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.
H V 38 (STTA 145) HIP: 80247 SAO: 65262
RA: 16h 22.9m Dec: +32 20°
Magnitudes: 6.41, 9.79
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:
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)
The 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:
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
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.
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):
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!
William 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):
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 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.
The 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.
In 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! 😎