Type-cD galaxy
A Type-cD galaxy, or cD galaxy, is a giant elliptical galaxy with a very large, faint halo of stars. It sits near the centers of some rich galaxy clusters and is often the brightest galaxy in its cluster. The letters “c” and “D” come from “supergiant” and “diffuse,” and the term can also stand for “central dominant.”
cD galaxies are among the largest galaxies known. Their envelopes can stretch over well more than a million light-years and blend into the surrounding galaxy cluster. They look elliptical, but their huge, low-brightness halos make up much of their size and may belong as much to the cluster as to the galaxy itself.
These galaxies are thought to grow mainly by merging with other galaxies at the cluster center, in a process sometimes described as galactic cannibalism. Some cD galaxies show multiple galactic nuclei or tidal features from eaten galaxies. They are typically the brightest cluster galaxies rather than ordinary field galaxies, and about 20% of brightest cluster galaxies are type-cD.
The surrounding halo can include intra-cluster light—the stars stripped from other galaxies—creating an envelope that can be up to about 3 million light-years across. The cD galaxy can contribute a notable fraction of the cluster’s total baryon mass within its region.
Dynamical friction helps this growth: as the big galaxy moves through the cluster, it slows and pulls in smaller galaxies, building up mass at the center. Because of their size and central role, cD galaxies help astronomers study how galaxies form and evolve in the universe.
This page was last edited on 2 February 2026, at 12:26 (CET).