After reading the background information and working through all exercises in the HR Diagram Simulator, the student will understand...
The HR diagram is an astronomer's most important tool for presenting data on stars. It is a graph with some measure of luminosity L (its energy production) on the y-axis and some measure of its surface temperature T on the x-axis. Because of the solar units equation L = R2T4, specifying a star's L and T determines its radius R.
Stars do not exist with parameters corresponding to all locations of the HR Diagram. In fact, 90% of stars are found in a wide band stretching from the lower right corner to the upper left corner known as the main sequence. Stars on the main sequence are fusing hydrogen into helium in their cores and do so for the vast majority of their lives. Stars found above the main sequence are fusing other nuclei in their cores near the ends of their lives.
We can rewrite the luminosity equation as . This equation indicates that stars are large when L is large and T is small -- the upper right of the HR Diagram. We can picture a "radius axis" running from the lower left to the upper right along which radius increases. (need graphic) Lines of constant radius run perpendicular to this axis and are shown in the simulator.
This astronomy "Little Big Picture" was programmed by REU student Nick Robe. It is an early effort of the UNL Astronomy Education Group to provide materials for mobile devices. More astronomy teaching materials can be found on the web at astro.unl.edu.
This simulation makes use of the Dojo tooklit available at dojotoolkit.org.
Background image by P. van de Haar at Wide field astrophotography.