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The Dolphin’s Double-Double – Gamma and Struve 2725

Struve 2725

RA: 20h 46m Dec: +15°54′
Mag: 7.2, 8.3 Sep: 6.1″ PA: 012°
Distance: 100 ly
Spectral Type: K, O

Gamma Delphini
RA: 20h 47m Dec: +16°07′
Mag: 4.4, 5.0 Sep: 9.1″ PA: 266°
Distance: 100 ly
Spectral type: K, F

So we should call Struve 2725 the Dolphin’s Double-Double  – it would seem so the way it pairs so nicely with Gamma. 😉

This reminded me of the Double-Double’s Double-Double – but with two significant difference. One, there is quite a contrast in terms of magnitude and two, the two pair, while bearing a real resemblance to one another, slant at different angles. But in my book it is another double-double.

In the famous – and still champion – Double Double , the stars line up like this. (All images that follow are flipped horizontally and are to different scales in order to display both pair at once.  But these are reasonable simulations of what you should see in a refracting or SCT scope at a power sufficient to split them.)

Separation between pairs is about 3.5 minutes. Click image to enlarge - derived from a Starry Nights Pro screen shot.

The “other” Double Double, aka Double-Double’s Double-Double, aka Struve 2470 and Struve 2474, in Lyra, line up this way.

Separation between pairs is about 10.5 minutes of arc. Click image for larger view - derived from Starry Nights Pro screen shot.

And now, the Dolphin’s Double-Double:

With 14 minutes between them this is the widest pair of pairs of the three. However, they still fit comfortably in the same field when using a power high enough to get a clean split. Click image for larger view - derived from Starry Nights Pro screen shot. .

Splitting Gamma, as described in another post, is easy. It’s a real gem in a 60mm  and in larger scopes the color comes out better. But I have not been able to split Struve 2725 with a 60mm – I tried last night after I split it easily with the ES 127mm APO, but no luck. Using th2 127mm scope and a 9mm Nagler, both pair split nicely in the same fov and they make a charming pair of pairs. Although they’re not physically related – though at about the same distance from us – it’s hard to avoid the feeling that Struve 2725 is a child – or younger sibling – of Gamma. or maybe you could think of it as a Gamma wannabe. Even though the oritenation isn’t the same, it’s close enough to the fainter little double brings a smile to your face – and when isolated in the 15-inch it really felt a lot like Gamma, though the colors aren’t as rich.

I had people looking over my shoulder last night, waiting to take a peak, so I didn’t make notes – or a detailed observation. But  will return to this many times. According to the Starry Nights chart, there’s actually an 11 magnitude air in the same fov I didn’t see.

So what makes a “double-double?” To me  it needs two things. First, you should be able to fit both pair in the same field of view while using enough power to split. Second, they need to all be physically related – as in the original Double-Double – or they should  feel like a pair of pairs. OK, what does that last mean? It means that when you  look at them they should at least look like they belong in the same picture.  Yep – I’m struggling with the definition, but they all qualify in my book 😉

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One Response

  1. OK – got it!

    But I would call Struve 2725 a challenge object for 60mm, not because of the separation, but because of the dimness. That’s interetsing because at 7.2 and 8.3 they’re well within the capability of a 60mm, but I guess seeing the 8.3 star just 6 seconds from a star a full magnitude brighter is what makes it a challenge.

    In any event, with excellent conditions I was able to do it last night with the Unitron 114. But it’s not the sort of thing I would call a visitor over to take a look at. I suspect they would be frustrated and not see it – and not impressed if they did. But double stars grow on you and I had a wonderfully long visit with these pairs, playing with different eyepieces to see the results. In all of them – even a .965 Unitron Ramsden of 25mm focal length – I could split Gamma with ease. OK – maybe “ease” isn’t the right word. You have to be real careful with your focus at 36X, but when you were the result was two pristine pinpoints of light. In fact, this made me go back and retry the 32mm TV Plossl. I had skimmed over it casually with this at first assuming I couldn’t get a split at 28X – but I could. It just required careful focus and careful positioning of my eye above the eyepiece.

    I was using a Unihex loaded with four original Unitron eyepieces, so I marched through them. Each, of course, split Gamma. But I didn’t get a split of Struve until I got to the 12.5mm Kellner,

    I also marched through the complete set of TV Plossls – these are the original design from Japan sold in the 1980s – and I felt all showed the colors better on Gamma than the .965 eyepieces did. The field wasn’t quite so cramped with the Plossls, either. (Hmmmm . .. maybe the .965 eyepieces need some cleaning. I hate to do that, but after more than three decades, who knows what has accumulated. )

    But when it came to Struve 2725 I need the 69X of the 13mm Plossl – or something in that neighborhood – to see it clearly. At that magnification the split was wide enough to make the fainter secondary come out of the murk. I probably could have seen it with less, but I would have been really straining.

    Now I have to go back and look at Struve 2470 and Struve 2474, in Lyra with the 60mm. I’m having great fun with this scope, but I think it is also refreshing and extending my observing skills.

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