One of my favourite star clusters resides high overhead in our winter sky. In fact, it is always in our sky, for our viewing pleasure, as it is circumpolar. It is close enough to the North Star (Polaris) that it never sets.
I am actually referring to a pair of clusters, the famous Double Cluster in Perseus. These clusters are close together in the sky and appear to the naked eye as an extended hazy patch—kind of a peanut shape, I’m told by someone with better eyes than mine. In reality, they are a couple-hundred light years (ly) apart, but, at the distance of about 7,500 ly away, it is difficult to see that separation without optical aid.
Before I describe their location in the sky, I recommend that everyone find a monthly sky chart online to print off and use to familiarize yourself with our night sky. A very good one is at Skymaps.com, a two-page sky chart with simple-to-use instructions and plenty of helpful hints.
You will notice, embedded in the Milky Way band shown on the chart, the prominent “W” shape of the constellation Cassiopeia. Cassiopeia is high overhead in our sky, and once you’ve found her, it is a cinch to locate the Double Cluster (also shown on the chart). I visualize the left arm of the “W”—the one that looks like its flopped over too far—as the base of an elongated triangle (kind of an arrowhead shape that hangs below the “W” from that left arm). The Double Cluster will appear right at the tip of the arrowhead, as a hazy patch.
When viewed through binoculars, the hazy patch of the Double Cluster is resolved into two distinct concentrations of brilliant blue-white points of light. Those sharp points of light are the largest stars of the clusters and are many times the mass of our Sun. They are supergiant stars, fusing furiously with luminosities thousands of times brighter than our home star, which is why we can see them at all over such a great distance. Each cluster is home to more than 300 of these supergiants.
The fact that there are still supergiants residing in the Double Cluster tells us that the clusters must be very young. Paradoxically, the larger the star, the quicker it goes through its nuclear fuel, and thus the shorter its life. Supergiants devour their hydrogen content so voraciously that their lives are measured in millions of years instead of billions, as with smaller stars like our Sun. A telescope reveals several red giant stars within the clusters— large stars that have swelled and cooled, and all but exhausted their nuclear fuel and are very near the end of their lives. One study determined the clusters harboured even larger stars that have already met their demise and have supernovaed into neutron stars or black holes. All of this suggests an age for the Double Cluster of less than five-million years—barely toddlers, astronomically speaking.
The Double Cluster is embedded within the Perseus Arm of our Milky Way galaxy. As our Sun resides on the inner edge of the Orion Arm, when we look at the Double Cluster, we are looking through the Orion Arm and into the Perseus Arm, the next spiral arm out from the center of our galaxy. Both clusters, having evolved from the same gas and dust cloud, are moving spatially together. At present, that happens to be toward us, at a leisurely pace of about 21 kilometres per second (km/sec). But don’t hold your breath on the Double Cluster ever becoming cozy neighbours with us. In the grand-time scale of things astronomical, the clusters will have long since dissolved before they could ever get close to our neck of the galactic woods. The largest stars of the cluster will pop off in supernovae, and gravitational dynamics will slingshot most of the rest of the stars out of the clusters, to live solitary lives like our Sun.
I quite enjoy viewing these beautiful clusters, without optical aid, just as a hazy patch. Everytime I’m out under our glorious night sky, I make sure to cast a glance upward and into the disc of our galaxy to wish the Double Cluster a good evening. It is never lost on me, even with just a quick glance, that those photons striking my eyeballs at that very moment left those massive stars shortly after the retreat of the last ice age, when wooly mammoths were just beginning their stompings about the Yukon, and had been travelling in space at 300,000 km/sec ever since. And there I was, just in the nick of time, to catch a few of them.
So, until next time, do get out and catch a few ancient photons yourself. Just in the nick of time.