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Clusters of Double Stars in Open Clusters: Σ 2212 and Σ2216 in IC 4665; Σ2342 and HJ 864 in IC 4756

I spent some time two years ago in a very intriguing area of Ophiuchus (see In the Realm of the Ophiuchan Triangle) and was so taken with the variety of objects in the area that it’s become one of my favorite late night haunts.  There’s an uncanny allure there I wish I could describe, but words just never quite capture the elusive quality of celestial magic.  From the very distinctive outline of Poniatowski’s Bull, a now defunct constellation, to the area surrounding the cryptically named Cebalrai, aka Beta (β) Ophiuchi, there exists a very Bermuda Triangle like tug – which is why the phrase Ophiuchan Triangle suggested itself for that earlier post.

There are two open clusters in the area, one that is very open, Collinder 350, lurking slightly south of Gamma (γ) Ophiuchi, and a relatively more condensed cluster, IC 4665, about a degree and a half north of Cebalrai.  Hidden west of Gamma (γ) and barely north of the double star 61 Ophiuchi is a small and very hard to see 10.9 magnitude globular cluster that will challenge your averted vision, NGC 6426 (you need nine or ten inches of aperture to have any chance at all to see it), and speeding along silently almost four degrees east of Cebalrai is the fleet-footed Barnard’s Star. And of course, there’s a suggestive scattering of double stars, which are covered in that earlier post.  If you like variety and a slightly unheimlich (uncanny) flavor to spice up an observing session, this is a good place for it.

To get back to double stars, I’ve always found open clusters great places to explore.  Like an untracked wilderness, you never quite know what sights you’ll find over the next ridge.  You’ll find more than a few duplicitous-appearing stars that look like they could be siblings, but frequently there are so many, or they’re packed together so tightly, any attempt to identify them is almost like assigning names to the grains of sand on a beach.  But as Steve McGee pointed out to me a few months ago, there are a couple of double stars buried within the relatively open confines of IC 4665, so I put on my star-seeking goggles and safari hat and went searching for them.

If you center Cebalrai in your finder, you should be able to see the subdued glow of IC 4665 about a degree and a half to the north. (Stellarium screen image with labels added, click to enlarge).

If you center Cebalrai in your finder, you should be able to see the subdued glow of IC 4665 about a degree and a half to the north. (Stellarium screen image with labels added, click to enlarge).

 And when you zero in on IC 4665 with a low power, wide field eyepiece, you’ll see an image something like the one above.  Navigation here isn’t too difficult since there are relatively bright stars located at key reference points.  Notice that 7.10 magnitude HIP 87002 forms a distinctive triangle with 8.0 magnitude Σ 2216 southeast of it and 9.45 magnitude Σ 2212 at the southwest corner, somewhat in the shadow of 7.30 magnitude HIP 86960 a bit further to the southwest.  (Stellarium screen image with labels added, click for a larger view).

And when you zero in on IC 4665 with a low power, wide field eyepiece, you’ll see an image something like the one above. Navigation here isn’t too difficult since there are relatively bright stars located at key reference points. Notice that 7.10 magnitude HIP 87002 forms a distinctive triangle with 8.0 magnitude Σ 2216 southeast of it and 9.45 magnitude Σ 2212 at the southwest corner, somewhat in the shadow of 7.30 magnitude HIP 86960 a bit further to the southwest. (Stellarium screen image with labels added, click for a larger view).

Σ 2212          SAO: 122730   (No HIP number assigned)
RA: 17h 46.4m   Dec: +05° 42’
Magnitudes: 9.51, 9.60
Separation:  3.1”
Position Angle: 342°  (WDS 2010)
Distance: ???
Spectral Classification: A0

Σ 2216          SAO: 122742   (No HIP number assigned)
RA: 17h 47.0m   Dec: : +05° 42’
Magnitudes: 8.01, 10.09
Separation:  26.8”
Position Angle: 27°  (WDS 2009)
Distance: ???
Spectral Classifications: B7, A2

 Σ 2212 and Σ 2216 offer an amazing study of contrasts in separation and position angles, with the secondaries pointing in equal and opposite directions from due north. (East & west reversed to match the refractor view, click on the sketch for a larger version).

Σ 2212 and Σ 2216 offer an interesting study of contrasts in separation and position angles, with the secondaries pointing in opposite directions from due north. (East & west reversed to match the refractor view, click on the sketch for a larger version).

The only color in this cluster of stars is white, all white, and lots of it, with the exception of 8.3 magnitude SAO 122746, which sports a slightly orange flavor.  On the night I made the sketch, the seeing  was so poor I was barely able to split Σ 2212, which normally shouldn’t be all that tough in a six inch refractor.

