Towards Coma Berenices, a tiny constellation between the handle of the Big Dipper and the haunches of the constellation Leo, the Lion, lies in a tiny expanse of sky an assembly of some of the most distant galaxies visible in a backyard telescope. These are the members of the Coma Cluster of Galaxies, a group of more than 1,000 big galaxies located so far away, their starlight left well before the first dinosaurs walked the Earth.
Located at an average distance of about 320 million light years, the thousands of members of the Coma cluster are packed into a space about 20 million light years across, a sardine-can density far greater than our sparse and underpopulated Local Group. It’s classified as “rich” or “spherical” galaxy cluster, and its mass anchors a larger agglomeration of galaxy clusters called the Coma Supercluster.
Unlike the much closer and sparser Virgo galaxy cluster, which has many beautiful spiral galaxies, most members of the Coma cluster are rather featureless elliptical galaxies. This is no accident. The mutual gravitation of the Coma cluster whipsaws its member galaxies at a high speed and pulls them frequently towards the cluster’s core where they collide with each other. As the galaxies interact near the cluster’s core, their gas and dust gets stripped out before their stars merge into a larger elliptical galaxy. There remain only a few slow-moving spiral galaxies in the cluster’s outskirts. The Virgo cluster, by contrast, which is more irregular and less massive than Coma, has a larger fraction of spirals because the galaxies move more slowly and don’t collide or interact as often.
More than 80 years ago, the astronomer Fritz Zwicky noted the speed and estimated mass of the galaxies in the Coma cluster. He found something remarkable. The mass of the cluster, which he estimated from the number and brightness of the visible galaxies, was far too low to allow the fast-moving members to remain bound in the cluster. So he deduced the cluster must have more mass, a lot more, that was not detectable with telescopes. This was the first hint at the existence of a weakly interacting material with mass but which emits no light. Zwicky called it “dunkle Materie”, German for “dark matter”.
So how can you see the Coma cluster? The group lies well overhead for northern observers in the late spring months, and low over the northern horizon for observers in the deep southern hemisphere. But you need a big telescope, at least 8″ (200 mm) aperture, to see even the brightest members. Thousands of galaxies lie within a one-degree field of view, and dozens of the brightest galaxies are accessible visually in a 15″ to 20″ telescope. The brightest, the elliptical galaxy NGC 4889 shines at magnitude 11.5 and its companion, the massive NGC 4874, is just a little fainter. If you can spot this pair, use averted vision to try to see some of the fainter members of this cosmologically important massive group of galaxies.