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Horsing Around in Equuleus: Delta (δ), Σ 2786, Σ 2765, β70, and Lambda (λ)

Equuleus is one of those small and obscure constellations that is difficult to locate under the typical urban sky.   You really need  skies of at least fifth magnitude in order to pick it out visually, although binoculars will make the task much easier for people who aren’t that fortunate.   For a dedicated star-splitter devoted to ferreting out those poor double stars that don’t get much attention, it’s a happy hunting ground.  Provided, of course, the weather cooperates.  Which it hasn’t …. for quite some time, now.

Let’s start with the pronunciation, which is enough to throw you off this diminutive celestial horse before you even locate it —   Ee-KWOO-lee-us, which is Latin for Little Horse.  The name has been traced back to the Greek astronomer Hipparcos, of the second century B.C, although some accounts credit Ptolemy with it as well.

Equuleus in context. Click on the chart for a larger view. (Stellarium screen image with labels added)

You’ll find Equuleus wedged between Pegasus’ nose, marked by the star Enif, and another small but better known constellation, Delphinus.  Or  ……  you could follow these ancient directions included in Admiral William Smyth’s discussion of Delta (δ) Equulei on page 498 of his Bedford Catalog:

When Pegasus within our view,           his spacious square doth spread,
Midway from Markab to Altair                 you’ll find the Horse’s head.”

There is only one constellation smaller, Crux, in the southern hemisphere, which makes Equuleus the smallest in the north.  Credit goes to the Night Sky Observer’s Guide, Volume Two, for this information.

Now, back to the weather.  There are close to a dozen stars in Equuleus worth looking at, although several require good seeing because of their tight separations.  I had hoped to get enough cooperation from the weather to spend some time on all of them, but now that the November rains have started it looks like I missed the window of opportunity this year.  Even if it clears in the next couple of weeks, the moon will make it difficult to do much here, so for now, I’ll stick with mainly the northern part of the constellation and save the southern half for a later time.

We’ll start with Delta (δ), located at the northeast corner of the constellation, and use it as a jumping off point.  And, as we go through the rest of these, you’ll see that in most cases there isn’t much separation between these stars.

That’s because this is a small constellation and there’s not a lot of space.  😉

Screen shot from Stellarium with labels added. Click on this and the photos below for a larger view, and click a second time to enlarge once more.

Delta (δ)  (Σ 2777)  (H IV 37)         HIP: 104858    SAO: 126643
RA: 21h 14.5m   Dec: +10° 00′
Magnitudes: 4.5, 10.2
Separation:  74.3″
Position Angle: 6°   (WDS 2011)
Distance: 60.3 Light Years  (Simbad)
Spectral Classification: F6, F6

October 13th, 9PM I was using two scopes on this particular night, an Astro-Tech 90mm f/6.7 APO and a Zeiss 63mm f/13.3 achromat. At first I needed to use averted vision to see the secondary because of the wide magnitude difference between the two stars, but once I picked it out of the glare, I had no problem seeing it in either scope with direct vision.  The 90mm did well with a 16mm Meade SWA (38x) in it, and I gave several eyepieces a try in the Zeiss – the 16mm Meade (53x), a 12mm Radian (70x), and a 7.5mm Tak LE (80x).  The primary star is white, but the companion was far too faint for me to detect any color.  With his 5.9 inch refractor, Admiral Smyth saw the primary as “topaz yellow” and the secondary as “pale sapphire” — judgement withheld for the moment on that last one.   On the east side of the low power eyepieces, I noticed an eye-catching asterism of three ninth magnitude stars lined up in a north-south direction, which added a little something extra to the scene.

Σ 2786        HIP: 105295    SAO: 126707
RA: 21h 19.7m   Dec: +09° 31′
Magnitudes: 7.5, 8.2
Separation:  2.7″
Position Angle: 189°  (WDS 2011)
Distance: 605 Light Years (Simbad)
Spectral Classification: A3

Now we’ll move one degree to the southeast — this is kind of like a small town, everything is within walking distance — to Σ 2786, which should easily be visible in your finder when Delta is centered.

