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Star-Hopping in the Northern Marches of Corona Borealis, Part One: H V 38, Sigma (σ) CrB, Rho (ρ) CrB, and OΣ 302

I’ve mentioned a few times in the past that Corona Borealis has always struck me as a small constellation dominated by double stars with little distance between them.  As it turns out, this curving constellation actually has a split personality — it’s mainly the southern half of the Northern Crown where claustrophobic separations rule.  When you venture up into its northern reaches, you’ll find the separations surprisingly spread out.  In fact, there’s quite a variety up here in the upper astronomical marches of this small section of sky, and we’re about to cut a wide swath right through the center of it.

So what in the name of Nusakan are astronomical marches, you ask?

Back in the pre-telescopic days of our European ancestors, those areas of Europe lying on the fringes of a king or an emperor’s domain were known as Marches — these days we would be more inclined to call them frontiers.  As you can see from the first chart below, the large area north of the etched outline of the Corona Borealis crown is a rather sparse wilderness of scattered stars – perhaps even a bit forbidding in appearance – and very much resembling a frontier.  But have no fear – it’s perfectly safe to grab a telescope and go marching around in it – which is just what we’re going to do.

 We’ll return to the scene of a prior visit to Corona Borealis and make our entrance from Hercules.  Using Epsilon (ε) and Zeta (ζ) Herculis as pointers, follow their directional arrow due west for about the same distance that separates them and you’ll find yourself sitting between two stars, one to the north (actually a pair, Nu-1 and Nu-2, which are labeled “v1, ν2”) and the other lying an equal distance to the south (Xi Coronae Borealis, which is labeled “ξ”).   (Stellarium screen image with labels added, click to enlarge).

We’ll return to the scene of a prior visit to Corona Borealis and make our entrance from Hercules. Using Epsilon (ε) and Zeta (ζ) Herculis as pointers, follow their directional arrow due west for about the same distance that separates them and you’ll find yourself sitting between two stars, one to the north (actually a pair, Nu-1 and Nu-2, which are labeled “v1, ν2”) and the other lying an equal distance to the south (Xi Coronae Borealis, which is labeled “ξ”). (Stellarium screen image with labels added, click to enlarge).

 While you’re parked at that midway point, take a peak in your finder and you should see a tight triangle of stars.  The two fainter ones are both 7.6 magnitude class K stars, so you might detect a slight hint of orange in your finder. The southernmost of the three is the brightest and just happens to be our first target, Sir William Herschel’s H V 38.  (Stellarium screen image with additional labels, click for a larger view).

While you’re parked at that midway point, take a peak in your finder and you should see a tight triangle of stars. The two fainter ones are both 7.6 magnitude class K stars, so you might detect a slight hint of orange in your finder. The southernmost of the three is the brightest and just happens to be our first target, Sir William Herschel’s H V 38. (Stellarium screen image with additional labels, click for a larger view).

H V 38  (STTA 145)                 HIP: 80247   SAO: 65262
RA: 16h 22.9m   Dec: +32 20°
Magnitudes: 6.41, 9.79
Separation:  31.0”
Position Angle: 17°  (WDS 2012)
Distance:  515 Light Years
Spectral Classifications: A4, A2

Now I hope you like white light, because there’s lots of it here:

My observing notes describe the primarial glow of H V 38 as “a beautiful pure white.”  (East & west reversed to match the refractor image, click for a better view).

My observing notes describe the primarial glow of H V 38 as “a beautiful pure white.” (East & west reversed to match the refractor image, click for a better view).

Herschel on H V 38

As you can see from Sir William’s catalog entry above, he added this pair to his catalog on September 21st, 1781, measuring the separation at the time as 36.27” (link to volume 72 of Philosophical Transactions).  Somehow red managed to get into his eyepiece once again, this time in his description of the secondary — “S[mall].  w[hite]. inclining to r[ed].]” – which I’ve seen so many times in his observations that I can’t help but think the frequent mention of that color is associated with either the speculum coating on his telescope mirror, or possibly has something to do with his eyepiece construction.

