An extremely bright giant elliptical galaxy near the centre of a cluster of galaxies, in which it will be the first-ranked (i.e. brightest) member. They are assigned the letter D on Morgan's classification, which signifies that they are dustless. The more extreme supergiant examples at the centres of rich clusters are classified as cD galaxies. D galaxies are often radio sources, and are surrounded by extended envelopes of faint stars.
Subjects: Astronomy and Astrophysics.