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Four for Canis Major: S 537 and S 538, S 534, Nu1 (ν1)

I’m never sure where I’m headed next when I finish up in one area of the sky. Sometimes something in a star atlas will catch my eye and I’m off like a photon shot from a laser in hot pursuit, and other times I come across a book or internet reference to an intriguing multiple star that piques my curiosity.   There are other times, though, when an email lands squarely in the middle of my computer screen and grabs my full attention.

That’s what happened this time, and it led me on a chase down the same star-strewn path traveled by Sir James South almost two hundred years ago in Canis Major.   Whereas he was accompanied by a five inch f/16.8 refractor with a Tulley objective, I had the companionship of both a five inch f/9.3 Meade refractor and a six inch f/10 refractor equipped with a Jaeger’s objective – which is not to ignore the constant presence of Herr Klaus, the astro dog. I suspect the transparency of our skies was about the same, though – mine subdued with coastal moisture and his with the smoky murk of early nineteenth century Paris (the observatory he used for these observations was located at Passy).

And while I was roaming those star fields of two hundred years ago, my eye was caught by a distinctive trio of stars southwest of Sirius.   Those turned out to be the three members of the Nu (ν) family — ν1, ν2, and ν3 — one of which is a dazzling double star.

First, a look at the wide view of where we’re going:

 You’ll find Nu-1 (ν1), the last star on our tour, three degrees southwest of Sirius, wedged between its stellar siblings, Nu-2 (ν2) to its south, and Nu-3 (ν3) to its north.   But since I’m writing this in late April as Sirius sinks into the southwest, we’re going to start further south with S 537 and S 538, located west of Omicron-1 (o1) CMa, which is almost eight full degrees south and slightly east of Sirius. Hopefully by the time we work our way back up north, Nu-1 (ν1) will still be high enough for you to catch.   (Stellarium screen image with labels added, click on the chart to enlarge it).

You’ll find Nu-1 (ν1), the last star on our tour, three degrees southwest of Sirius, wedged between its stellar siblings, Nu-2 (ν2) to its south, and Nu-3 (ν3) to its north. But since I’m writing this in late April as Sirius sinks into the southwest, we’re going to start further south with S 537 and S 538, located west of Omicron-1 (o1) CMa, which is almost eight full degrees south and slightly east of Sirius. Hopefully by the time we work our way back up north, Nu-1 (ν1) will still be high enough for you to catch. (Stellarium screen image with labels added, click on the chart to enlarge it).

Here’s a close-up view of our starry hunting grounds with Omicron-1 (o1) parked down in the bottom left hand corner of the view. A short hop of three degrees due west from Omicron-1 will put both S 537 and S 538 in your field of view. If you have time, don’t neglect the ethereal starlight of M41. (Stellarium screen image with labels added, click on the chart for a larger view).

Here’s a close-up view of our star-studded hunting grounds with Omicron-1 (o1) parked down in the bottom left hand corner of the view. A short hop of three degrees due west from Omicron-1 will put both S 537 and S 538 in your field of view. If you have time, don’t neglect the ethereal starlight of M41. (Stellarium screen image with labels added, click on the chart for a larger view).

S 537/Bu 324           HIP: 32744   SAO: 172389
RA: 06h 49.7m   Dec:  -25° 05’
Identifier        Magnitudes       Separation     Position Angle      WDS
Bu 324    AB: 6.56,   7.93               1.80”                  211°               2008
S 537      AC: 6.56,   8.28             30.40”                 282°               2008
S 537      AD: 6.56, 13.00            30.60”                      3°               1999
Distance: 584.5 Light Years
Spectral  Classifications:  “A” is A0, “C” and “D” are A2

S 538                HIP: 32729   SAO: 172383
RA: 06h 49.6m    Dec:  -24° 09’
Magnitudes:  7.16, 7.18
Separation:   26.8”
Position Angle:  4°  (WDS 2007)
Distance: 622.4 Light Years
Spectral Classifications:  A6 and A2

Back to the email I referred to earlier, which is a description of S 537 and S 538:

A beautiful formation of two doubles and a single star. The top double is S 537 which is perpendicular to the line of stars, a bright and faint white double which is wide, then underneath (231″ away) is S 538, a faint and fainter white double slightly less wide, which is in line with the main line of stars, and finally adding to the overall beauty of the asterism is a faint star (mag. 8.4) which is CD-24 4476, about as away again (actually 253″). The main star of S 537 (BU 324) was resolved intermittently (‘popped out’) at 136x. The popping out was most odd. With rather poor seeing at the moment (the jet stream is very fast which makes the air turbulent) the main star was unresolved at first, then just as I was giving up, it suddenly ‘popped’ into being a pair, and then fell back to a single star again. I was rather doubtful but kept on looking and it happened again (but not thereafter).”