But the real surprise in his observation is his location of H V 38 in Hercules, even though he places Nu (ν) and Xi (ξ) in Corona Borealis.  He refers to this star with a Flamsteed number, 23, which I couldn’t find on any of the on-line versions of Flamsteed’s various atlas editions.  Finally, though, I did discover the Bright Star Catalogue refers to it as “23 Her in CrB.”

And in fact, if you go searching for this star in the venerable Admiral William H. Smyth’s handy mid-nineteenth century observing handbook, The Bedford Catalogue, you’ll find it filed in Hercules under the title “23 Herculis”.  (pp. 362-363 of the Willmann-Bell edition)

South on H V 38The Admiral adds two interesting pieces of information to this discussion of H V 38 (aka 23 Herculis in CrB): he saw the secondary as “violet” (an odd color to associate with a star), and also corrected another case of mistaken identity, pointing out that Sir James South had it confused with H V 88 (which is actually the AB pairing of Lambda (λ) Aurigae). Click on the thumbnail at right to see Sir James’ catalog entry.

So let that be a lesson about the dangers of flirting on the fringes of two constellations – it can scramble the photons in even the best of observers.

Meanwhile, we’ll move on to a less confusing stellar light, Sigma (σ) CrB, for which we’ll return to our second chart (click here to open it in a second window).  From H V 38, a two degree move to the north will take you to the Nu (ν) CrB twins, and a right angle turn to the west followed by another two degree hop will put you right on top of our target.  Or, if you’re adventurous, try an almost three degree northwest leap through interstellar darkness from H V 38.  Either way, all three stars should be visible at the same time in the average fifty degree 8×50 finder field, so you’ll have to work real hard to get lost (not that it can’t be done, trust me).

Sigma (σ) Coronae Borealis     HIP: 79607   SAO: 65165
(17 CrB)  (Σ 2032)  (AB is also H I 3, AC is also OΣ 538/STT 538)
RA: 16h 14.7m   Dec: +33° 52’
.     Magnitudes       Separation        Position Angle       WDS
AB: 5.62,   6.49               7.16”                  238°                  2013
AC: 5.62, 13.10            24.60”                    95°                  2009
AD: 5.62, 10.78            92.10”                    83°                  2009
AE: 5.62, 12.31          634.60”                  241°                  2002
BD: 6.49, 10.78           98.50”                     81°                  2009
Distance: 71 Light Years
Spectral Classifications: “A” is G0, “B” is G1
Notes: “A” and “B” are an orbital pair (orbit can be seen here)
AB and AD are optical pairs

Now this is a considerably more complicated star than H V 38, and it also offers a pleasing contrast in color, shifting from the previous pure white to a cream-colored white with a subtle hint of yellow hiding within:

The G0 and G1 colors of the AB pair combine to form a soft creamy glow.  (East & west reversed once more, click on the sketch for a larger view).

The G0 and G1 colors of the AB pair combine to form a soft creamy glow. (East & west reversed once more, click on the sketch for a larger view).

I find myself rather attracted to this family of five stars the more I return to it, which is partly due to its restrained coloration and partly due to the difficulty of prying “C” and “D” away from the primary-secondary glow.  After tackling the fainter stars of Sigma (σ) with the 9.25 inch SCT used for the sketch, I gave a five inch refractor (Meade AR-5) a shot at them and found “C” was invisible.   I did better with a six inch refractor, digging it out of the primary’s glare with concentrated averted vision.  “D”  was no problem at all in the five inch scope – in fact, William Herschel also saw it in his six inch reflector on August 7th, 1780.  (Philosophical Transactions, 1782, vol. 72, p. 116)  And then of course there’s the distant “E”, radiating it’s weak 12.3 magnitudes of light so far away (almost a full eleven minutes of arc) from “A” and “B” that it escapes their powerful glow.

At the opposite end of the aperture possibilities, the AB pair is a real delight to behold in a 60mm refractor.  That cream-colored glow still prevails, and if you need a challenge, you might try picking 10.78 magnitude “D” out of it.  It’s an averted vision affair only, but it’s a real treat to catch a glimpse of “D” flickering in and out of view with phantom-like stealth.

