Thousands of galaxies pack into the Coma Cluster, one of the densest known galactic groupings in the universe. Located in the constellation Coma Berenices, this region of sky is best viewed in April and May from the Northern Hemisphere. Also known as Abell 1656, the Coma Cluster spans 20 million light-years and is filled with galaxies of all shapes and sizes, from giant ellipticals to dwarf galaxies.
An old but beautiful name for this region of sky is the Realm of the Galaxies.
The central part of the Coma Cluster of galaxies covers a roughly circular area about 1 1/2 a degree across (nine times the area of a full moon, which is about 1/2 a degree across). The full cluster may extend farther, and numerous other galaxy clusters are in the same area of the sky.
How can you see it? The constellation Coma Berenices lies between the constellations Leo the Lion and Boötes the Herdsman. This part of the sky is the site of a famous open star cluster known as the Coma star cluster, and also of the more distant galaxy cluster, visible through telescopes. Both the star cluster and the galaxy cluster need a dark sky to be seen. For a dark sky location near you, see EarthSky’s Best Places to Stargaze page.
The galaxy cluster is near the northern border of Coma Berenices, midway along a line drawn from Rho Boötis to Delta Leonis (Zosma), near the North Galactic Pole.
Science of the Coma Cluster. The center of the Coma Cluster is about 320 million light-years away. This cluster as a whole is flying away from us at the rate of about 6,900 km/second (more than 15 million miles per hour)!
One of the most populated galaxy clusters known, it contains as many as 10,000 or more members by some estimates. There are more individual galaxies in this cluster than there are stars visible to the unaided human eye on a clear, dark night.
Most galaxies in the central part of the cluster are elliptical, which is a type of galaxy created through galaxy mergers. The two brightest members are NGC 4889 and NGC 4874, both of which are giant ellipticals at least two to three times larger than our own Milky Way galaxy. Farther out from the center are several spiral galaxies.
Meanwhile, most galaxies in the Coma Cluster are dwarf galaxies, perhaps similar to the Milky Way’s companions, the Large and Small Magellanic Clouds.
Coma Cluster in history. Too faint to be seen by the human eye (or binoculars or even small telescopes), the ancients could not have seen the galaxy cluster and hence no mythology is associated with it. However, the Coma Cluster still has an interesting history.
Not only is it one of the largest and most densely populated clusters of galaxies known, it is also the source of our first ideas about the dark matter in our universe. Unseen and mysterious, this matter greatly increases the total mass and gravitational strength of the universe, affecting its evolution and fate.
Dark matter was unknown and unsuspected until Swiss-American astronomer Fritz Zwicky discovered it in the Coma Cluster in the 1930s. Zwicky tallied up the visible galaxies in the cluster and estimated its mass. Then he observed the motions of galaxies near the edge of the cluster, which are determined by the total gravity (and hence mass) of the cluster. Zwicky found that the mass derived from the latter method greatly exceeded that from visual inspection.
Zwicky knew that if the law of gravity is correct – and there is no reason to doubt it – the only answer could be an additional source of mass, which he called dunkle Materie in German.
Today, the imprint of dark matter has been found throughout the universe and is at least five times more prevalent than the more familiar visible matter, such as the stars and galaxies we can see.
The center of the Coma Cluster is approximately RA: 12h 59m, dec: +27° 59′.
Bottom line: The Coma Cluster of galaxies is packed with up to 10,000 galaxies and can be spotted with the help of medium to large telescopes. The cluster was instrumental in the discovery of dark matter.
Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.