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Touring the 50mm/60mm Skies, Tour Number Four: In the Northwest Corner of Taurus — Σ I 7, Σ 401, OΣΣ 38, and Σ 427

High in the sky, tucked between the Pleiades and the southern border of Perseus, in a visually barren, triangular shaped corner of the sky where eyes seldom pry, is a group of four double stars that have been waiting patiently for a few Star Splitting enthusiasts, such as you and I, to re-discover them.

Which is why we’re out here cooling our toes tonight in the brisk winter air.  And we’re going to rediscover them with a pair of small-apertured refractors — the first being a 60mm f/13.3, and the other a 50mm f/10.8, both of which are described in the introductory post for this series.

So how in the heck did we ever end up here in this barren stellar desert?  A darned good question.

First, immediate credit goes to a dedicated Star Splitter who frequently aims a six inch Celestron refractor into his New Jersey skies.  It was Javier who stoked the stellar flames of my curiosity when he called my attention to this area about a month ago.  Second, when I checked Sissy Haas’s book to see if she had spent time up here, I discovered she had looked at three of the four stars with a 60mm refractor, which stoked those stellar flames into a raging fire.  That’s because — and this is third — the first and second just happened to coincide with the time I was planning this series.

So  —-  I suppose you could say the stars happened to be aligned just right. 😉

Grab your 50mm or 60mm scope and prepare to turn it towards Taurus, and we’ll let the four stars of this tour demonstrate just how capable an instrument a small refractor is. We’re head for this North Taurus Triangle, the corners of which are marked by Omicron (ο) Persei, HIP 15549, and the Pleiades. (Stellarium screen image with labels added, click to enlarge).

Now, by way of preparing you for what will soon be obvious, I need to emphasize one thing here first — these aren’t bright stars.  They’re not particularly difficult — one of them is separated by enough space to chase the Taurus bull through (or for him to chase you through) — but they aren’t bright.  And that means you won’t find yourself dazzled by an enormous flood of photons.  Instead, this tour that will provide an opportunity to enhance your appreciation of those aspects of viewing that are sometimes lost in the glare of those flooding photons.   Because there’s another aspect of double stars that — if you keep at this long enough — can be every bit as aesthetically pleasing as the more visually stimulating sights.

This is a closer view of the area within the triangle …………. (Stellarium screen image with labels added, click to enlarge).

…………. and here it is again, without the green triangle super-imposed on the sky. (Stellarium screen image once more, with labels added).

We’re going to begin with Σ I 7 and Σ 401, which appear as one star to the naked eye (the other two stars of this tour are below the naked eye threshold), and it turns out that the easiest way to get there is to go completely around the triangle.  So, using the chart above that works best for you, we’ll start at the Pleiades (found at the bottom center of either chart), which is the first corner of our triangle.

Then look north about four degrees and locate 2.8 magnitude Zeta (ζ) Persei and 3.9 magnitude Omicron (ο) Persei.  From Omicron (ο), the second corner of the triangle, extend a line about three degrees to the southwest and you’ll find your eyes drawn to 4.5 magnitude HIP 15549, the third corner.  Now extend a line from it to the Pleiades, and not quite halfway along that line you’ll see a faint star, which is the fifth magnitude glow of the combined light from Σ I 7 and Σ 401.

If you can’t see it because of sky glow, point your scope at that halfway spot and chances are you’ll land right on it — I’ve actually done it twice in a row!  Binoculars will help also, so don’t hesitate to use them if you have them.

Σ I 7  (STFA 7)  (HJL 49)          HIP: 16386    SAO: 75964
RA: 3h 31.1m   Dec: +27° 44′
Magnitudes: 7.4, 7.8
Separation:  43.9″
Position Angle: 234°   (WDS 2010)
Distance: 311 Light Years
Spectral Classification: B9
Rating: Easy

Σ 401  (STF 401)  (BC is SHY 445)         HIP: 16411    SAO: 75970
RA: 3h 31.3m  Dec: +27° 34′
Magnitudes  AB: 6.6, 6.9     BC: 6.9, 10.9
Separation   AB: 11.4″         BC: 999.9″
Position Angle   AB: 270°  (WDS 2010)   BC: 129°   (WDS 1997)
Distance: 776.5 Light Years
Spectral Classification   A: A2    B: A3     C: F5
Rating   AB: Moderate     AC: Moderate to Difficult

