NGC7789 (Ron Brecher)

The Finest Cluster You Can Hardly See

Try for a cluster that even Charles Messier missed.

Cassiopeia open cluster NGC7789. (Ron Brecher)

Roughly 7,500 light-years from Earth, open cluster NGC7789 is situated in western Cassiopeia. The 6.7-magnitude group packs far more stars than Cassiopeia’s two Messier clusters, M52 and M103, yet it’s tough to see in a grey city sky. That’s because few of the nearly 600 members of NGC7789 are brighter than magnitude 11.0, and their collective glow is spread across ¼ degree of sky. Might this dim and delicate spray of stars be visible from my suburban yard? One night last fall, I decided to find out.

My star-hop to NGC7789 began at second-magnitude beta (β) Cassiopeiae. Using 8×56 binoculars, I drifted 2½ degrees southwest to a wide pair: the orange-yellow, semiregular variable rho (ρ) Cas, which fluctuates between fourth and sixth magnitude, and the dimmer variable V373 nearby. Almost two degrees south of this pair, I sighted a right angle triangle dominated by fifth-magnitude sigma (σ) Cas. Between sigma and rho is a chain of seventh- and eighth-magnitude stars. East of the chain, closer to rho than sigma, I spotted an 8.5-magnitude star guarding a faint haze farther eastward. Bingo! My hefty tripod-mounted 15×70 binoculars improved the haze to a grainy patch.

NGC7789 chart
Beside the “W” of Cassiopeia, a celestial treasure hunt awaits. Visible in binoculars as a dim cometlike puffball, NGC7789 is easy to overlook, but in a telescope of any size — though the larger, the better — the faint fuzzy reveals itself as a delicate yet dense cluster of distant suns. (Glenn LeDrew)

Time for some telescopic exploration. Squinting into the 6×finder of my 4¼-inch f/6 Newtonian, I followed the same star-hop from beta Cas through rho to sigma. I stopped at sigma because it’s a close binary worth seeing. Sigma’s 7.2-magnitude companion, separated from the primary by only 3.2 arc seconds, challenged my small reflector, but I was able to detect it at 186×. Before continuing on to the cluster, I backed off to 27×. Right away, I noticed something near sigma, just above the right-angle triangle. What had appeared in the 8×binoculars as an eighth-magnitude star showed in my low-power eyepiece (and even in the 15×70s) as an evenly matched pair.

At 27×, the telescope framed the entire sigma-to-rho star chain. Spaced well apart near the middle of the chain was a 7.2-magnitude red star and an 8.2-magnitude blue star, which formed the base of a triangle pointing eastward to the aforementioned 8.5 magnitude “guardian.” Beyond that red­dish orange star, I spotted a 10.3-magnitude blue white star on the cluster’s west edge. Barely perceptible along that side was a north-south zigzag of 11th-magnitude stars I dubbed “red giant row.” Overall, the cluster was a textured haze whose graininess intensified with increasing magnification. At between 50×and 100×, my averted vision caught a few dim stars across the mottled patch, plus others spilling past red giant row to the 10.3-magnitude star. Charts plot that star and the 8.5 magnitude guardian inside the cluster boundary, though they are almost certainly foreground objects.

Karlsson-NGC7789 sketch
John Karlsson made this sketch of the dim but incredibly rich star cluster NGC7789.

I turned to my 10-inch Newtonian and centred sigma again. The tight twosome split nicely at 140×. And remember the 7.2-magnitude red star between sigma and rho? The 10-inch at 58×revealed an 11th-magnitude companion 36 arc seconds to the northwest. From there, I nudged the scope to NGC7789. At low power, it displayed numerous stars over a powdery haze. The raggedy red giant row was distinct, with two main star pairs highlighting the zigzag line. A 155×ocular pulled in dim dots all over, but resolving the background powder into even fainter stars remained beyond reach.

A night in the country solves that problem. The dark sky view of NGC7789 in my 10 inch is stunning. At 47, the cluster is a blizzard of suns that blends into the Milky Way all around, except along the sharp-edged west side. Even there, faint outliers extend west of red giant row almost to the 8.5-magnitude guardian star. Also, I can pick out a few roughly parallel dark lanes, or voids, running eastward from red giant row. John Karlsson’s sketch of NGC7789 (above) captures much of this detail.

I recall one perfect dark rural night when I observed NGC7789 with 7×50 bin­oculars. The cluster materialized as a tiny, round, featureless patch of light — a dead ringer for a comet without a tail. Charles Messier would have loved it.