You may know the Greek myth of Perseus, a young hero who slew the petrifying Medusa and the sea monster Cetus, in order to rescue the beautiful Andromeda from her cruel mother Cassiopeia. The ancient Greeks saw fit to immortalize the story in the sky, by tracing constellations resembling these characters. We recognize these constellations today-- in particular, Andromeda with its legendary naked-eye galaxy M31, our closest spiral galactic neighbor. But Andromeda’s hero also sparkles with its own celestial gem-- the striking Perseus Double Cluster of stars.
To the naked eye, the Double Cluster appears as a single hazy patch roughly between Perseus and Cassiopeia, in the low northwest sky around 9 pm. Low-powered binoculars or telescope will immediately resolve the two parts of the double. Each is a 70 light-year-wide cluster, brimming with hundreds of blue-white and yellow-orange stars. A light year is about 6 trillion miles. The members of each cluster are bound together by mutual gravitation, but the clusters themselves are gravitationally independent of each other, separated by several hundred light-years. They appear close together only because they happen to have the same line of sight from Earth.
The Double Cluster is about 7700 light-years distant, much farther than other well-known clusters like the Pleiades in Taurus. It is one of the most distant clusters you can still see unaided. Yet, the Double Cluster resides well within our home galaxy. You can see the Milky Way flowing faintly past the cluster. The Milky Way is faint now because we are viewing the galactic edge; by summertime, it will brighten as the Earth’s orientation faces the galactic core.
Perseus and the Double Cluster will still be visible then. In fact, from the Western Slope, they always are visible because they are circumpolar. This means they revolve close enough around the celestial north axis (marked by the “pole star” Polaris) that they never set below the horizon, unlike the Sun, Moon, planets, and most stars. As long as you have clear, dark skies, you have a clear shot at the Double Cluster. Hopefully, maybe even tonight!
Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written and recorded by Michael T. Williams.
