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Touring the 50mm/60mm Skies — Tour Number Five: Iota (ι) Orionis and it’s Jewels: Σ 745, Σ 747 and Σ 754

Majestically huge and one of the finest sights in the sky to grace the eye, Orion is absolutely captivating on a dark, moonless night — and even on a full moon night it’s distinctive outline is guaranteed to capture your attention. (Stellarium screen image with labels added, click for a larger view).

If ever there was an area of the sky just calling out to a telescope of small aperture to come and take a long look at it, the area surrounding Iota (ι) Orionis would be it.  And if ever there was an area that is frequently over-shadowed by more spectacular sights, this is it as well.

Well beyond the power of words to do justice to it, M42 has kept me glued to the eyepiece of a telescope for more hours than I can count. It and the diamond-like stars of the Trapezium draw my attention in the same way a bar of steel becomes riveted to a magnet. (Stellarium screen image with labels added, click to enlarge).

The problem is M42, the Orion Nebula.   It’s a spectacular problem, actually  —  because it’s an awe-inspiring sight.  But it’s that very quality which draws your eyes away from the beauty of the area just barely south of it, which under less competitive circumstances would rank right up there with the grandest views in the night sky.

In fact, if you can free yourself from the magnetic attraction of M42, you’ll find Iota (ι) can even be a bit of a problem as well.  On a dark night, it sits at the center of a cloud of glowing nebulosity which tends to divert your attention away from the gleaming jewels surrounding it.  My eyes have inched over to Iota more than once, and yet I’ve never taken the time to identify any of those stars —  nor did I realize how many of them were multiple stars.

But all that’s about to change as of tonight!

Grab your 50mm or 60mm scope and come along with me.  (The introductory material for this series, including the scopes being used, can be found here).  I’ll show you three very easy doubles — one of which is a tantalizingly tough triple and one a ghost-like triple — plus a genuinely difficult double.  And all of them fit very gracefully within the field of view of a small telescope, even at moderate magnification.

Let’s start with two views of the area.  This first one is the conventional view — although the one shown here goes a bit deeper, the orientation matches what you’ll see in an 8×50 correct image finder:

The three multiple-star jewels that surround Iota (ι) are seen here just to its south and southwest. If you happen to have a pair of binoculars, Σ 747 is easy to split. And if the binoculars happen to be mounted, you may even be able to pry apart Σ 745. In fact, the binocular view of this entire area of Orion is a stunning sight! (Stellarium screen image with labels added, click for a larger view).

Here it is again — and this time the view has been flipped horizontally (east and west reversed) to match what you’ll see in a refractor or SCT using a diagonal . . .

. . . and you can see that all four of our stars for this tour fit very comfortably into the field of view of the yellow circle, which is about one full degree in diameter. (Stellarium screen image, click to enlarge the view).

Na’ir al Saif: The Bright One in the Sword

Iota (ι) Orionis  (Σ 752)  (H III 12)      HIP: 26241    SAO: 132323
RA: 05h 35.4m   Dec: -05° 55′
Magnitudes    AB: 2.9, 7.0       AC: 2.9, 9.7
Separation     AB: 10.8″           AC: 49.4″
Position Angles    AB: 138°  (WDS 2009)     AC: 103°  (WDS 2002)
Distance:  1326 Light Years
Spectral Classification   A: O9   B: B   C: A or F
Rating   AB: easy to moderate     AC: difficult

If you look up at Orion on a crisp winter night, your eyes will quickly be drawn to the glow of this brightest star in the sword (although, as I mentioned above, it’s M42 that gets all the attention in a telescope).  That naked-eye brightness is helped more than a little by the cluster of stars surrounding Iota (ι), as well as by the nebulosity it illuminates so skillfully — all of which goes by the collective name of NGC 1980.

Iota (ι) consists of four stars, one of which is a spectroscopic binary — meaning it’s well beyond our reach, whether we’re wielding a 60mm scope or a 60 inch scope.  But it was Sir William Herschel who first dissected Iota (ι) — on October 7th, 1779 — into the three stars we see in our telescopes.  Of those three, the primary is a bright blue-white 2.9 magnitude star which does its absolute best to obscure the other two.  The seventh magnitude secondary is really not all that difficult, but you’ll probably need to use averted vision in a 50mm or 60mm scope to see it the first time you look for it.  If you happen to catch it on a night when the seeing and transparency decide to work together, producing both stable seeing (a III on this chart) and matching transparency, you should be able to see it with direct vision.  In the 50mm Zeiss, I need the 15mm TV Plössl (36x) to see it at all (direct vision) — the 20mm (27x) just wouldn’t pull it out of Iota’s glare — while in the 60mm f/13.3 refractor, the 20mm (40x) is more than enough to do the trick.

