One of the central characteristics of disciplines is that they create their own particular ways of knowing the world. This process is facilitated by the specialization and refinement of disciplinary-specific semiotic resources over time. Nowhere is this truer than in the sciences, where it is the norm that disciplinary-specific representations have been introduced and then refined by a number of different actors. As a consequence, many of the semiotic resources used in the sciences today still retain some traces of their historical roots.

In this paper we analyse one such disciplinary-specific semiotic resource from the field of Astronomy—the Hertzsprung-Russell diagram. We audit the potential of this semiotic resource to provide access to disciplinary knowledge—what Fredlund et al (2012) have termed its disciplinary affordances. Our analysis includes consideration of the use of scales, labels, symbols, sizes and colour. We show how, for historical reasons, the use of these aspects in the resource may differ from what might be expected by a newcomer to the discipline.

We suggest that some of the issues we highlight in our analysis may, in fact, be contributors to alternative conceptions and therefore propose that lecturers pay particular attention to the disambiguation of these features for their students.