Astronomy Jargon 101: Galaxy Clusters

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll be the biggest thing in town after today’s topic: galaxy clusters!

Galaxy clusters are the single largest gravitationally bound structures in the Universe. The “gravitationally bound” part means that the gravity of the individual components of a galaxy cluster is strong enough to hold it together. While larger structures, known as superclusters, exist, they are not gravitationally bound and will eventually disperse.

Galaxy clusters can host a thousand galaxies or more and are typically at least 3-4 million light-years in diameter. The smallest galaxy clusters weigh 10¹⁴ solar masses, while the largest are ten to a hundred times more massive.

By far the dominant component of every galaxy cluster is dark matter, the mysterious form of matter that is invisible to light and does not interact with normal matter. Dark matter makes up around 90% of the mass of every cluster.

Of the remainder, 90% is in the form of an extremely hot but incredibly thin plasma called the intracluster medium. This medium is so hot that it emits brightly in the X-ray part of the electromagnetic spectrum. Even though the intracluster medium is so hot, it would register as a vacuum in Earth-based laboratory experiments.

Only 1% of the mass of every galaxy cluster is in the form of galaxies themselves. Those galaxies typically move through the cluster like bees in a beehive with speeds of up to a thousand kilometers per second. Those speeds can be used to weigh the cluster – more massive clusters will support faster galaxy movements without the cluster breaking apart.

The nearest cluster to Earth is the Virgo Cluster, which contains 1,000-2,000 galaxies and sits about 50 million light-years away.

Another famous cluster is the Coma Cluster, about 321 million light-years away. The main galaxies of the cluster are visible in amateur telescopes. In 1933 the astronomer Fritz Zwicky used observations of galaxy motion within the Coma Cluster to discover the existence of dark matter.