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Hanging out in the Coat Hanger with Herschel and Struve: Σ2521, h 2871, Σ2504, and Σ2540

I can’t think of a more satisfying thing to do on a warm summer’s evening than to dangle from the end of a coat hanger located between the two lanes of the Milky Way as it splits in the southern reaches of Cygnus.  The view from up there is out of this world.

And if you’re going to hang out with two inveterate Star Splitters, you really can’t do much better than these two — Sir John Frederick William Herschel and Herr Friedrich Georg Wilhelm von Struve.

Of course we don’t want to leave out Per Collinder, who left his name here in the form of the designation frequently applied to this area, Cr 399.  Nor do we want to leave out D. F. Brocchi, who’s name hangs up here as well — as in Brocchi’s Cluster.

And above all, we don’t want to leave out the source of the inspiration for the evenings I’ve spent here recently, which comes from Down Under.  Specifically, it was an enthusiastic email to Greg from two very sharp-eyed observers, Daphne and Dom Gonsalves of Sydney, Australia, that reminded me I better get busy in Vulpecula before the summer slips past once again.

So come on along with me and we’ll spend an evening exploring the intricate and subtle delights of double stars Coat Hanger style.

The Milky Way splits in southern Cygnus, one branch flowing through Sagitta, the other past Albireo and around the north edge of Vulpecula, while the Coat Hanger sits between the two branches. (Stellarium screen image with labels added, click to enlarge the chart)

If you’ve got magnitude five or six skies, you can spot the Coat Hanger between Sagitta and Albireo using your optically unaided, unclothed, naked pair of eyes.  Draw a line from Alpha (α) and Beta (β) at the southwest end of Sagitta to Albireo at the south end of Cygnus, and at about a third of the way along that line, if you look to the west you’ll see a small, glowing area of stars — and that’s your target.  Or if your eyes are really good, extend the line from Alpha (α) and Beta (β) Sagittae directly to 4.8 magnitude 1 Vulpeculae and you’ll come across the Coat Hanger right at the halfway point.  And if your skies are too bright for any of that, a pair of binoculars will pry the unmistakeable hook-shaped asterism out of the sky with ease.  Which is really the easiest way, regardless of how dark or how bright your skies are.

Our first gem of subtle starlight, Σ2521, is really a compact delight, and you’ll find it located right at the neck of the hook. When I mentioned dangling from the end of the Coat Hanger ……. I wasn’t kidding.  (Stellarium screen image with labels added, click for a larger view)

Σ 2521        HIP: 95582    SAO: 104839
RA: 19h 26.5m   Dec: + 19° 53′
*****    Magnitudes   Separation  Position Angle   WDS Data
AB:         5.8, 10.5            27.9″                  33°                1997
AC:         5.8, 10.5            74.1″                325°               2002
AD:         5.8, 10.6          152.1″                  63°               2002
AE:         5.8, 14.5            26.5″                   71°               1934
Distance: 455 Light Years
Stellar Classifications:  All K5

As the chart above shows, Σ 2521 is located at the south end of the Coat Hanger asterism, just before the hook curves to the west.   But, before we put it under the eyepiece, let’s look at Admiral William H. Smyth’s description of it:

A very delicate double star on the Goose’s foot, in a small group to the north-north-west of Altair and at one-third the distance between that star and Vega.  “A” 6 1/2, topaz yellow; “B” 13, deep blue; and “A” is the apex of a scalene triangle with two other stars to the north of it.”  (Bedford Catalog, Willman-Bell: 1986, p. 447)

And after we center this aptly described “delicate double star” in our eyepiece, we find there’s a lot to look at, compact though it is — and to think about.

“A very delicate double star on the Goose’s foot …”  Actually, it’s a quintuple, four of which are shown here. (East and west reversed to match the refractor view, click for a larger view)

First, the colors: I see the primary as a combination of red and gold, with red being the dominant color, and a few times I’ve even seen it as orange.  Call it what you will, it contrasts very nicely with the black background of the sky.  When I saw Smyth had referred to the secondary as “deep blue” and measured its magnitude at 13, I thought maybe he was looking at another star.  But he measures “B” at a distance of twenty-two arc seconds from the primary, so apparently he’s referring to the same “B” listed above in the Washington Double Star Catalog (WDS).  It certainly isn’t a 13th magnitude star — not now, anyway.   The 10.5 magnitude listed in the WDS matches up well when compared to its companions, but it’s too faint and too close to the primary for me to see any color in it.

