1. Cluster Merger - Early Stages
An illustration of the early stages of the merger of a two galaxy clusters. Giant galaxy clusters filled with vast clouds of hot, X-ray producing gas are assembled through supersonic collisions over billions of years. (Illustration: CXC/D. Berry)

2. Cluster Merger - Last Stages
An illustration of the last stage of the merger of two galaxy clusters. The multi-million degree gas has more mass than all the galaxies combined. (Illustration: CXC/D. Berry)

3. Illustration of Fornax Cluster
This illustration depicts the motions of a gas cloud in the core of the Fornax cluster and a group of galaxies on the outskirts of the cluster. A collision between the cluster core and the group will occur in a few billion years. The Fornax core and the galaxy group may lie along part of a large filamentary structure of dark matter (shown as a faint gray structure) that is collapsing and flowing in toward a common center. The swept back appearance of the gas in the core of the cluster is due to its motion through the lower density gas spread over a much larger volume. (Illustration: NASA/CXC/M.Weiss)

Photo Album: Fornax Cluster 08 Sep 04)