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Touring the 50mm/60mm Skies, Tour Number Six: Orion’s Belt Stars — Alnitak, Alnilam, and Mintaka

The three stars that gleam and glow and grace Orion’s belt are about as distinct as can be on a cold winter night, or even in the early spring before they start their slide out of sight into the west.

Majestic, fantastic, extraordinary — take your pick of one, or take all of them — Orion is simply in a class all by itself. (Stellarium screen image with labels added, click to enlarge).

Alnitak (Zeta – ζ), at the east edge of the belt, is probably the best known of the three, mainly because it hovers just north of the Horsehead Nebula (lower left center of photo) and southwest of the Flame Nebula (at the left edge of that photograph), even though it does its best to hide both of them behind its 1.85 magnitude blue-white light.  Second best known is Mintaka (Delta – δ), which anchors the west side of the belt at a magnitude of 2.40.  And least known — at least by name — is Alnilam (Epsilon – ε), which is strange since its in the center, and even stranger because it’s the brightest of the three at a magnitude of 1.65, which almost makes it a first magnitude star.

Now the fact that Alnitak is a double (actually a triple) is reasonably well known, and at least as well known is Mintaka’s duplicitous status — but I suspect there are only a few people on the planet that know Alnilam is also a double.  Not a knock-your-socks-off, spin-your-focus-knob dancing pair to be sure, but no less a double despite that.  But don’t let that deceive you — it’s a pretty pleasing challenge for a small aperture scope.  The name is derived from an Arabic word which means “The String of Pearls” — certainly an elegant description of the three Belt Stars.

At any rate, what all of that means is that all three of the Belt Stars are multiple stars!  Which is kind of neat, at least to me.

Now that might produce this kind of reaction  —  🙄  —  but I can explain.  I live on the northwest coast of Oregon.

For the uninitiated, what that means is I spend most of the winter getting drenched by rain and whipped around by wind.  So — when the rain runs dry, and the wind wears itself out, and the first stars return to claim the sky for a whole thirty minutes or so — it doesn’t take a whole lot of stellar electricity to jolt my damp and rusted Star Splitter mind into motion.

But tonight I see a chance to actually spend an entire hour — maybe more if the Sky Gods will look the other way for a while — under the newly rejuvenated sky, and I intend to see just how much I can see of those Belt Stars — and a couple of other gems — with a 50mm and a 60mm refractor.  So follow me — and bring your rain coat, just in case.  (If you haven’t been here before and are curious about the scopes, you might take a look at the introductory piece for this series here.)

I left the labels off this time to avoid cluttering the image with text (these stars are labeled in the image of above), so what you see here, starting on the left (east) side is Alnitak, Alnilam in the center, and Mintaka on the right (west). The brightest of the stars to the south and slightly west of Alnitak is Sigma (σ) Orionis, which is where we’ll eventually end this tour. (Stellarium screen image with labels added, click to enlarge).

Let’s start on the west side and work our way across to the east, which will give us a chance to begin with an easy split, Mintaka.

Mintaka – Delta (δ) Orionis   (Σ I 14)  (H V 10)     HIP: 25930   SAO: 132220
RA: 05h 32.0m   Dec: -00° 18′
Magnitudes:  2.4, 6.8
Separation:   53.0″
Position Angle: 359°   (WDS 2011)
Distance: 916 Light Years
Stellar Classification: BO
Rating: Easy

Greg has already covered the particulars of Mintaka rather well in a DSC-60 post , so I’ll stick mainly with my observations of it in the 50mm and 60mm refractors …………

The secondary is a very sharp pinpoint of bluish light . . . (East & west reversed to match the refractor view, click for a larger look).

………………  and my first observation was how easy this pair is to split.  In fact, it’s so easy that my first view of this pair of stars came several weeks ago in a pair of Canon 10×30 image-stabilized binoculars, and it was a real delight — the secondary was a very sharp pinpoint of bluish light silhouetted against a sky that was more blue than black, thanks to a nearby full moon.