Back to the chart again (click here), and we’ll negotiate our way from Sigma (σ) to Rho (ρ) by leaping three degrees to the west with a slight inclination toward the south.

Rho (ρ) Coronae Borealis  (S 676)  (H VI 93)     HIP: 78459   SAO: 65024
RA: 16h 01.0m   Dec: +33° 18’
Magnitudes: 5.47, 10.51
Separation:  141.4”
Position Angle: 46°  (WDS 2011)
Distance: 57 Light Years
Spectral Classifications: G2, K0
Notes: Optical pair, first separation was 87.7” back in 1782

It took a couple of return visits before I warmed to this widely separated star.  Many of Sir James South’s catalog entries are wide pairs that tend toward the visually uninspiring side, but this one is saved from that fate by an endearing soft hint of glinting gold in the primary.  There’s just enough there to soften what would otherwise be a harsh white light bent on trying to suffocate the secondary’s feeble photons.

Just pale gold on a black background, but a lovely pale gold it is! (East & west reversed to match the refractor view, click to enlarge).

Just pale gold on a black background, but a lovely pale gold it is! (East & west reversed to match the refractor view, click to enlarge).

I sifted through my dusty archives and found Sir James’s 1825 observations of Rho (ρ), which I included here for your perusing pleasure (clicking on either page will get you a larger view and save the eye strain for the eyepiece end of a telescope):

Click to enlarge the image.

Click to enlarge the image.

Starting near the bottom of the left-hand page, you’ll see he had a very difficult time measuring the secondary.  There are four separate nights of observations listed on the two pages, all of which are labeled as “excessively difficult.”  He made five measurements of the PA and separation during each observation, and included the differences between them, which are actually pretty significant.  His separation differences run from 0.962” to 0.529” to 2.043”, all of which were made at 92x because he lost sight of the secondary at higher magnifications.

Each of the observations was made at South’s observatory in France (Passy) with a five inch refractor, which may seem like it should be sufficient for the task.  But from my own experience, trying to avoid overpowering a 10.5 magnitude secondary while still having enough weak red light available to measure the separation is a tough chore, even with a six inch refractor and an astrometric eyepiece equipped with an adjustable LED for illumination.   And he was using a device which used either a candle or an oil lamp for illumination!

Herschel on S 676William Herschel, whose observation South refers to, also had a tough time with measuring the secondary in his six inch reflector.  His observation can be seen by clicking on the thumbnail at the right, where he includes the comment “a little inaccurate.”

As I scrutinized these two-hundred year old observations, the increase in separation really stood out.   William Herschel’s first measurement in 1782 was 87.7”, which has widened to 141.4” as of 2011.  A quick check of the Washington Double Star Catalog (WDS) shows a rather high rate of proper motion for the primary, -199 in RA and -774 in declination (no figures are shown for the secondary).  Translated into annual motion, those numbers shown the primary is moving .199” west and .774” south per year, imparting a decisive southerly motion with a slight westerly component, which is quite visible when plotted (source):

(Simbad plot, click to enlarge)

The primary’s motion is the long red arrow — no idea why “Planet?” is shown here.   (Simbad plot, click to enlarge)

And in fact, you can actually see that motion in the mere forty-three years between Herschel and South’s observations.  Sir James even comments on it — a “surprising change . . . which can hardly be real.” –- which he attributes to a micrometer error by Herschel.  In reality, using the WDS proper motion numbers, the primary had moved 8.56” further west and 33.3” further south in those intervening forty-three years.  That motion is also very evident as the primary “zips” past the secondary, the position angles changing from 144.5 degrees in 1782 (Wm. Herschel) to 125.1 degrees in 1825 (South) to the 2011 WDS figure of 46 degrees, all of which gives the mistaken impression the secondary is moving north.

Neat stuff, and possibly a very interesting project for anyone interested in making annual measurements to show the observable change in separation and PA as it takes place each year – although you may find the measuring process as frustrating as Wm. Herschel and James South did.