Now these two stars were a revelation to me when I first laid star prying eyes on them in the 60mm scope  —  mainly because I found them without seeing them.  There was some hazy moisture in the air that night, which meant I couldn’t quite glimpse this pair via the naked eye approach, so I estimated their location, found what looked like it might be the wider of the pair, Σ I 7, in the 6×30 finder, bent over the 20mm TV Plössl (40x) — and darned if both of them weren’t in the field of view.  Admiral Smyth, who had a demonstrated talent for it, would have been proud of the results of my celestial navigation.

Well separated in a dim field, these two stars offer a contrast in spacing and position angle. Haas describes them as a “lovely double double” — and I don’t disagree in the least! (East & west reversed to match the refractor view, click to see a view without this caption).

My eyes were drawn first to the wider Σ I 7, which is also the dimmer of this pair.  But they quickly found their way to the brighter and tighter Σ 401, although they had to stare hard for a few seconds before I realized they were looking at two tightly spaced stars instead of one.  Once their dual nature became apparent, I had no problem keeping the 6.6 magnitude primary and the 6.9 magnitude secondary separated.  But when I returned later on a night with rather poor seeing, I found the two of them merging and parting in a very eerie ir-rhythmic dance.

In the 50mm Zeiss on that first night, Σ I 7 was certainly no problem in the 15mm TV Plössl (36x).  It’s more tightly spaced partner, Σ 401, required a steady eye, but its two gleaming, very closely spaced stellar pinpoints of light were a real delight to behold.  Even with the 20mm TV Plössl, weakly magnified at 27x, Σ 401 was still distinctly separated.

The “C” component of Σ 401 poked its way into view in the 60mm refractor without a struggle, even using the 20mm Plössl, but it took some persistent peering in the 50mm Zeiss to prod it out of the dark sky.   I got it, though, with the 15mm Plössl, thanks to the fact that the 60mm had already shown me where it was.  It’s hardly stunning, to say the least, but it’s not all that tough to find since it lies on a line that runs almost directly through Σ 401 and Σ I 7.

Haas describes these as “a wide white pair (Σ I 7) and a close yellow pair (Σ 401)”, but the closest I could come to finding any color in either the 50mm or the 60mm scope was pale white in Σ I 7 and bright white in Σ 401.  I did get a chance to look at these in a 102mm Celestron refractor a few nights later, but the seeing and transparency were both so poor I still couldn’t detect any color.

Next, let’s wander east to our third star, OΣΣ 38, and we’ll use a new chart to zoom into this area:

To get to OΣΣ 38, move east right between 7.35 magnitude HIP 16791 and the 8.1 magnitude star just north and slightly west of it, for a distance of about two degrees. You shouldn’t have to move very far at all before it appears at the east edge of your eyepiece field. You’ll see it as a pair of closely spaced stars in a 6×30 or 8×50 finder, and it’s easily separated in a small pair of binoculars as well. (Stellarium screen image, click for a larger view).

OΣΣ 38  (STTA 38)              HIP: 17470    SAO: 76122
RA: 3h 44.6m   Dec: +27° 54′
Magnitudes: 6.8, 6.9
Separation:  133.8″
Position Angle: 51°   (WDS 2003)
Distance: 192 Light Years
Spectral Classification: F4
Rating: Easy, Easy, Easy

Nothing difficult here — you’ll see them right away.  As I said earlier, the Taurus bull could race between these almost seventh magnitude stars with no problem.

My first visual encounter with them was in the 50mm Zeiss/20mm TV Plössl (27x) combination, which is the ideal way to see them.  I detected a very slight hint of some  kind of color trying to escape this pair, but I wasn’t sure what it was.  Haas saw “whitish gold” in her 60mm at 25x, so maybe it was a hint of yellow I saw trying to get loose.

Can’t miss these in the center of that dim field! (East & west reversed, click to lose the caption).

As I mentioned, the low power view seems to works best here.  Usually, too much magnification of a widely spaced pair of stars weakens their visual impact.  But in this case  ………….  we’ll see that it reveals another quality lying dormant within the field of view.