So I could hardly fail with either the 50mmm f/10.8 Zeiss or the 60mm f/13.3 Carton-lensed refractor.  Using a 20mm TV Plössl (27x in the 50mm scope, 40x in the 60mm) in dark skies, the secondary is an intense point of light that seems as if it’s shining through a pin-sized hole poked in a piece of black velvet.  You can’t miss its relatively bright 6.8 magnitudes of light, and it’s far enough away from the glare of the 2.4 magnitude primary that it doesn’t suffer in the least from being overwhelmed.

With all that light available, I decided to give the 7.5mm Celestron Plössl (72x) a chance to perform in the 50mm scope ….. and it responded by etching a beautiful diffraction ring around the primary, and pushing the secondary far enough away that the resulting image looked almost like another pair of stars entirely.

William Herschel was the one to first recognize Mintaka’s duplicitous state, spying it for the first time on October 26th, 1779.  He described the primary as “w.” (white) and the secondary as “bluish” — which matches exactly with what I saw in both scopes, as well as in the binoculars.

Now one of the advantages of the area we’re in right now is you can move from one object to the next without using your finder.  Position Mintaka in the northwest corner of your eyepiece (use a low magnification to insure a large field of view), give your scope a slight nudge to the east, and you’ll be rewarded with the blue-white brilliance of Alnilam as it invades the field of view.

Alnilam – Epsilon (ε) Orionis    (BUP 81)    HIP: 26311     SAO: 132346
RA: 05h 36.2m   Dec: -01° 12′
Magnitudes:  1.7, 10.5
Separation:  179.3″
Position Angle: 58°   (WDS 2011)
Distance: 1342 Light Years
Stellar Classification: BO
Rating:  Difficult

And then see if you can find that sneaky secondary.  It shouldn’t be as tough to find as it is, but part of the problem is the overwhelming brilliance of the 1.7 magnitude primary, and the other part is that the slippery secondary has relatives that look suspiciously similar.  In fact, if I hadn’t looked up their magnitudes, I would swear they’re identical triplets.

Now the first time I looked for the secondary was with the 50mm Zeiss, and I saw what I thought was it right away.  But then I became suspicious — it was too easy.

If you look very closely at the sketch below, you’ll see there are three faint stars lined up at about, or very close, or right at, the correct position angle.  So which one is the secondary?

The magnitudes of the three faint stars to the northeast of the primary, which are discussed here, are shown in the inset at the lower right, and a bonus binary -- Σ 751 -- is visible at the north edge of the field of view. (East & west reversed to match the refractor view, click on the image and you'll lose this caption!).

The magnitudes of the three faint stars to the northeast of the primary are shown in the inset at the lower right, and a bonus binary — Σ 751 — is visible at the north edge of the field of view. (East & west reversed to match the refractor view, click on the image and you’ll lose this caption!).

STScI photo, plotted using Vizier.  East & west reversed to match the sketch above of Alnilam.

STScI photo, plotted using Vizier. East & west reversed to match the sketch above of Alnilam.  Click on the image for a better view.

It’s a heck of a good question, and I racked my mind and my focuser in and out for several weeks — when conditions would permit — in a determinedly ferocious attempt to forge an answer.  One of my many attempts began with looking up the magnitudes of that line of three stars extending to the east of the primary.  From that effort, it looked as if the last of the three, the one labeled 10.5 in the inset of the sketch, was the star I was after.  But I had a sneaking suspicion that one was much too far away, so eventually I discovered I could plot the distance and position angle of the closest of the three stars (the one labeled 11.6 in the inset) using the plotting feature of the Vizier Catalogue Database, and sure enough, that turned out to be the secondary I sought.  You can see the distance plotted in the negative image above (179.3”/60 = 2.99’); the position angle isn’t shown on the chart, but it measured 57.8 degrees.  As for the magnitude discrepancy, the 11.6 magnitude appears to have been the photographic magnitude, and the 10.5 magnitude listed in the WDS is normally the visual magnitude.  (No, photographic and visual magnitudes are not the same, but that’s a story for another day).