Gloss white, or pale white, this pair dominates a faint field. (East & west reversed to match the refractor view, click for a larger version)

October 29th, 9:00 PM This pair is much tighter than Delta, so it requires a bit closer scrutiny at first.    On this particular night, I was using my five inch Meade refractor.  It was an easy split with a 16mm eyepiece (53x), but I found the view through a 12mm Radian (98x) was the better of the two.  Both of these stars are similar in brightness, although a closer look reveals that “B” is just a bit fainter.  Haas describes them as “gloss white” – I saw a pale white without the gloss.  This one is a pleasing sight in a five inch scope, but probably less so in a 60 to 80mm, although it would be worth the effort to attempt to separate them at those apertures.

Σ 2765   (H I 63)          HIP: 104570    SAO: 126601
RA: 21h 11m   Dec: +09° 33′
Magnitudes: 8.47, 8.50
Separation:  2.8″
Position Angle: 78°  (WDS 2011)
Distance: 495 Light Years (Simbad)
Spectral Classification: A3

Now we’ll go back to Delta (δ), move one degree to the west with a slight tilt to the north, and find ourselves at fourth magnitude Gamma (γ), which again should be easy to see in your finder when Delta (δ)  is centered.  Gamma (γ)  is also a double, with a tight separation of 1.5″ — and, since the seeing hasn’t been that good in these parts for a couple of centuries, we’ll skip it for now (but if you succeed with this one, by all means let me know!).  You’ll notice a sixth magnitude star clinging to its southeast side, though, and if you follow that line half a degree further, you’ll come to Σ 2765.  Actually, the view of Gamma (γ) and that close sixth magnitude star, with Σ 2765 in the same field, is one of the highlights of this star-hopping tour.

I was able to get a look at these on two different nights:

I was able to squeeze pale white out of this close, dim pair of stars in a four inch refractor. (East & west reversed, click to enlarge)

October 13th, 9:30 PM This pair has the same 2.8″ separation as our previous pair, but is a bit fainter, although both stars are almost of equal magnitude.  Using the 90mm APO, I needed a 5mm Tak LE (120x) and a 4mm AT Plossl (150x) to see black space between them.  I was just barely able to split the pair in the Zeiss 63mm with a 10mm Radian (84x), but the seeing had deteriorated in the interval between scopes and it was much harder to get it to come to focus.  My notes for that night describe them as very close together and too dim to detect any color.

October 29th, 9:30 PM This time I used the AR-5 and was able to get a split at less magnification using the 16mm Meade SWA (74x), but it was very tight.  The separation was more pronounced in a 10mm Radian (118x), but I preferred the view in an 11m TV Plossl (107x), mainly because it looked better in the smaller field of view.  Seeing conditions, however, were going south quickly once again, so I moved on.

β70  (Bu 70)            HIP: 104035    SAO: 106818
RA: 21h 04.6m   Dec: +12° 01′
Magnitudes      AB: 7.72, 9.80     AC: 7.72, 11.2      BC: 9.80, 11.2
Separation:      AB: 81.3″            AC: 74.4″             BC: 8.9″
Position Angles:   AB:  241°    AC:  237°    BC:  96°   ( All data WDS 2012)
Distance: 659 Light Years  (Simbad)
Spectral Classification: “A” is K0, “C” is G5
NOTE: Magnitudes are from Simbad, not WDS

Now you wouldn’t think it would be possible to get lost within the confounded confines of this very small constellation … but it is.  Or at least I did.  More than once.

Your star hop to β70 can be made a little easier by starting at 18 Delphini. There is a line of three stars — HIP 103869 (magnitude 8.1), HIP 103858 (magnitude 7.2), and HIP 103730 (magnitude 7.7) — that lie about two thirds of the distance (about a degree) to B70. (Stellarium screen image with labels added, click to enlarge)

This little devil is very hard to find.  Delta (δ) and Gamma (γ) form a line that points west to 5.5 magnitude 18 Delphini, so the best way I’ve found to locate β70 is to start by moving Gamma (γ) over to the east side of your finder until you can see 5.5 magnitude 18 Delphini come into view three degrees to the west.  β70 is midway between Gamma (γ) Equulei and 18 Delphini, and about two degrees to the northeast of the line that joins those two.  If you picture an equilateral triangle with the last two stars at the east and west edges, you should see β70 at the apex of the triangle – not perfectly equilateral, but darn close. However, there are a few other faint stars right near it, so it’s easy to pick the wrong one.  Go for the brightest of the bunch and you should have it.

