Lost in the rich, star-dusted fields of Cassiopeia, about two point five degrees northwest of 1 Cas, lies NGC 7790—a charming, tightly knit open cluster that rewards careful star-hoppers. At magnitude 8.5, this compact stellar grouping spans just five arcminutes across, offering a delicate dusting of faint suns that can easily be overlooked in a casual sweep of the Queen’s realm. In small refractors, it reveals itself as a subtle, shimmering knot of light set against a dense backdrop of Milky Way stars.
Centering the view in a 4-inch refractor at around 80x magnification transforms this hazy patch into a distinct, arrowhead-shaped gathering of roughly two dozen faint stars. Its brightest member, a 10th-magnitude star, marks the apex of the cluster, guiding your eye toward a dense, rectangular core where dimmer, 11th- to 13th-magnitude gems lie closely packed together. Larger apertures, like an 8- or 10-inch reflector at higher powers, untangle the spray of faint points into a rich, granular cluster, revealing delicate chains of stars radiating outward like faint stellar tendrils.
What makes NGC 7790 particularly captivating for variable star enthusiasts is its trio of Cepheid variables—CF Cassiopeiae, CE Cassiopeiae A, and CE Cassiopeiae B. The latter two form a famously tight double Cepheid binary system near the cluster’s core, a rare astrophysical pairing that astronomers have used to refine the cosmic distance scale. Observing these distant yellow supergiants pulses through their subtle cycles within such a compact cluster provides a wonderful link between an enjoyable night at the eyepiece and the grand mechanics of astrophysics.