Having settled the question of which star was the secondary, there was no question whatever about how difficult it was to see it in both the 50mm and the 60mm scopes — it takes a fiendish delight in fading in and out of the primary’s glare.  What worked best for me was to look directly at the 8.2 magnitude star to its south, which seemed to prompt the secondary to make momentary averted vision appearances.  Stick with the intermediate focal lengths, in this case the 11mm TV Plössl (49x) in the 50mm scope, and the 15mm Plössl (53x) in the 60mm scope.   It takes the diligence and persistence possessed by a more than slightly obsessed Star Splitter to see it — but trust me, you can do it, too!

But hang on a minute!  There’s another double star that’s been sitting very quietly in the field of view all this time, too!

Σ 751          HIP: Not assigned in Simbad    SAO: 132337
RA: 05h 35.8m   Dec: – 00° 59′
Magnitudes: 8.0, 9.0
Separation:  15.8″
Position Angle: 123° (WDS 2003)
Stellar Classification: B8
Rating: Easy

Now these two stars, which can be seen north of Alnilam in the sketch above, aren’t the brightest bulbs in the fixture, but they do have one redeeming feature — you can split them with little effort!  So after that wrestling match with Alnilam, you cam see how that’s a very attractive feature.

You should be able to separate these two at the low magnification the 20mm TV Plössl provides in both of the scopes (27x in the 50mm, 40x in the 60mm), although in the smaller scope it’s possible you might need to jump ahead to something close to the 36x provided by the 15mm Plössl.

But now it’s time to return to the land of the difficult ………………

We’ll follow a familiar procedure  to get there — position Alnilam in the northwest corner of your eyepiece, nudge to the east, and be prepared for the brilliant blue-white light of Alnitak when it invades the field of view from the opposite corner of the eyepiece.

Alnitak – Zeta (ζ) Orionis    (Σ 774)  (H IV 21)     HIP: 26727    SAO: 132444
RA: 05h 40.7m   Dec: -01° 57′
Magnitudes (AC): 1.9, 9.6
Separation:  58.0″
Position Angle: 10°   (WDS 2006)
Distance: 817 Light Years
Stellar Classification: O9.5
Rating:  Difficult in the 60mm; Very Difficult in the 50mm

The secondary is seen here just barely north of the primary. It’s easy to be mislead into thinking one of the fainter stars to the north is the secondary because they’re much easier to see, but they’re much too far away. (East and west reversed once again, click to enlarge).

Alnitak, which was also discovered by Sir William Herschel (on October 10th, 1780), suffers from the same shy syndrome that Alnilam does, but even more so since the secondary, even though almost a full magnitude brighter, is more than three times closer.  And again, it’s easy to be misled by two stars of similar magnitude to Alnitak’s north, one that’s about four arcminutes away with a magnitude of 9.7, and another that’s a bit more than twice that distance with a magnitude of 9.5, as seen in the sketch to the left.

I’ve already covered Alnitak in more detail  with larger apertures, but since we’re dealing here with apertures of fifty and sixty millimeters, there’s considerable more challenge.  To be specific, I can’t claim with certainty that I saw that ghost-like secondary in the 50mm Zeiss, but I may have had a glimpse of it through the 7.5m Celestron Plössl (72x).  I did a bit better in the 60mm f/13.3, catching it with averted vision several times in the 11mm TV Plössl (73x), which tempted me to see what I could do with the 7.5mm Celestron (107x).  But all I saw was the glare glaring back at me, as if to say:  “Not today, go away.”   Honestly, this one just needs more aperture — at least 80mm would be a better choice.

So if we’ve learned anything at all from this struggle with Alnitak and Alnilam, it’s that they offer a pretty good reference point for what the limits are for the 50mm and 60mm scopes we’re using, at least in regard to large magnitude differences.  Alnilam is right at the point where it’s possible to catch sight of the secondary with decent cooperation from the atmosphere, while Alnitak is seriously pushing the limit of what is possible at these small apertures.  Of course, if you’re attempting to pry these two stars apart at a higher and drier altitude than the moisture saturated sea level air I peer through, you could probably extend those limits a bit more.