Apparently the Admiral was unaware of “C” and “D” as being included in this system, but it appears that he may have seen “C” in his remark above about the scalene triangle.  The WDS lists the first dates of measurement (which I believe are the dates of discovery) for these two stars as 1879 and 1878, respectively,  so it appears the senior Struve didn’t recognize them as associated with the primary, either, since his catalog was composed prior to those dates.

Now that we appear to have cleared up those minor mysteries, let’s quit quibbling and see what we can see.

I’ve looked at this one numerous times now, with everything from a 60m scope to a six inch refractor.  In each case, I was struck by how delicate these stars appear.  In the 60mm scope, “B” is pretty much lost in the glare of the primary, but I did catch brief glimpses of it.  I tried an 80mm f6 refractor, and found I could maintain a visual hold on “B” most of the time at 60x with an 8mm Radian, but it was a close thing.  If you’re viewing from under magnitude four or five skies, you’ll probably need to go to 70x or 80x to get it.  In either case, though, 60mm or 80mm, “C” and “D” were easily seen.

The view improves, of course, with more aperture.  A four inch refractor is ideal, and even better is a five or six inch.  At lower magnifications, “B” is again a bit elusive, but it comes into its own with a little magnification.  Try about 150x in a five inch or six inch refractor, though, and this little system suddenly sparkles and makes a grab for your affection.  At that magnification, all four stars (“A” through “D”) give the impression of floating in space.  “B” tends to fluctuate a bit in brightness because of the glare from “A”, almost as if it’s quivering.  “C” and “D” add to the floating effect as they hover at their progressively increasing distances from the primary.  And “E”  —  well, at a magnitude of 14.5 we’ll leave “E” for larger apertures and more magnification.  Notice that the last date of observation for it in the WDS is 1934, so if you spy it, get out your measuring tools and report the darn thing to the Naval Observatory!

As you can tell, I’ve found myself a bit taken in by this comparatively dim system.

And now I’ll show you how that happened  ——————->

h 2871 (HJ 2871)   (4 Vulpeculae)       HIP: 95498    SAO: 104818
RA: 19h 26m   +19° 48′
Magnitudes     AB: 5.2, 10.0        AC: 5.2, 11.7
Separations    AB: 14.3″              AC: 51.6″
Position Angles   AB: 88°  (WDS 2008)       AC: 209°   (WDS 1997)
Distance: 237 Light Years
Stellar Classification: Both G9

h 2871 is shown here at the bottom of the field of view, where it was lurking in the sketch above, and Σ 2521 still sits at the center. (East & west reversed, click for a larger view)

Lurking at the west corner of the field of view in the sketch shown above is our second star, h 2871, which is identified now in that same sketch, shown here again at the right.

Who woulda thought it.

Not me.  And the reason is I couldn’t see anything but the single reddish-orange 5.2 magnitude primary.  I spent more than a few minutes looking for this deceptively dim devil before my Star Splitter circuits connected and the light of recognition came on.

Now for this one you’re going to need at least four inches of aperture.  Even at that, I had quite a bit of trouble picking out the 11.7 magnitude “C” component because it wanted to hide behind the glare from the primary — averted vision was the only way.  It was slightly easier at the larger apertures, but without averting my vision, it was still very vexing and vague.  I can lay claim to having seen it with direct vision a couple of times, but only for a couple of seconds in my six inch f/10.

But the one that is really darned difficult is 10th magnitude “B” at what seems like a reasonable distance of 14.3″.  If you go back to Σ 2521 again and look at the statistics, you’ll see that the secondary in that system is half a magnitude fainter at 10.5, and more distant, at 27.9″.  But the differences between the two secondaries don’t offset each other — not one single bit.

I’ll spare you the descriptive details of the nights of failure I spent trying to pull that speck of light out of the primary’s glow. All I can say is Sir John Frederick William must have possessed a slightly sadistic sense of humor when he listed this one.  I tried every trick in the book and still failed to find those photons.  Averted vision, direct vision, micro-movements of the fine focus knob, a hood over my head, ominous oaths muttered to the Sky Gods — nothing worked.