That picturesque description, written by Peter Morris from his London observing site, landed in my inbox on the Saturday afternoon of January 25th (2014).   And at 9PM that night, I had S 537 and S 538 cornered in a 10mm Radian:

And there they were, exactly as Peter described them – S 537 at the top of the view, perpendicularly contrasting with the vertical symmetry of S 538, and at equal distance south of S 538 is the star Peter referred to as CD-24 4476, which also goes by SAO 172832, among other designations. The primaries, by the way, were all white. (East & west reversed to match the refractor view. Click on the sketch for a much better version).

And there they were, exactly as Peter described them – S 537 at the top of the view, perpendicularly contrasting with the vertical symmetry of S 538, and at equal distance south of S 538 is the star Peter referred to as CD-24 4476, which also goes by SAO 172832, among other designations. The primaries, by the way, were all white. (East & west reversed to match the refractor view. Click on the sketch for a much better version).

Click to enlarge the image.

S 537, click to enlarge the image.

My experience with trying to separate Bu 324 was uncannily similar to Peter’s. I had an elongated glimpse of it with the 10mm Radian (152x), which led me to see what I could coax from an Astro Tech 6mm Plössl (253x). The seeing was so poor that it wouldn’t come to focus, so I reluctantly went back to the 152x view and sat as still as rock, waiting for the elongation to transform itself into two distinct stars.

I eventually got the “popping out” effect Peter mentioned, but only briefly, and only twice.   Rats – foiled again by uncooperative skies. Bu 324, by the way, was discovered in 1875 by S.W. Burnham with his six inch f/15 Clark refractor. It was first measured at a separation of 1.88” in 1877, so it has changed little, if any, since.

Click to enlarge.

S 538, click to enlarge.

The “S” designations are the result of several observations made by James South in February and March of 1825, which I’ve included at the right.  A close look at those numbers shows little change in the almost two hundred years since Sir James made his initial observations.

Translating his early nineteenth century position angles into present usage and leaving the separations as is, he measured what is now the AC pair of S 573 at 281° 51’ and 30.305” (the 2008 WDS numbers are 282° and 30.40”). For S 538, his final results were 3° 16’ and 27.806” (versus the 2007 WDS data of 4° and 26.8”).  The source for South’s observations shown at the right can be found here.

Click to enlarge.

Click to enlarge.

It took some determined detective work, but I also tracked down the first measurement of what is now the AD component of S 537 on pages 46 and 47 of this source.   Eric Doolittle made that one in 1898 from the Flower Astronomical Observatory, which was associated with the University of Pennsylvania near Philadelphia.  The “Sh” designation Doolittle used in the page shown at the left is an error — it should be shown as S 537.  “Sh” was only used to refer to the joint John Herschel-James South catalog (scroll down to the last title) published in 1824, which we’ll get to when discussing Nu11) CMa.  (The WDS has replaced “Sh” with “SHJ”).

The instrument used by Doolittle was an eighteen inch refractor with a Brashear objective, which he described as having “proved in every respect more than satisfactory.”   Again, there is little difference between his figures for the AD pair (2.45° and 30.27”) compared with the 1999 WDS figures (3° and 30.60”).

Now we’ll follow Sir South north by moving one degree northwest from S 537/538 to 6.0 magnitude HIP 32368 and then another degree northwest (slightly more north) to S 534 (here’s that earlier chart again).

S 534               HIP: 32144  SAO: 172204
RA: 06h 42.8m    Dec:  -22° 27’
Magnitudes:  6.27, 8.30
Separation:   18.2”
Position Angle:  144°  (WDS 2003)
Distance: 160 Light Years
Spectral Classifications:  “A” is F2
Note:  “A” is a variable star, V 350 CMa

You’ll find the field of view surrounding S 534 is quite a contrast to the dazzling white lights of S 537 and S 538:

 In a five inch refractor S 534 is surrounded by a rather sparse field, although it compensates with that curving arc of eighth and ninth magnitude star arrayed along its west edge. In fact, while sketching this idyllic scene I grew rather attached to that arrangement of starlight. Once again, the only color visible here was the white light of the primary. (East & west reversed to match the refractor view, click on the sketch for a larger version).

In a five inch refractor S 534 is surrounded by a rather sparse field, although it compensates with an inner and outer arc of curving stars arrayed along its west edge. In fact, while sketching this idyllic scene I grew rather attached to that delicate arrangement of starlight. Once again, the only color visible here was the white light of the primary. (East & west reversed to match the refractor view, click on the sketch for a larger version).

The outer arc of stars guarding the west side of S 534 all have names, magnitudes, and even spectral classifications. From north to south they are SAO 172191 with a magnitude of 8.86 and a spectral classification of B9; SAO 172183, 8.12, B3; SAO 172181, 9.05, AO; and SAO 172187, 9.48, K5. That second star, SAO 172183, is also a double, designated as B 1963 (magnitudes of 8.10 and 10.98, 130°, 1.2”, WDS 1991), that would be an interesting challenge for a night of steady seeing.