As you’ll quickly recognize, the background stars in the field of view here are on the dim side, which is consistent with the general them of this triangular slice of Taurian sky.  But if you look closely, the two main attractions of that field are surrounded by a very distinctive — and dim — cluster of five stars.  The trick to teasing them away from their shy disposition is to add a little magnification and a bit of aperture.  So I gave the 60mm f/13.3 a try, with the 15mm TV Plössl (53x) to tempt them out of hiding, and it did wonders for their appearance, as shown in this sketch:

With a bit more magnification and a 10mm increase in aperture, the surrounding field comes to life and provides an interesting backdrop for the two stars that brought us here in the first place. (East & west reversed once again, click for a caption-less view).

Now no doubt you could do even better with a larger aperture, but we’ve done just fine here with a 60mm scope.

This is one of the subtle aspects of double star gazing I was referring to at the beginning of this post.  It’s no secret that two relatively dim, evenly matched, widely separated stars can be a bit boring.  But the context — in this case, the surrounding field of view — can make all the difference in the world between boring and interesting.  And the main thing with regard to the view we’re looking at right now is to recognize that it has the potential to get even better the second or third time you come back to it.

What seems to happen is this:

Somehow that visual image gets imprinted in the dark depths of your memory in such a way that you’re not the least bit aware of it — until you come back for another look.  And when you do, all of a sudden the light bulb of recognition gets switched on, and you begin to find something not quite describable, yet still very compelling, about seeing that same view once more.  So on one of those visits, maybe about the third time, you find yourself lingering over the eyepiece for several minutes, and gradually you realize the noisy cares of this terrestrial world have been banished from your thoughts.

Trust me, it happens.  It may not be this star that creates that magic, but if you keep at this pursuit, sooner or later that uncanny magic will reach out and grab you one night.

And after that night, the skies will never quite be the same for you again.

Meanwhile, back at the tour center, we’ll start on our way to our last star, Σ 427.  This is the pair that Haas didn’t view with a 60mm refractor (she used a 125mm scope), so we just might be blazing a new trail across the sky with both the 50mm and the 60mm apertures!

Looking again at our last chart, you’ll find Σ 427 lies a very short two degrees to the northwest of our current location at OΣΣ 38.  In fact, it’s probably right at the edge of the field of view in your eyepiece, so just nudge your telescope a bit to the northwest.  If it’s not already there, you’ll see 6.85 magnitude HIP 17220 come into view first, and immediately after that, the two stars of Σ 427 will grace your eyepiece.

Σ 427  (STF 427)  (H N 23)           HIP: 17168    SAO: 76071
RA: 3h 40.6m   Dec: +28° 46′
Magnitudes: 7.4, 7.8
Separation:  7.0″
Position Angle: 208°   (WDS 2007)
Distance: 502 Light Years
Spectral Classification: A1, A2
Rating: Moderate

Or at least they should.

The first time I cast eyes on Σ 427, I could see each of the components very distinctly in the 60mm f/13.3 using the 20mm TV Plössl (40x).  They were a pale white pair, very close, but very clearly separated  ……….  and then the seeing quickly slipped down the scale to about a I, and the two stars became an indistinguishable, slightly elongated blob of light.  I repeated that experience with the 50mm Zeiss, using both the 20mm TV Plössl (27x) and its 15mm sibling (36x) — tight, but not difficult — until the seeing threw up its hands and surrendered to bad influences.

So if your seeing conditions don’t match the III (average) on this scale, you may not get the two-star-view.  I took that into account by rating it as moderately difficult.  Otherwise, the pair will be tight, but quite distinct.

This is really a pleasing pair for the eyes, contrasting well with the previous star, the much wider OΣΣ 38.  Few sights in the sky can compete with a very closely spaced, but still very clearly separated, pair of equal magnitude stars.  I’ve seen similar pairs as faint as 11th magnitude in larger scopes, and they all have an ethereal beauty that is unlike anything else in the starry firmament — NOT better, but unique.

And there they sit, all alone except for each other, right at the center of the view. But wait! Feast your eyes on that pleasing parallelogram in the southeast corner of your eyepiece! (East & west reversed once more, click to see the parallelogram without this caption).

