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The “Other” Double-Double in Lyra: Struve (Σ) 2470 and Struve (Σ) 2474

Struve (Σ) 2470        HIP: 94043     SAO: 67870
RA: 19h 8.8m   Dec: +34° 46′
Magnitudes:  7.03, 8.44
Separation:   13.9″
Position Angle: 267°   (WDS 2013)
Spectral Type: B5
Distance: 1320 Light Years
Status:  Not determined

Struve (Σ) 2474     HIP: 94076     SAO: 67879
RA: 19h 9.1m   Dec: +34° 36′
Magnitudes  AB: 6.78, 7.88     AC: 6.78, 11.42
Separation   AB: 15.9″            AC: 96.80″
Position Angle   AB: 263° (WDS 2013)    AC: 123° (1998)
Spectral Type: “A” is G1, “B” is G5
Distance: 160 Light Years
Status:  Physically related (WDS, codes U & Z);  AC is WAL 105

(WDS data updated 10/6/2014)

This system of stars has earned a limited amount of fame as the “other” double-double in Lyra.  Located to the south and east of Epsilon (ε) Lyrae, the better known system of that name, these four stars would very likely get more attention if they were located in a constellation with fewer objects to demand your attention — such as Ursa Minor ……………

Lyra, showing Σ 2470 and Σ 2474 to the east. These two stars from a triangle with Delta (δ) and Gamma (γ) Lyrae — or, you can locate them by landing halfway between Iota (ι) and 17 Lyrae. (Stellarium screen image with labels added, click on the image for a larger view)

…………….  which is where I had planned to be on this night.  But plans are never seriously made with intentions of following them, at least not when searching the sky for 60mm delights.

In a 60mm f16.7 with a 20mm Plössl, giving me 50x, I could see four stars immediately.  But what really grabs your attention is the orientation of  each of these pairs — they’re parallel to each other!   And symmetrical as well!  In my 60mm, each one of the pairs consists of a bright component with a fainter one hanging on for dear life at the edge.

Σ 2470 is shown at the top left, and Σ 2474 is to the right and below it. Also shown is the “C” component of Σ 2474, which goes by the name of WAL 105 in the WDS. East and west are reversed to match the view in a refractor, click for a larger view. (STScI photo)

Really an unusual sight, and much more deserving of attention than they get.  (In case you’re wondering, the pairs of Epsilon (ε) Lyrae are oriented at roughly a ninety degree angle).  To my eyes, all four of these are white in color, but Haas sees some yellow in Struve 2474.

This is a sketch done by Steve McGee at the summit of Haleakala in Hawaii.   East & west are reversed here to match the SCT view, click on the sketch for an eye-pleasing larger version.

This is a sketch done by Steve McGee at the summit of Haleakala in Hawaii. East & west are reversed here to match the SCT view, click on the sketch for an eye-pleasing larger version.   For those who prefer an uncluttered view, I included an un-labeled version at the end of the post.

There’s more than meets the eye, though, with this pair of stars.  Σ 2470 is a spectroscopic binary, but it’s the other one Σ 2474, that’s really complicated.  It’s primary consists of two stars lying one-tenth of an arc second apart.  If you have a pressing urge to find them, the Washington Double Star Catalog (WDS) statistics for 2008 show the secondary (Ab) at a magnitude of 8.96 with a position angle of 258 degrees — but don’t get your hopes up too much unless you’re sitting at the eyepiece of a hundred inches or so of aperture!  That pair of stars carries the designation CHR 84.

But hang on — there’s yet another star listed in the WDS, and this one lies within reach of most telescopes.  Identified as WAL 105, it’s the “C” component of Σ 2474.  It sits at a distance of 96.8″ with a position angle of 123 degrees, and a moderate magnitude of 11.4 — those figures were updated in the WDS in 1998 — and it’s labeled on the photo above.   First identified in 1940, by 1998 it was determined that it’s not physically related to either Σ 2474 “A” or “B.”

The uncluttered view!   Many thanks to Steve for the use of the sketch.

The uncluttered view! Many thanks to Steve for the use of the sketch.


8 Responses

  1. The pair of pairs definitely deserves more attention than it gets -I was here several years ago and I’ll be back!

    I got a real nice view of it in the ES127mm APO – what a delight! Perhaps it’s especially charming because you are used to the Double-Double, which is much harder to split and closer together. Like that better known pair of pairs, this even has a third (unrelated) companion star that forms a triangle with the two pairs. But it is the near perfect symmetry both in terms of PA and brightness, that is so cool.

    I agree with Haas – I saw yellow in the southern-most primary, Struve 2474 and perhaps it was my imagination, but I felt I saw a bit of rose in the secondary. These split easily with a 24mm at about 40X and were best when I used the 13mm Nagler – 73X.

    I didn’t find them that easy to track down – especially when I switched to the 60mm Unitron 114. I had a 32mm Plossl (28X) in it which gave me a fov of about 1.5 degrees – maybe a bit more. I used that to drop “down” – well, go east from Delta to Iota, a fifth magnitude star almost 3 degrees away. The problem is I found three pair of roughly 7th magnitude stars, each with roughly the same east/west orientation, in the neighborhood. These confused me so much that I went inside, keeping my right eye closed to protect my night vision – ever done that? It’s really weird to then walk around with two eyes dilated much differently – and look at the chart on my computer. Aha! I could put Iota on the northeast side of my fov with the 32mm and the new double double showed up on the southwest side – with a closer pair of uneven brightness inbetween them.

    Having established all that my skies really went bad, though, and using a 13mm (69X) just revealed the 8th magnitude companions. But that was a problem of transparency, not seeing. In fact, just a few minutes before I had been able to split the Double Double using the 7.2mm Plossl in the 60mm – 125X. Satisfying.

  2. Greg’s comment about this pair of stars not being easy to find reminded me of an observation I made of them last year.

    I think it was late August or early September. It was about 5AM or so and the sky was becoming bright. I had been out since about midnight and was about to wrap it up, when I saw Lyra hanging in the western sky. For some reason, I remembered there was another pair of double stars on the east side of it, so I took my scope – I believe it was a 90mm refractor – and started searching that area of the sky. It took a few minutes, but I finally found them.

    The sky had become a light shade of blue at that point. I could see them easily enough, though – four white stars all alone against a slightly purple background. I just watched them for about fifteen minutes until they began to fade and become difficult to see. Really a pleasant sight and quite different than seeing them with a black sky as background.

  3. this double is well worth a look.

  4. John, I sketched these two pair just last month and I’ll send it to you. Its quite complicated and I’m pretty sure Wal 105AC is there in the sketch. The sep of 96″ has me a little thrown off – there’s two stars in that vicinity to choose from in my sketch…
    I get pretty good colors for the two pairs: STF 2470, primary blue/white, comp. ruddy brown. STF 2474, primary golden yellow and comp. ruddy brown.
    Hope you like the sketch. I’ve got a lot of “armchair” time these days.
    Looking UP, Steve McG

    • I just added the sketch of STF 2470 and 2474 mentioned above by Steve. Beautiful work — they’re a great addition to the post!


      • Thanks, John!
        This is a beautiful pair of doubles, with a triple out of one of them!
        I love the colors, too.
        Steve McGee, A.K.A. Stephen McGaughey, Haleakala Amateur Astronomers.

  5. I’ve observed Struve 2470 and 2474 many times over the years through a variety of telescopes. For my money, this “double double” is the best multiple star in the sky.

    • Thanks for the comment, Dave.

      I have little doubt these two pairs would be much better known if they were in another constellation. Their symmetry is quite a contrast to the better known double-double, Epsilon Lyrae.


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