -about 12″ away -looked like a phantasmally maroon. Crazy or what? Hard to see any colour in the smaller scopes though.

    Best wishes,


    Ps. Did ya hear about all the europium churned up in the atmosphere of Alpha 2 CVn?

    • Howdy Neil,

      Thanks for the tip on both Y CVn and 2 CVn. I haven’t been back to that area of sky since I wrote that post two years ago, so maybe it’s about time for a return trip. I’ll pencil it into my star calendar.

      And hadn’t heard or read about the Europium in the atmosphere of Alpha 2 CVn. I just checked — it has an atomic number of 63, which means there must some rather high temperature heating going on in the heart of that star.

      Wonder if that’s another version of extra-constellating? Still sounds like something that would be a good idea to avoid.

      Cheers! 😎


  5. Hi John,

    Eye. The ex-brewer Johannes Hevelius must have been sipping on some seriously strong Jopen beer to see a pair of hunting dogs under the Great Bear. Buy hey, that’s history. As for Europium; a next to useless element demonstrating nature’s proclivity for squander. Yet, without Europium, there would no Universe to contemplate. It’s all part of the plan.



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