University of Oregon
Determining Properties of Stars
An excellent website detailing how a star's properties can be determined, and how these properties lead to the way a star is classified.
NASA
Nasa: Imagine the Universe: How Big Is That Star
This 2-5 day lesson plan investigates the diameter, radius and mass among several stars in order to understand stellar behavior, particularly in a binary system.
NASA
Nasa Star Child: Stars
This page details the life cycle of a star, and includes information on medium-sized and massive stars, plus a glossary and a question.