Struve on STF 2212 and 2216This is a great exercise in identifying stars in a relatively well-populated field of view, but I would warn against using F.G.W. Struve’s directions pertaining to Σ 2212: Duae majores praecedunt (click on the thumbnail image at the right).  For those Latin-challenged readers (which includes me), the translation is “Two larger preceding.”  I’m not sure which stars Herr Struve was referring to, 7.30 magnitude HIP 86960 and 7.35 magnitude HIP 86944, or the latter star and 7.65 magnitude SAO 122709.

I’ve read that something like seventy or eighty percent of all stars are double or multiple (for example, see page one of The Cambridge Double Star Atlas), a figure I’ve always thought was rather high, based on personal experience.  Whatever the case, there are two more double stars in this field, which are listed below, giving the percentage here a badly needed boost.  Good luck, though – both boast of sub-arcsecond separations, one has a 1.4 magnitude difference between primary and secondary (A 1161), and the secondary of the second pair, CHR 157, is shown with no magnitude in the WDS.  But at least you know they’re there now.  :mrgreen:

A 1161         SAO: 122738   (No HIP number assigned)
RA: 17h 46.8m   Dec: +05° 34’
Magnitudes: 8.5, 9.9
Separation:  0.7”
Position Angle: 284°  (WDS 2005)
Distance: ???
Spectral Classification: B9

CHR 157    HIP: 86954   SAO: 122723
RA: 17h 46.1m   Dec: 05° 32’
Magnitudes: 6.86, ???
Separation:  0.3”
Position Angle: 40°  (WDS 1996)
Distance: 1348 Light Years
Spectral Classifications: B8

And for those who like their sketches minus the clutter of labels, here’s the sketch above again in a purer form:

(East & west reversed once more, click to enlarge).

(East & west reversed once more, click to enlarge).

When you get bored in the realm of the Ophiuchan triangle, try panning to the east about ten degrees to two more open clusters, the beautiful, concentrated, and condensed NGC 6633 and the comparatively more sparse IC 4756.  As I’ve already mentioned, identifying double stars in areas of dense concentrations of stars is like trying to distinguish one grain of sand from another, so I passed on NGC 6633 and dropped southeast to IC 4756.  Even there, I ended up staying on the southern fringes of the cluster to avoid getting pulled into its hypnotizing maelstrom of starlight.

To find your way to IC 4756, start from the south end of Poniatowski’s Bull, which is marked by 4.4 magnitude 68 Ophiuchi, and extend a line five degrees northeast to 74 Ophiuchi, slightly fainter at a magnitude of 4.85.  Now extend that line another five degrees northeast and you’ll find yourself in the middle of our destination, IC 4756.  Both IC 4756 and NGC 6633 are unmistakable balls of glowing starlight in an 8x50 finder.

To find your way to IC 4756, start from the south end of Poniatowski’s Bull, which is marked by 4.4 magnitude 68 Ophiuchi, and extend a line five degrees northeast to 74 Ophiuchi, slightly fainter at a magnitude of 4.85. Now extend that line another five degrees northeast and you’ll find yourself in the middle of our destination, IC 4756. Both IC 4756 and NGC 6633 are unmistakable balls of glowing starlight in an 8×50 finder.

You’ll find both Σ 2342 and HJ 864 located at the southwest corner of IC 4756, less than half a degree apart.  6.35 magnitude HIP 91499 and 6.65 magnitude HIP 91418 can be used as reference points to orient the field.

You’ll find both Σ 2342 and HJ 864 located at the southwest corner of IC 4756, less than half a degree apart. 6.35 magnitude HIP 91499 and 6.65 magnitude HIP 91418 can be used as reference points to orient the field.

Σ 2342          HIP: 91142   SAO: 123673
RA: 18h 35.6m   Dec: +04° 56’
Designation         Magnitudes      Separation     Position Angle     WDS
AB (Bu 643):         6.46, 12.50           12.90”                  325°              2000
AC (Σ 2342):         6.46,   9.63           34.10”                  359°              2011
AD (Fox 9031):     6.46, 13.90           34.30”                  323°              2000
Distance: 335 Light Years
Spectral Classifications: A2

HJ 864          HIP: 91220   SAO: 123688
RA: 18h 36.5m   Dec: 04° 57’
Magnitudes: 6.76, 11.0
Separation:  19.6”
Position Angle: 326°  (WDS 2006)
Distance: 614 Light Years
Spectral Classification: K2

Σ 2342 is on the west side (left) of this sketch and HJ 864 is opposite at the east edge (right) of this view.  You’ll need to enlarge it by clicking on the image in order to see HJ 864’s secondary, and especially to get a look at the 12.50 magnitude “B” companion of Σ 2342.  “D” is faintly absent from the view.  (East & west reversed once more).

Σ 2342 is on the west side (left) of this sketch and HJ 864 is opposite at the east edge (right) of this view. You’ll need to enlarge it by clicking on the image in order to see HJ 864’s secondary, and especially to get a look at the 12.50 magnitude “B” companion of Σ 2342. “D” is faintly absent from the view. (East & west reversed once more).

