IC 1101, the Largest of All Galaxies
An elliptical supergiant with a diameter of just under eight million light-years
In the Yerkes classification system, the abbreviation cD is reserved for the largest galaxies [1]. The lowercase ‘c’ indicates that the galaxy is a supergiant, while the letter ‘D’ means that a large halo of diffused light surrounds it. It implies that cD galaxies have immense widths, up to millions of light-years, much greater than the effective radius, that is, the radius of that brighter inner region from which 50% of the total light they emit emanates.
cDs are usually the central galaxies of clusters made up of hundreds or thousands of galaxies, so astronomers often use this abbreviation, redefining its original meaning, as an acronym for central Dominant galaxy. But since cDs are also the brightest in the cluster they belong to, they are also mentioned with the abbreviation BCG, which stands for Brightest galaxy in a Cluster.
Real monsters of the sky, these galaxies are formed, according to the theory currently most accredited, by way of successive mergers with smaller galaxies. Over time, thanks to the material torn from the latter, they become increasingly massive and inevitably “fall” to the center of the cluster. In fact, the gravitational frictions with other galaxies allow the cDs to…