Click to enlarge.

S 534, click to enlarge.

James South paid three visits to the pair of stars he cataloged as S 534 around the same time he looked at S 537 and S 538. Those dates can be seen in his observation shown at the right. And again, when we compare the averaged measures made during his three observations (143° 13’ and 18.252”) with the most recent WDS measure of 2003 (144° and 18.2”), we see virtually no change in the relative positions of the primary and secondary.

The S 534 primary is also a variable, but depending on where you look you’ll find two different designations assigned to it.   Stelladoppie, which pulls its data from the WDS, lists it as V3181 (the AAVSO designation is NSV 3181), but I found the AAVSO site now lists that as obsolete.   Simbad uses the current designation, which is V 350 CMa.  The AAVSO web site shows a rather narrow magnitude range, 6.18 to 6.27, with a short period of .91041 days.

Meanwhile, on to our last star, which also has a connection to James South. Nu11) CMa lies four degrees north and slightly east of S 534, but it’s actually easier to get there by starting at Sirius, which is impossible to miss with its -1.50 magnitudes of blazing white light.  Going back to our first chart (here), you’ll find the trio of Nu (ν) stars lying from two to three degrees southwest of Sirius.  Nu1 1) is located halfway between Nu33) and Nu2 2).

Nu11) Canis Majoris  (6 CMa)  (H IV 81)  (SHJ 73)          HIP: 31564  SAO: 151694
RA: 06h 36.4m    Dec:  -18° 40’
Magnitudes:  5.79, 7.38
Separation:   17.8”
Position Angle:  264°  (WDS 2011)
Distance: 277 Light Years
Spectral Classifications:  “A” is G9

Another surprisingly sparse field, at least in a five inch refractor, but this time with what almost looks like a pair of headlights coming straight at you.   There’s a very slight, but very noticeable, yellow-gold light streaming out of the primary.   SAO 151712, over in the northeast corner, is a 7.83 magnitude K4 star. (East & west reversed, click on the sketch to improve the view).

Another surprisingly sparse field, at least in a five inch refractor, but this time with what almost looks like a pair of headlights coming straight at you. There’s a very slight, but very noticeable, yellow-gold light streaming out of the primary. SAO 151712, over in the northeast corner, is a 7.83 magnitude K4 star. (East & west reversed, click on the sketch to improve the view).

Sir William Herschel had a look at this star on September 30th, 1782, describing it with a mysterious Latin phrase “In dextro genu.”   I plugged that into the Google translator, which spit out “on the right knee” – and sure enough, that’s where Nu11) CMa is (take a look at the first chart again).   Sir William measured the separation of the two stars at 18.32” but was more elusive with his position angle, describing it as “very near directly preceding”, which actually describes it reasonably well.

Wm. Herschel on Nu-1 CMa

Click to enlarge.

Click to enlarge.

I mentioned a connection between Nu11) and James South, which is a result of a March 22nd, 1821, observation he made jointly with Sir John Herschel.   He felt there was an obvious change in position angle, a result of his measuring it at 17.24”, but with the 2011 WDS data falling almost exactly halfway between his measure and that of William Herschel forty years earlier, it’s hard to say who was closest to being correct.

As to whether those differences indicate orbital motion, or are just variations between observers, it’s possible to gain some insight by looking at the proper motions of the primary and secondary.   There’s a slight difference between the data listed in the WDS (-013 +018 for the primary, -009 +022 for the secondary) and Simbad (-010 +013 and -002 +006), but they both indicate the two stars are pretty much moving through interstellar space in parallel with each other, meaning there’s no indication of an orbital relation.   Here’s what that motion looks like on Simbad’s chart:

For those not familiar with proper motion numbers, the -013 +018 mentioned above for the primary means .013” per year west and .018” per year north, while -009 +022 for the secondary translates as .009” west per year and .022” north per year. (Click on the chart to enlarge it).

For those not familiar with proper motion numbers, the -013 +018 mentioned above for the primary means .013” per year west and .018” per year north, while -009 +022 for the secondary translates as .009” west per year and .022” north per year. (Click on the chart to enlarge it).

As for the position angle measurements by Wm. Herschel and the Herschel/South duo (their 10.8° south preceding translates to 259.2°), Sir James took issue with the senior Herschel’s description of the position angle, maintaining his “very near directly preceding” was “irreconcilable with a deviation of 10° from the parallel.”

And considering it was only an estimate, I actually thought Sir William had described it rather well.  Which just goes to show perception, as well as beauty, lies in the eye of the person at the eyepiece.

Off to the east again for the next tour as I race to catch sight of one more winter constellation before losing it to the western horizon.

Clear Skies! 😎

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