University of Central Florida (UCF) AST2002 Astronomy Final Practice Exam

A galaxy cluster is defined as a structure of galaxies bound together by gravity. In such clusters, numerous galaxies—often ranging from a few dozen to over a thousand—are held in proximity due to their mutual gravitational attraction. This gravitational bond is strong enough to not only draw galaxies together but also to keep them in a stable formation over cosmic timescales.

Galaxy clusters are significant in the study of cosmic structure because they can provide insights into the distribution of dark matter, the evolution of galaxies, and the large-scale structure of the universe. Additionally, clusters may contain hot gas that's seen in X-ray emissions, contributing to our understanding of the thermal dynamics of the universe.

The other choices differentiate in their meanings: A collection of stars orbiting a single star refers to a stellar system or possibly a star cluster and does not imply the gravitational binding of multiple galaxies. A cloud of interstellar gas speaks to regions of space filled with gas and dust, not gravitationally-bound collections of galaxies. A region with high levels of dark matter relates to locations in the universe where dark matter concentration is significant but does not accurately characterize the group of galaxies that make up a cluster. Thus, the essence of a galaxy cluster specifically lies in the gravitational binding of multiple galaxies.

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