What is the difference between a galaxy group and a cluster?

The distinction is mainly one of size. Groups contain up to a few dozen galaxies and are less massive, while clusters contain hundreds to thousands of galaxies. The Milky Way belongs to a small group called the Local Group.

How do astronomers weigh a galaxy cluster?

They use several independent methods: the motions of member galaxies, the temperature of the hot X-ray gas, and the gravitational lensing of background galaxies. All three consistently show that most of the mass is dark matter.

Galaxy Groups and Clusters

Galaxy groups and clusters are the largest gravitationally bound systems, gatherings of tens to thousands of galaxies bound together with hot gas and dark matter.

Definition

Galaxy groups and clusters are gravitationally bound associations of galaxies, ranging from a few members in groups to thousands in rich clusters, embedded in dark matter halos and permeated by hot, X-ray-emitting gas.

Scope

This topic covers the definition and richness classes of groups and clusters, methods of measuring their masses from galaxy dynamics, X-ray gas, and gravitational lensing, the dominance of dark matter in their mass budget, and their use as tracers of cosmic structure growth.

Core questions

How are groups and clusters defined and classified?
How are cluster masses measured, and what do they reveal?
Why is most of a cluster's mass invisible?
How do clusters trace the growth of cosmic structure?

Key theories

Dynamical mass and missing mass: Zwicky applied the virial theorem to galaxy velocities in the Coma cluster and found the dynamical mass far exceeded the visible mass, the first strong evidence for dark matter.
Cluster catalogs and richness: Abell's systematic catalog classified rich clusters by the number of member galaxies, establishing a framework for studying their abundance and distribution.
Clusters as cosmological probes: The number and growth of massive clusters over cosmic time are sensitive to the amount of matter and the nature of dark energy, making clusters precise cosmological tools.

Clinical relevance

Clusters are signposts of the densest regions of the universe and sensitive probes of cosmology; their mass measurements were historically pivotal for dark matter, and counting them constrains how structure has grown under gravity and dark energy.

History

Zwicky's 1937 measurement of the Coma cluster revealed missing mass. Abell's 1958 catalog systematized the study of rich clusters, and the advent of X-ray astronomy and gravitational lensing in later decades provided independent mass measurements, turning clusters into precision cosmological instruments.

Key figures

Fritz Zwicky
George Abell
Andrey Kravtsov
Stefano Borgani

Seminal works

zwicky1937
abell1958
voit2005

Frequently asked questions

What is the difference between a galaxy group and a cluster?: The distinction is mainly one of size. Groups contain up to a few dozen galaxies and are less massive, while clusters contain hundreds to thousands of galaxies. The Milky Way belongs to a small group called the Local Group.
How do astronomers weigh a galaxy cluster?: They use several independent methods: the motions of member galaxies, the temperature of the hot X-ray gas, and the gravitational lensing of background galaxies. All three consistently show that most of the mass is dark matter.