But, having put you through that one, I’ll see if I can get back into your good graces by taking you for a look at one of the more stunning triple stars in Orion, which is even shadowed by another triple.  And they’re both easy!

Sigma (σ) Orionis (Σ762)         HIP: 26549    SAO: 132406
RA: 05h 38.7m   Dec: -02° 36′
Magnitudes       AB: 3.8    C: 8.8    D:  6.6     E: 6.3
Separation         AB-C: 11.4″          AB-D: 12.8″        AB-E: 41.2″
PA     AB-C:  238°  (WDS 2008)    AB-D:   84°  (WDS 2011)    AB-E:   62°  (WDS 2011)
Distance: 1148 LY
Spectral Classification: O9.5 (A)  B2 (B)
Rating: AB, D, and E — Easy    C: Not Likely

Σ 761          HIP: Not assigned in Simbad    SAO: 132401
RA: 05h 38.6m   Dec: -02° 33′
Magnitudes:         A: 7.9    B: 8.4    C: 8.6
Separation –        AB:  67.8″     AC: 71.8″   BC:  8.5″
Position Angle –  AB: 203°   AC: 209°    BC: 269°  (All WDS 2011 data)
Distance: ?????
Spectral Classification: B5
Rating: Easy, except for the BC split, which is difficult in the 50mm scope

Between Greg and I, we’ve worn a path through the sky to these two groups of stars.  I described my experiences with them in the post on Alnitak mentioned above , and Greg covered Sigma (σ) in a DSC-60 post.  So I didn’t have any problem getting here — I just followed the groove through the sky between Alnitak and Sigma!

But if you don’t see our well worn path, move Alnitak over to the east corner of your eyepiece and then move your scope south slowly and you’ll be rewarded with the appearance of Sigma (σ) on the west side of your field of view.

Sigma (σ) sits in the center of this view, with “D” and “E” seen to its east, and the three stars of the much fainter Σ 761 hover northwest of it. (East & west reversed again, click for a larger and more pleasing view).

And you’ll find yourself looking at a bright 3.8 magnitude primary with a pair of sixth magnitude stars guarding its eastern approach.  Those are the “D” and “E” components, and they’re easily seen in either the 50mm or the 60m scope at low power.  What you won’t see — unless you have very good conditions and a very sharp eye — is the “C” component, which is lost behind the glare of the primary.  That one is difficult to see even in an 80mm scope.  At just a bit less than two magnitudes fainter than “D” and “E”, and almost the same distance from the primary as “D” is, you would expect “C” to be easier to see!   The cause of the difficulty is that the 3.8 magnitude primary is a full one hundred times brighter than “C”, which is a full five magnitudes fainter.  (That figure comes by multiply 2.51 times itself five times).

The much fainter Σ 761 group glimmering northwest of Sigma (σ) in your field of view will show up as a distinct pair of stars at low magnification in both of our small scopes.  I had very little problem splitting the BC pair in the 60mm scope using the 11mm TV Plössl (73x), but it eluded me in the 50mm scope with that eyepiece (49x), and the poor seeing conditions defeated my efforts every time I tried to catch it with the 7.5mm Celestron Plössl (72x).

On a night of very steady seeing, it’s a pure joy to behold those two stars, almost matched in magnitude, sitting there in the black sky so close together you could barely force a photon between them.  They have a very delicate, bead-like quality in a fifty or sixty millimeter scope that is unlike anything seen in larger apertures.  And that deliciously delicate quality is one of the frequent charms of these small scopes.

So there you have it — a diagonal trip through the center of Orion, with a dazzling flourish of multiple starlight to top it off.

Next destination — who knows?  But stay tuned for a surprise or two   ……….    and Clear Skies!


One Response

  1. I observed the belt stars years ago in an 8″ SCT, this article made me want to take a fresh look with my 4″ refractor. Great article – thanks!!

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