The elusive β70 (BU70) of Equuleean fame (STScI photo)

The first time I looked at this one, I made the mistake of thinking I had separated the two very close “B” and “C” components.  I’ve included a photo of the area, which shows them separated, as well as a fainter star just to the southeast of them.  It was that fainter star that I thought was the “C” part of this system because I couldn’t see the split in what was actually the “BC” pair.  I was using the 90mm APO and the Zeiss 63mm, and neither of them showed it at the magnifications I was using.  Actually, if the search hadn’t warped my usual discerning gaze, it would have dawned on me that I wasn’t likely to split that 5.3″ pair of tenth magnitude stars with those two scopes.

What made β70 so hard to find was that the PA’s don’t quite match when the wrong star is substituted for “C,” so I kept looking past my actual target for something that was a closer match.  Rather a humbling experience.

However — I went back on October 29th with the AR-5 in hopes of splitting the real “BC” pair — but by the time I got to β70, a dense fog creeping up out of the Pacific had removed it from view.  So, until next year ………….

And, if you’re wondering what the β in β70 stands for, it’s for S.W. Burnham, who discovered this terrible triple.

(Stellarium screen image with labels added, click to enlarge)

Lambda (λ)  (Σ 2742)
HIP: 103813    SAO: 126482
RA: 21h 02.2m   Dec: +07° 11′
Magnitudes: 7.4, 7.6
Separation:  2.9″
Position Angle: 214°    (WDS 2011)
Distance: 264 Light Years  (Simbad)
Spectral Classification: F8

And now for something completely different – and the best view of the whole tour.  We’re going into the far southern reaches of Equuleus, which will take us a distant three degrees to the south.  Going back to Gamma (γ), extend a line to the southwest until you reach Epsilon (ε) Equulei.  Move southwest three degrees along that line, which is not quite two-thirds of the distance to Epsilon (ε), and you should see Lambda (λ) about thirty arc minutes to the west of it.  If you’re not sure you have the right star, note that it’s the southernmost of a line formed by it and two other seventh magnitude stars pointing to the northwest.

“A very neat double star closely preceding the Horse’s nose . . . both white”  was Admiral Smyth’s impression of Lambda (λ).  I saw a very tight pair of equal magnitude stars with a distinctive yellow color that made them easy to pick out from the surrounding field. Haas describes these as a close copy of Gamma (γ) Virginis, better known as Porrima. Although quite a bit fainter, they warrant the comparison, provided you use enough magnification to split them cleanly.

I saw these as yellow in October of 2010, but eleven months later they appeared to be a pale white. So take your pick! (East & west reversed, click to enlarge).

October 13th, 9:45 PM I was almost able to separate them in the 63mm Zeiss using the Meade 16mm SWA (53x).  I swapped that out for a 14mm Radian (60x) and got a clean split, and then did a bit better with a 12mm Radian (70x).  In the 90mm f 6.7 APO, I couldn’t quite get them using a 10mm Radian (67x), but moving up to a 7.5mm Tak LE (80x) did the trick.  I gave a 5mm Tak LE (120x) a try, but poor seeing sent me back to the 7.5mm eyepiece.  Long focus refractor afficianados take note: the 63mm Zeiss split this pair with less magnification than the 90mm APO.  I suspect the much wider field of the shorter focal length APO had something to do with that – the more narrow field of the Zeiss narrows your zone of concentration to a much smaller area.

October 29th, 9:45 PM Back again, and this time with the AR-5 under much worse seeing conditions, but I was still able to pick out both stars with the 16mm Meade SWA (74x).  Because it worked so well on Σ 2765, I used the 11mm TV Plossl (107x) again, and got a great view and a clean split.  I also had an 80mm f15 out that night and was just able to detect both stars with the 60mm Meade SWA (75x).  By that time, the seeing had once again become too poor to use more magnification.

So that’s it for Equuleus this year.  I’ll be back next year for the southern half, which for those keeping track, will consist of Beta (β), S 781, and Epsilon (ε)– and also, I hope, much better seeing.

Until then, Clear Skies!

(WDS info updated 12/5/2014)


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