Seeing multiple stars: Lyra’s other double-double

By Ade Ashford

The small but distinctive constellation of Lyra is easily found owing to its brightest star, magnitude-zero Vega, currently high in the southwestern sky as darkness falls in the UK. Lyra is famous for the Ring Nebula (M57) and the double-double star epsilon (ε) Lyrae, but the constellation harbours another ‘pair of pairs’ known by their Struve catalogue numbers Σ2470 and Σ2474. These stars lie almost 3 degrees northeast of gamma (γ) Lyrae, or 1½ degrees south of iota (ι) Lyrae. This simulated ten-degree field of view shows stars down to magnitude +8.5. AN graphic by Ade Ashford.
The illustration above is centred on the familiar parallelogram-shaped asterism of the constellation Lyra showing an area of sky 10 degrees wide, which is roughly the width of a fist held at arm’s length. Lyra’s easily found from its magnitude-zero primary luminary, Vega, a star some 25 light-years from Earth that currently lies high in the southwest as darkness falls seen from Northern Hemisphere temperate latitudes such as the UK.

Home to the fabulous Ring Nebula (M57), Lyra also contains the celebrated double-double star Epsilon (ε) Lyrae, some 1⅔ degrees to Vega’s upper right. However, if you find that components ε1 and ε2 are too close to split comfortably in your telescope, Lyra contains yet another double-double whose component stars are far easier to resolve — Struve catalogue Σ2470 and Σ2474 — located in the one-third-degree wide red circle in the illustration above and shown in greater detail below.

In this simulated telescope view at a magnification of about 150x, Σ2470 consists of a pair of magnitude 7 and 8.5 stars separated by nearly 14 arcseconds, while Σ2474 consists of magnitude 6.7 and 8 stars some 16 arcseconds apart. Users of smaller telescopes will appreciate that Σ2470 and Σ2474 are easier to resolve than ε1 and ε2 Lyrae some 7 degrees away to the northwest. AN graphic by Ade Ashford.
There are two ways to locate this attractive ‘pair of pairs’ by star-hopping with your lowest power telescope eyepiece. You can either move 3 degrees (or three fields of view at 40 to 50x magnification) to the northeast of gamma (γ) Lyrae, or track 1½ degrees south of iota (ι) Lyrae. For observers using a computerised GoTo mount or one equipped with digital setting circles, the equatorial coordinates of the mid-point between Σ2470 and Σ2474 is α = 19h 08.9m, δ = +34° 41′ (J2000).

However your find your way to Struve Σ2470 and Σ2474, see if you can detect that the stars of the more southern pair (Σ2474) have a yellowish cast, while those of Σ2470 are white. These stars are all at widely differing distances from Earth too. The brighter component of Σ2474 lies some 160 light-years distant, while that of Σ2470 is about 1,300 light-years away. The latter may be an optical double rather than a genuine binary star system.