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An Eta-fyingly Close Encounter: Eta (η) Coronae Borealis

I know I’ve said it somewhere before in these pages (let’s see if I can find it . . . . . . . ah, here it is!), but this little curved constellation sure seems to have more than its fair share of tight doubles, which may have something to do with the relatively small slice of stellar real estate it occupies.  Normally I cringe at the thought of a pair of stars separated by less than 1.5”, so in general I avoid even the slightest sight of this Northern Crown.

But every now and then a surprisingly cooperative night comes my way, which was the case a few weeks ago.  I had just spent a very rewarding hour experimenting with how far apart I could pry Phi (φ) Ursae Majoris, and considering I had done better with it than I ever expected, I began looking around the sky for other possible tightly-knit targets.  And then, as if it was a blazing bolide appearing out of the inky black darkness, my eyes were suddenly drawn to Eta (η) Coronae Borealis.

Wedged between Boötes on the west and Hercules on the east, this gracefully curving constellation is one of the few that really resembles its name.  (Stellarium screen image with labels added, click to enlarge).

Wedged between Boötes on the west and Hercules on the east, this gracefully curving constellation is one of the few that really resembles its name.  Eta (η) is shown just to the right of center in a pleasingly warm shade of yellow. (Stellarium screen image with labels added, click to enlarge).

Eta Coronae Borealis (Σ 1937)  (AB is H I 16)           HIP: 75312   SAO: 64673
RA: 15h 23.2m   Dec: + 30° 17’
Magnitudes  AB: 5.64, 5.95   AB,C: 4.98, 13.35   AB,D: 4.98, 11.00
Separations AB: 0.667”         AB,C: 73.7”              AB,D: 217.7”
Position Angles   AB: 191°   (WDS 2013)
.                           AB,C: 359°   (WDS 2006)
                          AB,D:   41°   (WDS 2006)
Distance: 61 Light Years
Spectral Classification: “A” is F7; “B” is G0
Status: Gravitationally linked, orbit can be seen here.

Now I labored rather hard on this double-star-posing-as-a-single-star about a year ago for several nights, so my memory guided me to it right away.  Since it’s a naked eye star under dark skies, I just aimed the ole cream-colored tube of my six inch f/10 refractor in its direction, took a peek into an 18mm Radian (84x), and thar she was again, a wonderful shade of sizzling white with judiciously placed tinges of faint yellow melting into a mouth-watering lemon meringue custard.  Dazzling, delightful, and down-right appetizing.

Having just inhaled the thin upper stratospheric atmosphere of magnification possibilities on Phi (φ) UMa, I knew where I wanted to start – the 18mm Radian was about as far from my destination as Mars is from Miami.  Into the eyepiece box I went and came out with a 3.2mm TMB Planetary, offering what Sir William Herschel would describe as a tempting  475 diameters, better known to us these days as 475x.

Last year when I attempted this, I clearly remember uttering some very un-astronomical-like words, a few of which only used four letters, and none of which were MARS.  This time another four lettered word materialized in my mind as soon as Eta’s image landed in it:  HOPE  -–  with an exclamation point (!) even (!!!) . . . . . . .

. . . . . . . because without straining my eyes, without holding my breath, without even muttering sweet stellar nothings, I could actually see two stars!  OK, they were elongated, and yes, they were draped one over the other – as in overlapping – but still, they were two distinct almost globes of light.

That one word, HOPE, led to a desire to cast my line further out into the dark rippling current of the star-studded night, so I removed the 3.2mm and replaced it with a 2.5mm version cut from the same cylindrical mold . . . . . . .

. . . . . . . which elevated my aim to six hundred and eight of Sir William’s diameters.

 And allowed me to net this wriggling little duo of wiggling light:

Still touching, but only just.  The star image is courtesy of Sky Safari.  (East & west reversed to match the refractor view).

Still touching, but only just. The star image is courtesy of Sky Safari. (East & west reversed to match the refractor view, click for a larger version).

So close!   Arggghhh!  I cast my eyes up into the vacuous night and begged and beseeched and implored the heavens – please! – just one quick glimpse of a thin black line of interstellar nothingness between Eta-A and Eta-B.   I promised the Sky Gods an Ethos I didn’t have, I promised my four-legged observing partner all the dog bones in the pantry —  I even swore (bad choice of words) never to use un-astronomical-like words for the rest of my life!

But it made no difference.

As I sat hunched over the eyepiece, imploring the impossible from the invisible, I could feel my feet sliding on the slippery wet rocks of seeing degradation.  What I mean is the seeing was slipping south faster than a barrel of hapless human hurtling over Niagara Falls, thanks to an approaching cold front swooping in from the Pacific.