And  ……… ………  if you look closely at the sketch, you’ll see that Σ 427 holds down the northwest corner of a well-shaped parallelogram.  It’s almost easy to miss because it’s off to one side of the field of view, as opposed to being at the center of it.  But that adds to the stellar context once again, and provides another attractive element to this view, one that just might whisper to you some dark night to make a return visit.

The only other scope I’ve used to look at this star was a 102mm refractor in very poor seeing conditions, but that parallelogram configuration held up well in it — so it has possibilities.

And thanks to the whim of celestial happenstance, two more of those stars that compose that parallelogram are doubles.  Both are beyond the reach of the small scopes we’re using tonight, but in the event you come back to this area with more aperture, here’s the basic info.  The one at the southeast corner, Σ 429, is the widest and brightest.  Its magnitudes are 9.3 and 11.8, separated by 16.7″ at a position angle of 105 degrees (WDS 2000).  HO 323 holds down the south corner, and has magnitudes of 8.3 and 13.8, which are separated by a much tighter 6.8″ at a PA of 211 degrees (WDS 1994).  That one will require at least a six inch refractor or an eight inch SCT, good seeing, and probably a minimum of average transparency.

I’ve got both of ’em on my six inch refractor list — along with that almost perfect parallelogram.


I hope you got a little something different from this tour than from the previous three.  Those tours were aimed at more visually stimulating stars for the most part, as well as a few difficult ones to challenge your observing skills.  This group of four, on the other hand, offers a window into the subtle aspects of double star viewing that frequently don’t get noticed because your attention is drawn to the more stunning qualities of the brighter stars.

Both kinds of observing are two sides of the same coin, but it takes time for the subtle side to ripen and reach maturity.  Once that begins to happen, you may find you prefer the more visually stimulating views on one night, and on others, you’ll hear the quiet, but persistent, call of the more subtle sights as they percolate upwards into your conscious thoughts.  More than likely you’ll find the two types of views are complementary, so you’ll switch back and forth on the same night.

And after you develop an awareness of and an appreciation for those subtle aspects of this stellar endeavor, the way you look at the brighter pairs will begin to change.   One night, as a pair of glowing globes illuminates all the glass in your eyepiece with their energetic blue-white photons, it will dawn on you that you’re experiencing an aesthetic thrill that wasn’t there before.

I find that happens now practically every time I turn a focus knob and see Castor — or Algieba, or Rasalgethi, or Meissa, or any one of countless others — come into sharp focus as separate and distinct spheres of pulsing light.  In one case it will be the color that will compete for my aesthetic attention, in another it will be a faint secondary nestled up closely to a brighter primary, or it might be the way a scattering of faint and bright stars are etched sharply into the dark sky.  It might even be the background itself — or it could just be that those globes of light are so distinct and perfectly shaped I can’t pull my eyes away from them.

Or it might be each one of those qualities, following rapidly one after the other, like dominoes cascading in a wave across a table.

Or it might be all of them at once  ——-  overwhelmingly all at once.

There’s nothing like it.


Before you leave this area, it would be a crime of astronomical proportions if you didn’t frame the Pleiades in a wide field eyepiece at low magnification.  There are half a dozen doubles shown in the photo available at that link, which comes from this piece Greg wrote a few weeks ago.  You’ll find enough there to keep you occupied for the better part of an evening.  I can’t help but wonder if the reason for the barren character of the triangle we just left is because all of the stars in this general area have been captured by the Pleiades!

Anyway, it’s a stunning sight — don’t neglect it!

Next time out, I think we’ll work our way through Orion and look at a mixture of both types of stars.  In fact, I suspect we’ll be in Orion for at least two tours.  Right now, though, I’m in a holding pattern until the infernal rain and wind wear themselves out — and if that doesn’t happen soon, I’m gonna have to buy a ticket for a ride on that ark being built by my neighbor at the end of the street.

May your skies be Clear, Dark, and Dry!   Tour Number Five starts   …………..   HERE!


One Response

  1. John,

    Thanks for a great post. Reading your description of each pair, especially STT 38, made me realize how much of the surrounding area I had not observed. I recall seeing the cluster around STT 38 but I did not pay much attention to it. I guess it’s back to that area for me.


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