Hubble catches a colossal cluster

Hubble catches a colossal cluster
Credit: ESA/Hubble & NASA, RELICS

This NASA/ESA Hubble Space Telescope image shows a massive
galaxy cluster glowing brightly in the darkness. Despite its
beauty, this cluster bears the distinctly unpoetic name of
PLCK G308.3-20.2.

Galaxy clusters can contain thousands of all held together by the glue of
gravity. At one point in time they were believed to be the
largest structures in the universe—until they were usurped in
the 1980s by the discovery of superclusters. These massive
formations typically contain dozens of galaxy clusters and
groups and span hundreds of millions of light-years. However,
clusters do have one thing to cling on to: superclusters are
not held together by gravity, so galaxy clusters still retain
the title of the biggest structures in the universe bound by

One of the most interesting features of galaxy clusters is the
stuff that permeates the space between the constituent
galaxies: the intracluster medium (ICM). High temperatures are
created in these spaces by smaller structures forming within
the . This results in the ICM being made up of
plasma—ordinary matter in a superheated state. Most luminous
matter in the cluster resides in the ICM, which is very
luminous in X-rays. However, the majority of the mass in a
galaxy cluster exists in the form of non-luminous dark matter.
Unlike plasma, dark matter is not made from such as protons, neutrons and
electrons. It is a hypothesized substance thought to make up
80% of the universe’s mass, yet it has never been directly

This image was taken by Hubble’s Advanced Camera for Surveys
and Wide Field Camera 3 as part of an observing program called
RELICS (Reionization Lensing Cluster Survey). RELICS imaged 41
massive with the aim of
finding the brightest distant galaxies for the forthcoming
James Webb Space Telescope to study.

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