How to see it
The Coma Cluster is a group of galaxies in the faint constellation Coma Berenices, visible in medium to large amateur telescopes. Coma Berenices lies between Leo and Bootes, and as such is most conveniently viewed in the evening sky of spring and summer. The cluster, the central part of which covers a roughly circular area about a degree and a half across (9 times the area of a full moon), is near the northern border of Coma Berenices. The full cluster may extend farther, and numerous other clusters are in the same area of sky. The Coma Cluster is roughly midway a long a line drawn from Rho Bootes to Delta Leonis (Zosma), near the North Galactic Pole.
Science
The center of the Coma Cluster is about 320 million light years away, and it may stretch 20 million light years from side to side. 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. In any case there are more individual galaxies in this cluster than there are stars visible to the unaided human eye on a clear, dark night. Most are elliptical, although there are a few spiral galaxies. The two brightest members are NGC 4889 and NGC 4874, both of which are giant ellipticals at least 2 to 3 times larger than our own Milky Way. However, most are dwarf galaxies, perhaps similar to the Milky Way’s companions, the Large and Small Magellanic Clouds.
A majestic face-on spiral galaxy located deep within the Coma Cluster of galaxies. Image Credit: NASA
History and Myth
Too faint to be seen by the human eye (or binoculars or even small telescopes), the ancients could not have seen the cluster and hence no mythology is associated with it. However, the Coma Cluster, also known as Abell 1656, is extremely interesting historically and scientifically. 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 of “dark matter.” Unseen and mysterious, this matter greatly increases the total mass and gravitational strength of the Universe, further affecting its evolution and fate. But dark matter was unknown and unsuspected until Swiss-American astronomer Fritz Zwicky “discovered” it in the Coma Cluster in the 1930′s. 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?
