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Star Hopping Across Aquila’s Eastern Wing – Altair, OΣ 198, Σ2587, and Σ2644

Altair (Alpha [α] Aquilae)   (Σ II 10)  (H VI 46)       HIP: 97649    SAO: 125122
RA: 19h 50.8m   Dec: +08° 52′
Magnitudes: 0.95, 9.8
Separation:  192.30″
Position Angle: 286°  (WDS 2011)
Distance: 16.8 Light Years
Spectral Classification: A7

Altair — a blazing blue-white diamond that occupies the southern corner of the Summer Triangle of Vega-Deneb-Altair — which, I’ll admit, not many people will think of when looking for interesting double stars.  It’s a star I frequently use when aligning digital or conventional setting circles — it works well for alignments because of its position near the celestial equator.  However  ……  let’s set aside the setting circles and get the GOTO unplugged because we’re going star hopping tonight — and Altair will be our starting point.  Hopping describes exactly what we’re going to do, too — first to the east of Altair, then to the west of it, then back to the east of it again — as we work our way south.

East wing of Aquila, click on image for a larger view. Stellarium screen image with labels added.

Despite having looked at Altair in a telescope so many times, I’m always struck by it’s beauty.  In a telescope of any size, it’s a bluish-white gem that just lights up the entire field — the larger the aperture, the more intense the color.  I had long forgotten, though, that it has a companion until I saw it listed in Haas’s Double Stars for Small Telescopes.

Now, in the case of Altair, the trick is not in seeing the faint companion, but figuring out which star is the correct one.  In this case, the less aperture, the better.  In a larger scope — say 100mm or more — there are too many candidates visible.  In a 60mm, most of those are lost in the glare of Altair, and so armed with the position angle, all of the competitors are eliminated.

But I made the mistake of looking at it in a 152mm refractor first, so in order to figure out which of the many visible faint stars was the correct one, I put a 5mm eyepiece in the scope, determined it’s field of view (37 arc minutes), converted the 192 arc second separation of the two stars to minutes (3.2′), and put Altair in the center of the field.  Then I mentally pictured a line running at a 290 degree angle (287 to be exact) from Altair to the edge of the eyepiece.  I was now interested in only that half of the field of view  which contained the line, reducing the field I was looking at to 18.5′ (37 arc minutes divided by two — or 18.5 arc minutes).   Starting at Altair, I moved 1/6 of the distance of that line (3.2′ being approximately 1/6 of 18.5′) until I found a faint star  — and sure enough, there was that 9.8 magnitude star.

But geez,  I could have saved myself a lot of time if I had started with the 60mm scope instead.  😉

OΣ  198  (STTA 198)         HIP: 99055    SAO: 125456
RA: 20h 06.6m   Dec: +07° 35′
Magnitudes: 7.1, 7.6
Separation:  64.10″
Position Angle: 186°  (WDS 2011)
Distance: 434 Light Years
Spectral Classification: A2, A3

Click on this image and those that follow for a larger view. Stellarium screen image with labels added.

OΣ 198 is an easy, wide pair of stars that are very similar in brightness.  The easiest way to get to it is by starting at Beta (β) Aquilae (also called Alshain), a third magnitude star you should be able to see three degrees southeast of Altair.  Place Alshain in the center of your finder, move east two degrees, then north one degree, and you’ll see fifth magnitude Tau (τ) Aquilae come into view.   Your destination is one degree north and east of it.

In the 60mm f/15 I have mounted on the back of my 152mm refractor, both stars were white.  Haas breaks that down into “lemon white and azure white,” which is a bit more precise than I could see — wish I knew how she does that.

At any rate, I could see both of them comfortably with 17mm (53x) and 15mm (60x) Plössls.  In the 152mm scope, a dense field of stars came to life at 84x, providing a beautiful backdrop for these two stars.

Σ 2587  (STF 2587)   HIP: 97709    SAO: 125141
RA: 19h 51.4m   Dec: +04° 05′
Magnitudes: 6.7, 9.4
Separation:  4.1″
Position Angle: 101°  (WDS 2006)
Distance: 2500 Light Years
Spectral Classification: A5, K2

Stellarium screen image with labels added.

To get to Σ 2587, go back to Alshain, center it in your finder once again,  move three degrees south and then one degree west, and your target will come into view.  There is a sixth magnitude star a degree to the northeast of it, though, so you’ll need to look in your scope to confirm you’ve got it.

In the 152mm refractor at 84x, I saw a very small pinpoint of light right up against the reddish-orange primary.  I increased the magnification to 109x with a 14mm Radian and decided that was a view worth spending several minutes admiring.  Then I tried moving up to a 10mm Radian (152x), but it magnified the moisture in the air to the point that the secondary became difficult to see, so I went back to the 14mm.  The hazy moisture proved to be too much for the 60mm – the secondary just wouldn’t emerge from the glare at 53x, 60x, or even a last ditch effort at 180x.

Σ 2644  (STF 2644)    (H II 96)       HIP: 99585    SAO: 125566
RA: 20h 12.6m   Dec: +00° 52′
Magnitudes: 6.9, 7.1
Separation:  2.5″
Position Angle: 206°   (WDS 2011)
Distance: 603 Light Years
Spectral Classification: B9

So, now it’s time to move off to a true gem, Σ 2644.  Third magnitude Theta (θ) Aquilae marks the southern tip of Aquila’s east wing – just move two degrees north of it and you’re there!

Stellarium screen image with labels added – click on the image for a larger view.

And what you’ll see at first glance in a 152mm refractor at 84x is two beautiful white globes of equal brightness that are just barely separated – this pair looks very much like it could be either half of Epsilon (ε) Lyrae, of Double Double fame.  I was able to go as low as 58x and still see two distinct white orbs.  A few nights later I got a very clean split in my 90mm F10 refractor at 61x.

Having just failed to get a split in the 60mm scope on the previous star, I was prepared for a repeat here since this pair is only 2.6″ apart versus 4.3″ on the previous pair.  Using the 26mm Plössl (35x) in the 60mm scope, I could see one white star that was clearly elongated.  Replacing that with a 15mm Plössl (60x), the elongation became two stars that were just touching.  The 11mm Plössl (82x) did the trick — cleanly separated and beaming back proudly at me.

Now what we have here is a study in contrasts.

The previous pair, Σ 2587, is 1.7 arc seconds further apart than this pair, Σ 2644.  And yet it was the closer pair that I split in the 60mm scope, not the wider pair!  The difference in magnitudes within the two systems (2.7 magnitudes between primary and secondary in the case of the wider first pair, two-tenths of a magnitude in the case of the narrower latter pair) is what made the wider pair more difficult.  You can see how that makes reasonably reasonable sense — call it the brightness gap.  But the other factor at work here was the moisture in the air, magnifying the glare in the gap, and that is essentially what put me out of business on the first pair.  I half expected that would be the case here — fortunately it wasn’t.

But  …….  you just never can be quite sure what the sky gods will have hidden halfway up the sleeves of their long white robes.  They hide a lot of tricks in those sleeves.  When you have clear skies, moisture laden or not, it pays to raise a Plössl or two in toast to them.  They don’t forget these things.


(WDS data updated 4/14/2013)


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