RATS!  (Whoops – said I wouldn’t do that).

But just on the off-chance I might reel in some real luck, I slipped a 4mm TMB Planetary into a 2.4x Barlow . . . . . . which brought me 894 diameters of smeared shimmering light with hints of oval-ing globes buried near the center.  It quickly became obvious they were never going to separate — not now.  Maybe if I had been thirty minutes sooner, I could have done it –- but definitely not now.  If I had just been a little less mesmerized by the view and cast my line right away -– but it was way too late now.

I knew darn well I had just missed a very rare chance of splitting this pulsating pair of grasping stars.  Which was all the more frustrating because I was also well aware this is the year to succeed with Eta (η) – because these two luscious globes of creamy white light are slowly moving closer together after this year . . . . . . .


 Theta  Rho        Theta  Rho      Theta   Rho      Theta   Rho     Theta   Rho
 .     2011                  2012                 2013                  2014                 2015
175.5  0.623     183.5  0.651    191.0  0.667    198.4  0.665     205.9  0.643

(Theta is position angle, Rho is separation)
from WDS Sixth Catalog of Orbits of Visual Binary Stars: Ephemerides


. . . . . . . not by a whole lot, as you can see, but in the miniscule range of separation this pair of stars inhabits, every thousandth of an arc second counts!

Now if you care to give Eta (η) Cor Bor a try, you can either go for it cold turkey like I did, or you can follow these well thought out instructions suggested by Sir William back in 1781:

 . . . when η Coronae borealis (one of the most minute double stars) is proposed to be viewed, let the telescope be some time before directed to α Geminorum, or if not in view to either of the following stars, ζ Aquarii, μ Draconis, ρ Herculis, α Piscium, or the curious double-double star ε Lyrae.  These should be kept in view for a considerable time, that the eye may acquire the habit of seeing such objects well and distinctly.  The observer may next proceed to ξ Ursae majoris, and the beautiful treble star in Monoceros’s right fore-foot; after these to ι Bootis, which is a fine miniature of α Geminorum, to the star preceding α Orionis, and to η Orionis.  By this time both the eye and the telescope will be prepared for a still finer picture, which is η Coronae borealis.  It will be in vain to attempt this latter if all the former, at least ι Bootis, cannot distinctly be perceived to be fairly separated because it is almost as fine a miniature of ι Bootis as that is of α Geminorum.”

Sir William Herschel: On the Parallax of Fixed Stars: Philosophical Transactions, Vol 72 (1782), pp. 100-101.

At any rate, it’s been an interesting past few weeks.  I’ve gone from a frustrating and de-φ-ing (de-PHI-ing) experience to a considerably more illuminating and η-fying (ETA-fying) experience  ——-  and somehow managed to not become totally π-eyed (Pi-eyed) during the process.  🙄

Although it is possible I was out in the cold dark damp night just a little too long.

Clear Skies!  😎


4 Responses

  1. Hi John,

    I was laughing my head off at this report. We’re nutters to say the least! LOL

    If at first you don’t succeed, try, try again! Incidentally, I’ve had similar issues this side of the great pond. The weather just won’t settle down. One minute it’s chucking it down with rain, the next, bright sunshine. Day and night, it’s the same!

    I have my 6″ f/8 Bazooka ready and armed for the killing.I also know that Belisarius will take at least 480 diameters (I’m a Herschelian at heart!) I know it’s possible but I just need the right kind of sky.

    Anyways. great report! And I do hope you gave Klaus his dividend of dog biscuits just for being patient!

    Best wishes,


  2. Hi John, Another great read your picture of Eta at 608 X reminds
    me of XI Ursa Major through my 80mm Zeiss at 200X. both
    beautiful doubles. Anyway congrats on almost splitting Eta Coronae


  3. Hi guys!

    Well, you either need a healthy sense of humor to do the things we do, or you’ll soon become a frustrated bundle of quivering nerve endings in search of a less torturous way to relax. There’s peace to be had under the stars provided you remember the huge dimensions of the monster you’re sometimes wrestling with.

    But I know I was within a hair’s breadth of splitting Eta CrB that night! If only I had made the decision to go after it about thirty minutes sooner! There’s lots of nights left this year and I don’t intend to miss another chance.

    I saw on CN where Rutilus has coaxed a split from this pair of stars with a six inch f/15 (Istar lens) at 643x and 900x. So I know it’s there if the heavens will only cooperate!


  4. Nice reading, you always put out a great story.

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