- Stephenson 1 Delta Lyrae Cluster
Stephenson 1 is described as a "probable" open cluster located in the constellation Lyra. It contains 77 member stars, the brightest of which is the magnitude 4.22 orange class M giant δ2 (HD 175588). The second brightest member is the magnitude 5.58 blue-white class B main-sequence star δ1 (HD 175426) located to the lower right of δ2 in my image. The distances to δ2 and δ1 are, respectively, 741 and 988 LY. Lying to the lower left of δ2 and making an isosceles triangle with it and δ1 is HD 175538, a magnitude 7.61 blue-white class B star located at a distance of 932 LY. Other dimmer members of the cluster can be seen within and around this triangle. The overall visual magnitude of the cluster is 4.5 and it has a visual diameter of around 20 arc-min.
This cluster was first described by the American astronomer Charles B. Stephenson in 1959. Later photometric observations of 33 member stars by Olin J. Eggen at the Palomar and Mt. Wilson Observatories confirmed that it likely is a true open cluster.
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