Tony Sizer of the Orpington Astronomical Society and the Royal Observatory, Greenwich, gave us an informative talk about “The Hertzsprung-Russell Diagram“. This diagram was developed independantly by Hertzsprung and Russell as a way of categorising stars based on two directly-measurable properties, surface temperature and absolute brightness. After some introductory explanation of the ways these properties are measured and how the appropriate scales work, Tony went on to show how information about stars could be deduced from their relative positions on the diagram. These relationships, when combined with information we know about our own Sun and its place in the diagram, allow things like stellar diameters to be estimated. By taking plots of stars of the same age from suitable clusters it is possible to chart the course of at least some stellar evolution. By the end of the talk it was clear that the diagram provides a firm base upon which theories about the birth, development and death of stars could be built.