The third star (“C”), even though almost five times farther from the primary than the secondary, is 2.7 magnitudes fainter, which is just enough make it a star with a difficult personality.  I can glimpse it with averted vision in the 50mm Zeiss at 49x using the 11mm TV Plössl, and I’ve glimpsed it a few times in the 60mm f/13.3 using a 20mm TV Plössl (40x).  Low power is best here — too much magnification amplifies both the glow of the nebulosity and the glare of Iota (ι).

This is what you’ll see in a 60mm f/13.3 (800mm focal length) refractor at 40x. All of the stars’ companions mentioned in this post are shown here, but some are pretty darn faint. You’ll find it may help to see all of them if you turn off any lights that are near your computer screen. This same sketch, minus the labels, is at the end of this piece. (East & west reversed, click to lose this caption).

And here’s a tip — your focus needs to be very precise!  Focusing until the Iota (ι) primary is sharp won’t necessarily pull either of the two companions out of its glare.  If Iota is pinpoint sharp and the secondary still isn’t visible, nudge your focuser just a bit more and you should see the secondary appear first — what you’ll see is a very tiny gleaming point of light.  When you have it, fine-tune the focus until it’s at its most distinct, and then cast an averted glance a bit beyond it and away from the primary.  That’s the point at which “C” should show up — if it doesn’t, let your eyes wander around the area without drifting very far from the primary.  That 9.7 magnitude micro-dot of light is prone to popping into view and then disappearing again, so be patient.  It’s sly, but not bashful.

Now I’ve never seen Iota (ι) as anything other than blue-white, and the two fainter companions I would characterize as fainter flavors of the same tone.  Haas describes the primary as yellow-white, and Admiral Smyth calls it white.  He saw the secondary as blue-white, and described the third star as — hang onto your focus knob now — grape red.  Sir William Herschel, who saw the primary as white, described both “B” and “C” as “dusky r.[red].”   And the other day I noticed that the Night Sky Observer’s Guide (scroll down to the fourth listing) also describes “C”as red!  So there must be more there than has met my eyes so far.  I think I hear “C” calling out for more aperture.

And Now the Jewels ………………..

Σ 747  (H III 14)          HIP: 26199    SAO: 132301
RA: 05h 35.0m   Dec: -06° 00′
Magnitudes: 4.7, 5.5
Separation:  36.0″
Position Angle: 224°  (WDS 2010)
Distance: 1864 Light Years
Spectral Classification: B0.5, B1
Rating: Easy

Just to the southwest of Iota (ι) is a very distinctive pair of stars of almost equal magnitude, which also make a very attractive pair in binoculars.  Because they’re wide and bright, there’s nothing difficult at all here.  Sir William Herschel discovered this pair on the same night he made Iota (ι) famous, which is where the designation H III 14 comes from.

Again, I see blue-white when I look at these two stars, but F.G.W. Struve, whose name now is attached to them, described the primary as whitish-yellow and the secondary as “bluish” when he looked at them back in 1825.

Wide field, low magnification works best here, especially since it keeps them in the same field with Iota (ι), as well as with our next star …………..

Σ 745  (H III 13)             HIP: Not assigned in Simbad    SAO: 132289
RA: 05h 34.8m   Dec: -06° 00
Magnitudes   AB: 8.3, 8.6      AC: 8.3, 10.4
Separation    AB: 28.6″          AC: 96.9″
Position Angles    AB: 347°  (WDS 2002)     AC: 304°  (WDS 2000)
Distance: 279 Light Years
Spectral Classification: A
Rating   AB: Easy   BC: Moderate

………… which is barely west of it.

You’ll see that the primary and secondary of this pair are almost matched in magnitude, and about the same distance apart as Iota “A” and “B”.  Still, in a small apertured scope, despite the almost equal magnitudes, the secondary has an eerie, ghostly quality about it.  You shouldn’t have any trouble separating the two stars on a night of average to even sub-average seeing (a II on this chart), but if you can’t quite get them with the equivalent of a 20mm eyepiece (27x in my f/10.8 50mm and 40x in my f/13.3 60mm), try something in the neighborhood of a 15mm eyepiece (36x and 53x, respectively). There’s not a lot of light here, so at the 50mm and/or 60mm apertures we’re working with, too much magnification will dim the stars to the point that they become harder to see.

And speaking of a ghostly presence, that describes 10.4 magnitude “C” right down to its last photon.  In the 50mm Zeiss/20mm TV Plössl (27x) combination, I can’t see it at all.  But it suddenly springs into averted vision view as soon as I increase the magnification to 36x with the 15mm Plössl, and it’s almost to the point of being detectable with direct vision in the 11mm TV Plössl (49x).  The 60m f/13.3 sees it with little problem, although I did have to look closely, which shows just how much difference ten millimeters of aperture can make under the right circumstances.

