Knowra Galaxy cluster Galaxy cluster A gravitationally bound structure of hundreds to thousands of galaxies, embedded in dark matter and hot, X-ray-emitting gas. It is the largest type of structure held together by gravity.
Dark matter : Matter inferred from its gravitational effects that emits, absorbs, and reflects no detectable light. It supplies most of a cluster’s mass and helps hold the system together.
Gravitational collapse : The contraction of matter under its own gravity, often leading to bound structures. It drives the formation of dense regions that become galaxy clusters.
Sunyaev–Zel'dovich effect : The change in cosmic microwave background radiation caused by its scattering off energetic electrons in hot gas. It detects clusters through their hot plasma, including at great distances.
Coma Cluster : A nearby, rich galaxy cluster in the constellation Coma Berenices. Its galaxy motions provided classic evidence for substantial unseen mass.
Galaxy group : A gravitationally bound association of relatively few galaxies, smaller than a galaxy cluster. Groups are less massive and typically contain fewer galaxies than clusters.
Intracluster medium : The diffuse, extremely hot plasma filling the space between galaxies in a galaxy cluster. This gas contains much of a cluster’s ordinary matter and radiates strongly in X-rays.
Hierarchical structure formation : The growth of cosmic structure as smaller gravitationally bound systems merge into larger ones. Clusters form near the endpoint of this process, through repeated accretion and mergers.
Cosmic microwave background : The relic radiation released when the early universe became transparent, now observed across the sky. Its distortions and measured properties help constrain the abundance and growth of clusters.
Bullet Cluster : A merging system of galaxy clusters whose hot gas and gravitational mass are spatially separated. Its lensing map and X-ray gas illustrate how cluster mergers can separate mass components.
Supercluster : A vast association of galaxy groups and clusters spanning tens to hundreds of millions of light-years. Unlike a cluster, a supercluster is generally not bound as a whole by gravity.
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