 |
| The Virgo Cluster of Galaxies |
Classification of Clusters
We refer to those clusters near the upper end of this range as rich clusters, while those near the lower end are termed poor clusters. We may further classify clusters according to whether they are regular (spherical with a central region of higher density) or irregular (amorphous in shape without a condensed central region). The adjacent image shows the Virgo Cluster of galaxies, which is a nearby rich, irregular cluster that we shall discuss more in the next section.Typical Properties of Clusters
Clusters typically have masses from 1014 to 1015 solar masses, the diameters are typically 8 Mpc, the velocity dispersions are 800-1000 km/s, and the average separation from other clusters is about 10 Mpc. In clusters the typical mass to light ratio is ~ 400 in units of solar masses divided by solar luminosities. Thus, they contain large amounts of dark matter.The adjacent image shows the cluster of galaxies in the constellation Hercules (Ref; source: Bill Keel). This cluster, which is 650 million light years distant, is rich in gas and dust and spiral galaxies with robust star formation, but relatively poor in elliptical galaxies. Thus it is a relatively young cluster of galaxies. In this image the star forming regions has a bluish cast, while the elliptical galaxies with little star formation are more yellow in color. We can also see evidence in this images for various collisions and mergers of galaxies in the cluster. These collisions and mergers are presumably altering the content of the cluster.
Information supplied by: http://csep10.phys.utk.edu