False-color image of IC 1101 at the center of the galaxy cluster Abell 2029. It was the cover image of the Science issue of October 26, 1990

IC 1101, the Largest of All Galaxies

An elliptical supergiant with a diameter of just under eight million light-years

Michele Diodati
Island Universes
Published in
8 min readJan 24, 2020

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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…

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Michele Diodati
Island Universes

Science writer with a lifelong passion for astronomy and comparisons between different scales of magnitude.