Hertzsprung–Russell diagram
Great A'Tuin is the Giant Star Turtle (of the fictional species "Chelys galactica") who travels through the Discworld universe's space, carrying four giant elephants (named Berilia, Tubul, Great T'Phon, and Jerakeen) who in turn carry the Discworld. The narration has described A'Tuin as "the only turtle ever to feature on the Hertzsprung–Russell diagram".