In the meantime, we’ll return to our second chart (available here) and move two degrees northwest from Rho (ρ), which will take us to our next star, located halfway between Rho (ρ) and Kappa (κ) Coronae Borealis:

 OΣ 302  (STT 302)           HIP: 77933   SAO: 64970
RA: 15h 54.9m   Dec: +34° 22’
Magnitudes    AB: 7.16, 10.42       AC: 7.16, 12.38
Separations   AB: 28.80”                AC: 65.60”
Position Angles    AB: 51°  (WDS 2012)     AC: 358°  (WDS 2002)
Distance: 491 Light Years
Spectral Classifications:  “A” is A3, “B” is K0
Notes:  AC is WAL 68

And we’re back to white light again, although this time it’s about three-quarters of a magnitude fainter than that of H V 38:

The primary forms a crisp right angle triangle with its two much fainter companions, see just to the east and north.  (East & west reversed once again, click on the sketch for a larger view).

The primary forms a crisp almost right angle triangle with its two much fainter companions, see just to the east and north. (East & west reversed once again, click on the sketch to really see “C”).

The secondary is listed with a spectral class of K0, but try as I tried, I saw no hint whatever of orange photons attempting to escape from it.  The 12.38 “C” component required some stretching of my visual apparatus, but with persistent use of averted vision and six inches of refractor glass, I finally plucked it out of the primary’s white glow.

J. Herschel on STT 302The AB pair was first measured by Sir John Herschel in 1827 (thumbnail at the left) with a distance of 15” and a PA of 50 degrees, resulting in a rather harsh clash with the WDS figures above.  They show a mere one degree change in position angle as of 2012, whereas the separation increases by 13.8” – meaning something somewhere is incorrect and askew.

Hussey on STT 302In the meantime, I found a list of observations in William J. Hussey’s  compilation of Otto Struve’s catalog (seen at the right) which shows a quick leap from Sir John’s 15” measure to Johann Heinrich von Mädler’s 1845 measure of 28.22”, followed a year later by Herr Otto’s 28.58”, both of which are very consistent with the subsequent observations.

So let that provide encouragement to anyone just getting started in this business – even the giants of mid-nineteenth century astronomy were plagued by errors lurking in the night.

Next time out, we’ll continue our northwesterly trek through the northern Marches of Corona Borealis, so don’t stray too far into the wilds of the wilderness.

Hope your skies are clearer than mine!  😎

4 Responses

  1. Hi John!
    I continue to marvel at the level of research that you bring to this blog. I have one question…in the observing notes from Sir John South, he is referencing to a “Five-foot” and “Seven-foot Equatorial”…telescopes. Are these “five & seven” refering to optical tube length, assuming refractors, or the equitorial mount?

    Cheers, Chris

  2. Hi Chris,

    That is an excellent question!

    For some reason I have yet to discover, the convention in the late 18th and most of the 19th century was to refer to a telescope by the length of the tube — which tells you absolutely nothing about the optical capabilities of the scope.

    Sir James South’s “five foot equatorial” had a 96mm (3.75″) f/15.8 objective lens made by Dolland (here’s a photo of it). His “seven foot equatorial” had a five inch lens, which works out to something like f/16.8 when you divide the length of the tube by the diameter of the lens (84″/5″). No idea who constructed that refractor for him, or what happened to it.

    The Newtonian telescope used by William Herschel for most of his early double star and deep sky observations is usually referred to as a seven foot telescope, but the actual diameter of the mirror was 6.2 inches (another photo here).

    What I’ve found when researching double stars is the current literature frequently mistakes the old convention (i.e, length of telescope tube) for the actual diameter of the objective or mirror. So you have to be a bit careful when reading some of this stuff — it can be very misleading, to say the least. At least Wikipedia has this description of Sir William’s Newtonian correct!

    Cheers!

    John

    • Hi John!
      The image you have for the 5 foot refractor is begging for some further scrutiny. I am seeing something that has to be longer than 5 feet…I am thinking closer to 8+. Am I missing something.

      Cheers, Chris.

      • Hi Chris,

        I wondered about that, too, when I came across that image. It was described as the Five Foot Equatorial, but it could well be it’s the seven foot version. Hard to tell from the photo, but the end of the tube looks like it might be large enough to hold a five inch diameter lens. I’ve tried to run down that source a few times since with no luck.

        John

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