………….  And then one dark morning — July 30th, 2011, at 1:10 AM Pacific Daylight Time, to be exact — it popped into view.  With direct vision.  And stayed there, too.  For several minutes.  In the six inch /10.  In a 10mm Radian.  152x.

I never woulda thought it.

It was just a very miniscule micro-dot of light.  I could even swear it was bluish-white.  But I won’t.  But it was there — and that was the main thing.  And after all that effort, it was a thing of indescribable beauty.  Ravishing, even.

Then  ——–  over on the other side of the field of view, at the very edge of it, right in the corner, wedged almost so far over they were out of sight, floating just as I had seen them doing once before  ——— were the four stars of Σ 2521.  And in comparison to what could be seen of h 2871, they were as welcome a sight as an oasis in a desert.  Suddenly that system struck me with all its force —  all four stars — wham!    Very neat, very orderly, not quite as compact as what they were the first time I looked at them — and for some strange reason, very comforting.  I felt like I should reach out, grab all four of them, put them in my pocket, and just — I don’t know — keep them with me for a while I guess.

Weird.  Very.

I noticed it was rather damp at the time.  A few drops of dew even dripped from the bill of my Star Splitter hat.  Mighta been a moisture-induced short circuit crackling somewhere beneath that hat in the cranial cavity.

But I sat there a while and let it all soak in — the moisture, the dew, and the view.

………………………………………    Dangling at the end of the Coat Hanger’s hook.


There are two more stars hovering just out of reach of the Coat Hanger worth quick looks —  mirror images of each other — as well as a surprise lurking along the way to one of them.

Σ 2504 (Sagitta)         HIP: 95116      SAO: 104753
RA: 19h 21m   Dec: +19° 09′
Magnitudes    AB: 7.0, 9.0      AC: 7.0, 12.2
Separations   AB: 8.9″            AC: 92.2″
Position Angles   AB: 281°   (WDS 2007)   AC: 101°  (WDS 2005)
Distance: 191 Light Years
Stellar Classifications: F5, F5

Σ 2521 and h 2871 point almost directly at Σ 2504.  So, using the second chart above, let’s finally get free of this hook and slide a slight bit more than a degree to the southwest, past 7.8 magnitude HIP 95293, which marks the halfway point, and across the border into Sagitta.  Better make sure you have your celestial passport — the Sagittans are known for shooting straight as an arrow and asking questions later.

The well-ordered beauty of Σ 2504 sitting in the center of the field, “B” and “C” complementing one another on opposite sides of the primary. (East and west reversed, click for a larger view)

You’ll find you probably are going to have to look twice to catch the ninth magnitude secondary of this one.  Dim as it is, I caught just a spark of barely discernible blue in it.  The primary struck me as being mainly white, but with a hint of yellow, lending a gold cast to it at times, which was easily seen when I de-focused the image a bit.  Haas describes it as “lemon-white” and “blue-green.”  We’re close on this close pair.

This really isn’t what you would describe as sixty millimeter material, but I did find I could get a glimpse of “B” in my 60mm f16.7 using a 20mm TV Plössl (50x).  “C” is invisible at that aperture, but it will pop in and out of view at larger apertures — just look on the opposite side of the primary from “B” and avert your vision slightly.

Let’s go back to the Coat Hanger now,  head for the horizontal line of stars which distinctively outline its north side, and gingerly step across it to the east.

The Coat Hanger – aka Cr 399, aka Brocchi’s cluster – in detail, with NGC 6802 shown en-route to Σ 2540. (Stellarium screen image with labels added, click for a larger view)

Σ 2540          HIP: 96171    SAO: 87342
RA: 19h 33m   Dec: +20° 25′
Magnitudes     AB: 7.5, 9.2        AC: 7.5, 12.8
Separations    AB: 5.1″              AC: 147.1″
Position Angles   AB: 146°  (WDS 2005)      AC: 221°  (WDS 1912)
Distance: 452 Light Years
Stellar Classifications: Both A3

Starting from either of the stars at the center of the Coat Hanger — 6.6 magnitude HIP 95584 or 6.3 magnitude HIP 95487 — step across carefully to 6.9 magnitude HIP 95700, continue to 6.3 magnitude 7 Vulpeculae at the Coat Hanger’s edge, and you’ll see the 7.5 magnitude dimness of Σ2540 about half a degree to the east and very slightly north.   It forms a low and wide triangle out there with 7 Vulpeculae and fifth magnitude 9 Vulpeculae to its south.

