How big is Fornax Cluster? | Scale of the Universe

The Fornax Cluster is a cluster of galaxies in the southern constellation Fornax, the Furnace. After the Virgo Cluster, it is the second-nearest galaxy cluster to Earth, about 65 million light-years away. Its main body is compact, about 4.6 million light-years across. That width is measured to the edge of the region where the cluster's gravity holds galaxies in orbit.

Size

Galaxy clusters do not have neat edges, so their sizes depend on what is measured. For the Fornax Cluster, astronomers use a virial radius of about 0.7 megaparsecs. Inside it, galaxies orbit the cluster's center. A megaparsec is a million parsecs, about 3.26 million light-years, so the main body is about 1.4 megaparsecs, or 4.6 million light-years, across.

Measured from front to back, the cluster is compact too. Hubble Space Telescope measurements of its elliptical and similar galaxies found them spread over about 2.0 megaparsecs along the line of sight, which fits how compact it looks on the sky. The researchers found no sign of galaxies falling in and described the cluster as compact and settled.

Some galaxies farther out are still joining the cluster. A group around the galaxy NGC 1316, 3 degrees from the center on the sky, is bound to the cluster and probably falling toward it for the first time. The mass within 1.4 megaparsecs of the center is about 70 trillion times the mass of the Sun.

Size comparisons

Seen from Earth, the main body is about 4.4 degrees across, about eight times the width of the full Moon.
The main body is about 1.8 times the distance from the Milky Way to the Andromeda Galaxy, about 2.5 million light-years.
It is about 70% as wide as the Local Group, the group of galaxies that includes the Milky Way, but it has about 37 times as much mass.
The Virgo Cluster's main body is about 2.4 times as wide as the Fornax Cluster's and has about nine times the mass.
If the Milky Way, about 100,000 light-years across, were shrunk to the size of a basketball, the main body of the Fornax Cluster would be about 11 meters (36 feet) across and about 160 meters (510 feet) away.

A cluster that has settled

At the center of the cluster is NGC 1399, a giant galaxy of the kind astronomers call a cD galaxy. Galaxies like it look like large elliptical galaxies with faint, extended outer layers. They grow by swallowing smaller galaxies that gravity draws toward the center of the cluster.

Images from the European Southern Observatory's VLT Survey Telescope show a faint bridge of stars between NGC 1399 and the nearby galaxy NGC 1387. It is a sign that NGC 1399 is still pulling material from its neighbors.

The brightest galaxy in the cluster is not at the center. NGC 1316, also called Fornax A, sits near the edge and is one of the strongest sources of radio waves in the sky. A 2018 survey found 590 likely members of the cluster in and around its main body, 564 of them dwarf galaxies.