These two stellar denizens of the lower reaches of IC 4756 can both be a bit difficult to coax into cooperating.  HJ 864 is a good test of your ability to ferret faint photons out of the glaring glow of a much brighter primary.  In this case there’s 4.24 magnitudes of difference, which means the secondary is about fifty times fainter that its brighter relative.  My notes from the night of the sketch describe the secondary as “very clear with averted vision, impossible without.”

If you succeed with HJ 864, then you’re cleared to grapple with the AB pairing of Σ 2342, also identified as Bu 643.  In this case, we’re wrestling with 6.04 magnitudes of difference (which makes the secondary a vision-numbing 250 times fainter than the primary), AND, it’s about 35% closer to the primary than was the case with HJ 864.  Here are a few cryptic comments from my observing notes:

Σ 2342: “C” component very difficult with averted vision, but saw it clearly with a 10mm Radian (152x); “D” was hopeless.  Seeing absolutely rotten.  Needed to sit and wait for moments of relatively calm seeing to see what I saw.”

Burnham on STF 2342-Bu 643That evasive “B” companion was added to this system in 1878 by the devilishly eagle-eyed S.W. Burnham with the aid of the 18 ½ inch Clark refractor at the Dearborn Observatory of Chicago, so don’t doubt your vision if you fail to extract it from the glaring primary.

In his observation at the left (click to enlarge it), Burnham comments on the proper motion of the primary, which the Washington Double Star Catalog (WDS) shows at .029” per year east in RA and .029” per year south in declination.

Lewis on STF 2342You can see how rapidly the primary (“A”) changes in relation to both “B” and “C” in the list of observations at the right from Lewis’s compilation of Struve’s catalog (click on the thumbnail to enlarge it).  If you compare those observations with the separation and position angle in the WDS data above, you’ll find “A” has moved from a distance of 8.86” from “B” in 1878 to 12.90” in 2000, and the distance between “A” and “C” has grown from 26.91” in 1830 to 34.10” in 2011.  Nothing stays put for long in this galaxy of ours, including stars!

Struve on STF 2342The 9.63 magnitude “C” companion, which was F.G.W. von Struve’s discovery, is not much of a problem to pry loose in a five or six inch refractor, although you’ll have to look closely (it could be a real battle in a four inch refractor, though).  I took a look at Struve’s 1827 catalog, Catalogus Novus Stellarum Duplicium et Multiplicum to see what I could find, and saw that he identified the pair as Tauri Pon. 55, referring to Poniatowski’s Bull (click on the thumbnail at left).  As large and spread out as that area east of the upper part of Ophiuchus is, I’ll cast my vote now to restore that defunct constellation to its former glory.

Fox on STF 2342 - Bu 643That leaves the invisible “D”, which requires a bit more aperture than the six inch refractor I was using (probably at least another two inches as a bare minimum).  The WDS credits Philip Fox with the first observation of it in 1913 (also at Dearborn with the 18 ½ inch Clark refractor), but when I dug out his observing notes (click on the thumbnail image at the right), I see it was actually first reported by Wilhelm Doberck in 1909, and by both Burnham and Carl Wilhlem Wirtz (of the Observatory of Strasbourg) in 1910,   (There’s a beautiful photo of the 500mm [19.7 inch] refractor apparently still in use at Strasbourg HERE).

Apart from the difficulty of prying it apart, there isn’t much to be said about HJ 864, which was discovered by Sir John Herschel in 1827.  The primary is classified as a K2 star, meaning there should be a reddish-orange tint visible, but if so, I didn’t see it.   At a magnitude of 6.76, it should have been noticeable, so I suspect the poor seeing that night had something to do with it.

Also in same field with Σ 2342 and HJ 864 are two more double stars, neither one of which I was able to do anything with on that particular night because of the seeing:

J 2138 (BAL 2930)       (No HIP or SAO numbers assigned)
RA: 18h 36.0m   Dec: +04° 52’
Magnitudes: 9.9, 11.8
Separation:  5.2”
Position Angle: 140°
Distance: ???
Spectral Classification: ???

A 354        (No HIP or SAO numbers assigned)
RA: 18h 36.0m   Dec: +05° 06’
Magnitudes: 9.08, 12.40
Separation:  3.3”
Position Angle:  6°  (WDS 2006)
Distance: ???
Spectral Classification: G0

Their locations can be seen here on the a labeled version of the previous sketch:

East & west reversed to match the refractor view, click to enlarge.

East & west reversed to match the refractor view, click to enlarge.

That’s it for this tour – next trip is back up north to explore a few Herculean surprises.  But before you wander away, feast your eyes on the photo below from Burnham’s A General Catalogue of Double Stars of the 18 ½ inch Clark Refractor used by both S.W. Burnham and Philip Fox at the Dearborn Observatory.

Clear Skies!   😎

18.5 Inch Clark at Dearborn

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