As you leave 7 Vulpeculae, look for a very faint, ghost-like smudge in the sky that is oriented along a north-south line in your eyepiece — which is the open cluster NGC 6802 (shown at the far right in this upside down photo).  It’s directly on your path of travel — you can’t miss it, provided you look closely.  In an 80mm scope it will be a very faint smudge under a dark sky.  With four inches of aperture you’ll see a few of its very faint stars, and several brighter areas will begin to stand out as well.  In five or six inch scopes, you’ll find yourself staring at a very pretty glowing cloud of starlight framed within a rectangle of seventh and eighth magnitude stars.  It’s well worth pausing here for a few minutes just to admire the sheer faint beauty of it.

A “fairly bright white star with a little nebulous globe on its edge” floating above a background of dim starlight. (East and west reversed once more, click … to … enlarge)

As for Σ 2540, you’ll find yourself looking at a mirror image of its  slightly brighter predecessor, Σ 2504.  The primary and secondary are a bit fainter, the separation is a little tighter — and they’re quite a pair in scopes of four inches or larger, floating in and standing out from a background of 10th and 11th magnitude stars.   Haas describes them as a “fairly bright white star with a little nebulous globe on its edge.”  I thought I saw a very faint tint of red in that “nebulous globe” of a secondary, and the primary struck me as silver-white.

You’ll have to look carefully — with at least five or six inches of aperture — to see 12.8 magnitude “C”, but it’s far enough away from the 7.5 magnitude glow of the primary to be see-able.  In the six inch scope, it popped into view in the 12mm Radian I used for the sketch and pretty much stayed there.  At least I think I saw it — the WDS data shows the last observation for it was recorded in 1912, so I’m presuming the PA and distance is still reasonably close to what it was a hundred years ago — but that may not be the case.

And thus ends our tour of the Coat Hanger, aka Cr 399, aka Brocchi’s Cluster.  I’ll leave you out here at it’s eastern edge while you take a look around — there’s plenty to see.  Five degrees to the northeast is M27, aka the Dumbbell Nebula (take a look at this very realistic sketch by Jeremy Perez!)  — and five degrees to the southeast is M71, a diamond-like scattering of stardust near the shaft of the Sagittan arrow.  And another ten degrees or so to the east and slightly south of that you’ll encounter the glorious technicolor beauty of Gamma Delphini.

And if you’re still poking around in the Coat Hanger area on September 2nd, you’ll find a visitor passing through.  Comet 2009 P1 (Garradd), which is at eighth magnitude as I write this during the first week of August, will be positioned practically on top of Sir John’s h 2871.  And THAT should be quite a sight!  Greg has a great post here on his other site with some additional information.

Meanwhile, I have to get the four stars belonging to Σ 2521 out of my pocket and back where they belong before I cause a controversy of astronomical proportions.  Gotta be careful with those position angles and distances, too.  😎

Clear Skies!


3 Responses

  1. I made this sketch of Comet Garradd back on September 19. It was going into Hercules.

  2. Nice sketch, Steve!

    My last look at Garrad was last week when it was south of and midway between 101 and 109 Herculi. I noticed the tail was slightly longer, and would guess it was brighter by about half a magnitude than what it had been three weeks prior to that.

    It’s pretty well lost to the moon now, not to mention the clouds and rain. I should get another shot at it during the next dark cycle of the moon,although it will have sunk lower into the hazy southwest. Another few weeks after that I’ll lose it in the trees.

    It has been a pleasant surprise, though. Looking forward to catching it again in the morning sky later this year.

  3. I was able to get this sketch of Struve 2521 on September 5th. The theta & rho are “pretty” close to your lit. It is a fun system. Like I noted, I wasn’t able to get the 14th mag. AA companion…

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