Brasch - Double Cluster

Sightseeing in Northern Perseus

Take a tour from Mirfak to the famous Double Cluster.

LeDrew-Perseus chart detail
Beside the “W” of Cassiopeia is one of the finest sights in the heavens: the Perseus Double Cluster, seen in close-up on the facing page. Visible to the unaided eyes from a dark site, the cluster pair is striking in binoculars and is especially memorable in a small telescope at 20× to 40× on a moonless night. (Main chart by Glenn LeDrew)

Employing second-magnitude Mirfak, or alpha (α) Persei, as a stellar staging point, we head almost eight degrees northwest, past third-magnitude Gamma, to the colourful binary Eta (η) Persei. Eta’s orangey 3.8-magnitude primary and bluish 8.5-magnitude secondary are 28.5 arc seconds apart. Eta is exquisite in my 10-inch Dobsonian at 47×.

From Eta, we shift two degrees west to a 20-arc-minute-wide, hat-shaped open cluster I call the Fedora Cluster (officially, Trumpler 2, or Tr 2) that’s worth trying on for size. My 116× ocular picks up a dozen 8th- to 10th-magnitude stars here. A 7.5-magnitude reddish one near the middle of the Fedora yields a 10th-magnitude partner 34 arc seconds southwest. A 10th-magnitude pair of similar separation and orientation decorates the east brim of the hat. The west brim is extended 10 arc minutes by two stars outside the cluster’s boundary. The westernmost ninth-magnitude pinpoint comes with a faint companion southward.

Immediately southwest of the Fedora is a ⅔-degree-wide asterism I’ve dubbed The Fan. Five of its eight stars curve southeastward to form the fan’s open end. The two westernmost stars are binaries 12 arc minutes apart. At 116×, Struve (Σ) 268 (6.7, 8.5, 3 arc seconds) resolves beautifully in the same field as Struve (Σ) 270 (7.0, 9.7, 21 arc seconds). Reducing to 47×, we hop over the third fan star to the 8.5-magnitude fourth star, which sports an 11th-magnitude flanker wide to the northwest, and, finally, to the ninth-magnitude fifth dot, which comes with a 12th-magnitude neighbour southward. But something awaits us 10 arc minutes farther southward. There, a trio of 11th-magnitude doubles of 33-, 22- and 18-arc-second separation forms a squat triangle almost 12 arc minutes wide. At 116×, these little fellows make a surprisingly attractive field.

Brasch - Double Cluster
PERSEUS DOUBLE CLUSTER: Two rich star clusters side by side in the same telescopic field are a rare sight.  (Klaus Brasch)

From The Fan, we drift slightly more than two degrees northwestward to sweep up the dazzling Double Cluster. This popular showpiece (actually, two showpieces) comprises side-by side clusters some 7,000 light-years from Earth, oriented east-west on the sky. The western one, NGC869, shines at fifth magnitude and is packed with dozens of young, hot, blue-white stars. NGC884, the eastern one, is almost one magnitude dimmer, contains somewhat fewer stars and is less concentrated. Together, they span almost one degree, but each core is less than 10 arc minutes across, and their centres are nearly 25 arc minutes apart. In my 10-inch scope at 47×, the tandem clusters sparkle against the Milky Way. Just westward, the low-power view includes the wide double Struve 25 (OΣΣ25), whose 6.5- and 7.4-magnitude stars are separated by 1.7 arc minutes.

The core of NGC884 contains two roughly triangular clumps of eighth- and ninth magnitude stars that emerge with increasing magnification. Zooming in to NGC884 also reveals some warm-hued stars. I can see an eighth-magnitude red star on its western outskirts and another one south­ward, between the clusters. Just east of the core is the ruby variable RS Persei, which fluctuates lazily between 8th and 10th magnitude. Another ruddy pair lies in the eastern periphery. I’ve often wondered whether those fiery five are true cluster members or field stars in the same line of sight.

The western cluster, NGC869, is denser than its sibling and features two 6.6-magnitude stars 2.4 arc minutes apart, one northeast of centre and the other at the cluster’s core. The latter star marks the beaming face of the Parachute Man, a three-arc-minute-long figure shown to me years ago by my late friend Lance Olkovick, who built my 10-inch Dob. The parachute is a sharp curve of five stars, while several more suggest a torso and legs. Ramping up to 155× makes the stick-man parachuter much easier to discern.

A small telescope can show every target I’ve described above. In my 4¼-inch Newtonian reflector, for example, the Double Cluster is stunning at 22×, though I need almost 100× to appreciate the tiny Parachute Man in NGC869. High power also reddens the “fiery five” dotting NGC884. Eta Persei splits at 27×. The Fedora displays its two main star pairs at 72×. In The Fan, the binary Struve 268 resolves at 144×, while the three dimmest doubles show at just 93×.