Exploring the HR Diagram
Topic/Concept: HR Diagram
Type of Activity: Thought Experiment
Prerequisite knowledge required: HR Diagram and luminosity-radius-temperature relation for black bodies.
Resources required: Chalkboard, writing instruments, paper.
Learning Objectives: Understand how changes in the luminosity, temperature, and radius of a star affect its position on the HR digram.
Common misconceptions:
Detailed description of activity:
I drew the HR diagram, explained the axes and main sequence. I then explained how you can estimate the radius of a star by assuming it behaves like a blackbody and reading the luminosity and temperature off the HR diagram. After I had covered these topics sufficiently, I drew a point on the diagram and broke the class into four groups. I assigned each group one direction (up, down, left or right) and asked them what would happen to a star’s luminosity, temperature and radius if it moved that direction on the HR diagram. The groups would usually try to explain it using an equation. I would let the first group get away with this, but of later groups I would demand progressively more robust physical explanation. I was looking for them to refer to the assumption of a blackbody and how the luminosity emitted from a square meter of blackbody surface depended on only one thing, its temperature. Many groups would simply say their explanation was the same as the previous group’s, but I made sure to demand that they at least verbalize the same explanation again, just to force them to think about these issues.
Assessment: Did they get the right answer? How good was their explanation?
Associated files and images: None
Comments: None
By Christer Watson (8/99)