Observing Galaxy Clusters

The eternal silence of these infinite spaces terrifies me - Pascal

Observing galaxy clusters is a way of looking directly into the unimaginable depths of space. Whether one finds this terrifying or not depends, I suppose, upon one's taste and temperament. Personally I find the experience exhilarating, rather like the bracing air and vast panorama from the top of a high mountain peak. The universe may dwarf man but, paradoxically, the very fact that we are observing its vastness reconfirms that man is the measure of all things. - Jim Shields

Nearby Galaxy Clusters

Virgo, Como Berenices, Ursa Major: this part of the sky used to be called the Realm of the Nebulae because of the concentration of deep sky objects here, including the enigmatic spiral nebulae studied by Lord Rosse and other nineteenth-century observers. It was not until the 1920's that Edwin Hubble used the newly-constructed giant telescopes on Mount Wilson to resolve Cepheid variables in M31 and confirm that some faint fuzzies were nearby clouds of gas within our own galaxy, and others were distant island universes similar to the Milky Way itself.

At a distance of roughly 40 million light years, the Virgo Cluster is the nearest rich aggregation of galaxies in the sky, and a target that will repay many evenings of careful study at the eyepiece. The Fornax Cluster is the next brightest and most easily observable galaxy cluster after Virgo. A winter object, the cluster is most concentrated around NGC 1399, an 11th magnitude elliptical. The one degree field around NGC 1399 contains at least eight galaxies. The Fornax Cluster is about 55 million light years away, a bit further away than the Virgo Cluster.


Another pioneering step in understanding the structure of the universe was taken by George Abell in the late 1950's. Using plates from the newly-completed Palomar Sky Survey, Abell compiled the first comprehensive catalog of clusters of galaxies. While galaxy clustering had been recognized before Abell's catalog, the data was fragmentary and not well understood. His catalog encouraged the search for large scale structure in the universe and helped confirm the suspicion that galaxy clusters were themselves clustered.

The Virgo Cluster thins out as it stretches north into Coma Berenices, Canes Venatici and Ursa Major. A southern arm heads down into Hydra and Centaurus. This combined "cluster of clusters" is dubbed the Local Supercluster and probably includes our own Local Group of galaxies on the outskirts.

An immense supercluster sprawls through the constellations of Hydra - Centaurus and can be observed from Northern California despite its southerly declination. Among the brighter galaxy clusters here are the Hydra I Cluster (Abell 1060) around NGC 3309 and 3311, the Centaurus Cluster (RCG 3526) dominated by NGC 4696 and 4709, and the cluster around IC 4329. The brightest galaxies in these clusters are around 11th or 12th magnitude. The IC 4329 Cluster (RCG 3574) also contains NGC 5291, a bright elliptical with a famous companion known as The Seashell because of its peculiar shape. These galaxy clusters are our cosmic neighbors, being a mere 150 - 200 million light years away.

One of the largest known structures in the universe is the Perseus - Pisces Supercluster. This huge filament of galaxy clusters extends from the Perseus Cluster (Abell 426) through Abell 347 and Abell 262 in Andromeda into the NGC 507 and NGC 383 galaxy groups in northern Pisces. Even at a distance of 250 million light years, this chain of galaxy clusters extends more than 40 degrees across the winter sky!

The Great Wall

Further understanding of the large scale structure of the universe had to await the daunting task of measuring the redshift distances of thousands of distant galaxies. When Margaret Geller and John Huchra analysed the first three sections of the CfA redshift survey in the late 1980's, they discovered the largest known structure in the universe: the Great Wall. This giant sheet or filament of galaxy clusters stretches across eight hours of right ascension from Leo to Hercules, a distance probably in excess of 500 million light years! It's pretty amazing that two so widely - separated regions of the sky could be interrelated parts of the same gigantic structure!

The best and brightest distant galaxy clusters in the northern sky lie in or near the Great Wall. These clusters are 350 - 550 million light years away. The Leo Cluster (Abell 1367) is centered around NGC 3842, a 13th magnitude elliptical. The one degree field around NGC 3842 contains at least eight galaxies. However, the Coma Cluster (Abell 1656) is the all around champion for most galaxies in a single field. The cluster is dominated by two giant 11th magnitude ellipticals, NGC 4874 and 4889. NGC 4874 is surrounded by a virtual cloud of fuzzy stars to the very limits of visibility.

The Hercules - Corona Borealis region is one of the richest areas of the sky for amateur observers with large scopes. The Hercules Cluster (Abell 2151) is a lovely linear grouping of galaxy triplets, one of my personal favorites. With my 17.5-incher, I've observed at least three other interesting Abell Clusters in Hercules, all in or near the Great Wall. Finally, there is the famous Corona Borealis Cluster (Abell 2065), twice as far away at a billion light years or more, whose brightest members (16th magnitude) are at the very threshold of visibility in a 17.5-inch scope.

Click HERE to see how it all fits together (at least for our local neighborhood, the region observable in amateur telescopes).