A Hairy Constellation

Or a Tale of Two Clusters

Copyright © 2007 by Bob Moler

Coma Berenices

The hairiest constellation of all is Coma Berenices, or Berenice's hair. In the image above, from Cartes du Ciel I've limited the magnitude to the stars visible to the unaided eye, at least for someone with excellent eyesight. The star group extending down from the upper right star kind of looks like several strands of hair. This is especially noticeable in the real sky, because these stars are rather dim.

The story as to how this constellation got its name is of Greek origin, apparently from after the time when Alexander the Great conquered Egypt. Berenice was a real person, Queen of Egypt, and wife of Ptolemy III Euergetes in the 3rd century BC. It is said that Ptolemy went off to war in Syria. Berenice was so concerned with her husbands safety that she cut off her hair and offered it to the Goddess Aphrodite, the Greek goddess of love, and counterpart of the Roman Venus, at her temple in Alexandria.

Upon the safe arrival of her husband, her beautiful tresses disappeared from the temple. Only then it was noticed that these strands were transported into the sky to form the constellation we see today, as a sign of Berenice's love.

Mythology aside, the star cluster, Coma Berenices, is the only star cluster to be its own constellation. True none of the cluster stars is part of the stick figure of the constellation, but it's obvious what stars make the strands of Queen Berenice's hair. The cluster appears so large that it was not counted except in one catalog. It has the designation Mel 111. It appeared as object 111 in Philibert Jacques Melotte's 1915 catalog of deep sky objects.

Actually Melotte's cataloging of this cluster was prophetic. These stars were not proved to be an actual star cluster until 1938. This cluster lies at a distance of 288 light years, making it the second closest star cluster after the Hyades, the face of Taurus, at 150 light years away. Hyades, by the way is Mel 25.

How many stars are actually in the Coma Berenices star cluster? It appears to be only 37 stars. It seems that the cluster is not massive enough to hold itself together so its member stars are slowly scattering. The cluster's age is thought to be about 500 million years old, or about a tenth of the sun's age. Coma Berenices star cluster is an open or galactic star cluster. These star clusters lie very near the plane of the Milky Way,a and tend to be young, compared to the sun's age. However if you check the chart on the next page, you'll see that Coma Berenices lies at the galactic pole, nearly 90 degrees from the galactic plane, as far as you can get from it. This, of course, is an illusion of perspective. In actuality both the sun and the cluster lie quite close to the galactic plane.

Galactic Pole

Coma Berenices is located near the galactic pole. I don't have a program to show galactic latitude and longitude, so I went to a location on the earth where I could put the galactic equator on the horizon. Created using Cartes du Ciel.

The galactic pole is where the Milky Way galaxy appears thinnest, because we're looking out 90 degrees from the thickest part of the Milky Way. We'd expect to see galaxies here, and we do. More famous than the star cluster, to astronomers that is, is the Coma Cluster of galaxies. When you hear astronomers talking about the Coma Cluster, they're talking about the cluster of galaxies. At the bottom of the previous column is an unlabeled chart of the Coma Cluster. The cluster is located, if you refer to the chart on the front page, about a third the way along the nearly horizontal constellation line from the left. The Coma Cluster is a difficult one for the amateur observer. Its brightest member NGC 4889 is 13th magnitude; it's the large elliptical galaxy near the center of the chart.

Come Cluster

The Coma Cluster of galaxies as displayed by Cartes du Ciel without labels

The Coma Cluster contains more than a thousand galaxies and lies about 320 million light years away. The galaxies in this cluster are embedded in hot intracluster gas. This hot gas is falling towards the center of the cluster, the temperature being a measure of its kinetic energy.

In Coma Berenices we have two clusters for the price of one.

Questions? Send Email to me at bob@bjmoler.org

Updated:  05/04/07