Astronomy 162: Professor Barbara Ryden

Wednesday, February 26


``So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite 'em;
And so proceed ad infinitum.''
- Jonathan Swift

Key Concepts

(1) Our galaxy is in a small cluster of galaxies called the Local Group.

The universe displays hierarchical structure. Just as there is a hierarchy of sizes among fleas in Jonathan Swift's poem (big fleas have smaller fleas which in turn have smaller fleas...) so there is a hierarchy of sizes in astronomical objects. (As far as anyone has been able to tell, however, the superclusters are not clumped into `superduperclusters'. Superclusters, which are about 100 Mpc across, are the largest known structures in the universe.)

Our own galaxy, the Andromeda Galaxy, the Large Magellanic Cloud, the Small Magellanic Cloud, and a couple of dozen other galaxies are all part of a small cluster of galaxies called the Local Group. (In fact, the total number of galaxies in the Local Group is not yet known. Most of the galaxies are hard-to-see dwarf elliptical galaxies and dwarf irregular galaxies. Some of them are hidden behind the dust in our own galaxy, and are thus extraordinarily difficult to discover.) The galaxies in the Local Group are all in orbit around the Local Group's center of mass. Most of the mass (as well as most of the luminosity) in the Local Group is in the two biggest galaxies, our own galaxy and the Andromeda Galaxy. Thus, the center of mass lies between us and Andromeda. The smaller dwarf galaxies in the Local Group are all moving at high speeds, implying that the Group contains a large amount of dark matter.

(2) Most galaxies are in gravitationally bound clusters of 10 to 10,000 galaxies.

Poor clusters of galaxies (of which the Local Group is one) contain less than a thousand galaxies. Most clusters are poor. However, there are also a few rich clusters, containing over a thousand galaxies. Where poor clusters are only 1 megaparsec (Mpc) across or less, a rich cluster will be about 3 Mpc to 10 Mpc in size.

The Virgo cluster (seen in the image below) is an example of a moderately rich cluster of galaxies. It contains over 2000 galaxies, including the giant elliptical galaxy M87. The Virgo cluster is approximately 15 Mpc away and 3 Mpc in diameter. Since the Virgo cluster is the nearest rich cluster to us, it occupies a large region of the sky, about 10 degrees across.

[Image credit: © Anglo-Australian Observatory]

The Coma cluster (in the constellation Coma Berenices) is an example of an extremely rich cluster of galaxies. The Coma cluster contains about 10,000 galaxies. It is approximately 90 Mpc away and 5 Mpc in diameter.

Within rich clusters, there are many elliptical galaxies, but few spiral galaxies. (Spiral galaxies tend to be found in poor clusters like the Local Group.) Near the center of a rich cluster of galaxies, there are usually one or two or three giant elliptical galaxies, containing up to a trillion stars. These huge galaxies sit near the center of the cluster like bloated spiders sitting near the center of their web.

When you look at a cluster at visible wavelengths of light, you see mainly the stars which it contains. However, clusters contain more than stars. For example, X-ray observations show that rich clusters contain very hot gas distributed evenly throughout the cluster. (Since the gas is very hot, the individual atoms are traveling faster than the escape speed from a galaxy.) The Coma cluster is estimated to contain about six times as much mass in very hot gas as in stars. Moreover, the observed high speeds of galaxies within rich clusters show that clusters must contain dark matter. (Without dark matter to anchor the galaxies, the clusters would have flown to pieces billions of years ago.) The Coma cluster is estimated to contain roughly six times as much mass in dark matter as in very hot gas. Thus, the starry galaxies that you see when you look at a photograph of a rich cluster represent only about 2 percent of the cluster's total mass!

(3) Clusters are joined into superclusters, which are separated by huge voids.

Clusters of galaxies are themselves clustered. Superclusters, or clusters of clusters of galaxies, are the largest structures known. The biggest superclusters are 100 Mpc long (or more) and have masses of 1016 Msun.

The Local Group and the Virgo cluster are both part of the Local Supercluster, a flattened structure which is about 50 Mpc in diameter (thus, it takes light over 160 million years to travel from one edge of the Local Supercluster to the other).

It is possible to map the distribution of galaxies in space out to distances of hundreds of Mpc. The procedure used in making such a map:

For a map of the large scale structure of the universe, click here. The large-scale structure of the universe, as seen in such maps, is ``spongy'' or ``bubbly''. Superclusters are not spherical, but form flattened walls or elongated filaments. The superclusters are separated by enormous, roughly spherical voids (which are the holes in the cosmic sponge). The voids are as much as 100 Mpc across, and contain very few galaxies, in striking contrast to the galaxy-rich superclusters.
Prof. Barbara Ryden (

Updated: 2003 Feb 26

Copyright 2003, Barbara Ryden