There seems to be some confusion about identity in the Herschel Double Star Catalog, which applies the H III 13 designation to our next star (Σ 754), but it certainly looks to me as though it was meant for Σ 745.  William Herschel looked at it on the same date as our previous stars, and combined it with a description of the star we just looked at, Σ 747.

Here’s his description (from the preceding link), with my comments in brackets:

Double-Treble. It is the preceding or smallest of the two iota’s {confusing, but Herschel tended to group nearby stars with a brighter star in other cases, too, as when he applied Epsilon to H III 111 (Σ 758), a multiple star just north of the middle belt star in Orion (Alnilam), which is the actual Epsilon Orionis}.  The preceding set (forming a triangle) consists of three equal stars {which describes Σ 745 very well — “preceding” refers to their movement through they eyepiece}. All dusky r. The following set (forming an arch) consists of three stars of different sizes. The middle star is the largest; that to the south is also pretty large; and the third is very small. L. w.; l. w.; S. pale r.”  {And that describes Σ 747 well if you include the faint star to its north, which also explains Herschel’s “double-treble” at the beginning of this description}.

Thomas Lewis also assigns Herschel’s H III 13 designation to Σ 745 in his 1906 compilation of Struve’s observations on page 152 of this book.  Whatever the case, F.G.W. Struve again is credited with them based on his 1831 observations of them.

Σ 754  (H III 13)             HIP: 26345    SAO: 132359
RA: 05h 36.6m   Dec: -06° 04′
Magnitudes: 5.7, 9.3
Separation: 5.3″
Position Angle: 288°  (WDS 2002)
Distance: 1919 Light Years
Spectral Classification: B1
Rating: Difficult

And that brings us to our last, and most difficult pair.  Raise a few eyepieces to the Sky Gods and say nice things to them   …………   ’cause you’re going to need some help here!  I’ve spent more time pursuing these diminutive dancing points of light than I’ll ever admit in public, but that’s only because I know they’re well within reach of at least the 60mm scope.  The element that has been lacking is good seeing that happens to coincide with a moonless night.

As it is, I’m fairly certain I glimpsed the 9.3 secondary in the 60mm f/13.3 with a 15mm TV Plössl (53x), but I’m not about to bet the farm — or a telescope — on it yet.  I admit to cheating a bit, however.  First I spied the little dancing devil in a 102mm Celestron f/10 refractor, which in itself was far from an easy task.  But once I had it’s location firmly fixed, I went back to it with the 60mm scope and came away at least 50% convinced I had seen it dance into view briefly.  Then again, a cold, desperate, and determined Star Splitting mind is apt to play tricks, too.

The best I can say for the 50mm scope is I can detect a hint of duplicity — meaning the primary won’t quite come to a sharp focus — at 49x with the 11mm Plössl.  The 7.5mm Celestron Plössl (72x) just about exhausts all the light available, but given very stable seeing conditions, it might be possible to catch the secondary clinging to the primary.

The very stable seeing conditions I need in order to pry this pair apart have been missing for the last eight weeks or so — seems they flew south for the winter.  So I’ll leave it here for now, but I’ll be back to update this entry just as soon the Sky Gods see fit to cooperate some evening — hopefully before April, when I’ll lose Orion to the ravenous Hemlocks which devour all stars at my location that edge too far to the west.

F.G. W. Struve was kind enough in his 1830 observation of these two close stars to leave us at least a description of colors: white and blue.  That matches up with their stellar classifications, and pretty well describes what I saw, even though my apparent glancing glimpse of the secondary was very brief.

Before you leave this area, take some time to slide your scope north a couple of degrees and look closely at the Trapezium in the center of M42, which Greg has already covered very well with a small scope in this piece.  You won’t get the stunning diamond-studded view that can be seen in apertures of four inches or more, but there’s something very addicting about the way those four stars are displayed in a 50mm or 60mm scope as a very tight knot.  The dimmest of the four has a way of fading in and out of view, offering a tantalizing challenge that’s easy to meet with a bit of close scrutiny.

And so another exciting adventure of Touring the 50mm/60mm Skies comes to a rousing conclusion.  If you’ve followed all of these tours, you should be leaning by now toward the realization that there really are quite a lot of double stars up there that are more than willing to meet with small aperture telescopes on a dark night.  And so far we’ve barely scratched the surface — or in this case — the sky.

Next time out we’ll take a stroll through the northern half of Orion.  Until then, may your skies be clear and dark and all the stars intense pinpoints of gleaming light!  😯

Same sketch as above, minus the labels! (Click to see it without this caption).


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