The Romans called the Milky Way the via lactea or “milky road.” It’s a road on which I frequently find myself wandering. I want to understand the structure of our galaxy — not just where the nebulae and star clusters are located in the night sky as seen from Earth, but where they are in three-dimensional space and how they relate to each other.
A quartet of clusters in the constellation Cassiopeia is a good place to start figuring this out. From Delta (δ) Cassiopeiae, scan 1° northeast to M103, and then onward for an additional 1.5° to NGC 663 (Caldwell 10). NGC 654 lies just under 1° to the north-northwest of NGC 663, and NGC 659 is a little closer still, to the south-southwest. All four clusters will fit comfortably in the same field of view.
NGC 663 is the brightest and easiest to pick out. On a good clear night, NGC 663 looks grainy with 8th- and 9th-magnitude stars that tremble on the threshold of resolution. Resolving stars in M103, NGC 654, and NGC 659 is more of a challenge, in part because bright foreground stars dominate each of those clusters. The clusters themselves appear as knots of slightly denser light in the galactic band.
It shouldn’t be surprising that these clusters are hard to resolve with handheld binoculars. The average distance to the Messier and Caldwell open clusters is just over 3,000 light-years, but these four in Cassiopeia lie between 6,300 and 8,200 light-years away, in the Perseus Arm of the Milky Way. NGC 663 isn’t any closer than the rest. It’s the brightest because it’s the largest: 30 light-years in diameter, compared to roughly 10 light-years for the other three. All four are signposts along the “milky road,” part of the structure of the galaxy made visible.
This article originally appeared in print in Sky & Telescope's October 2017 issue.