For IB Physics SL and HL, read a Hertzsprung-Russell diagram by using the vertical axis to find a star's luminosity and the horizontal axis to find its surface temperature. Then identify the region containing the star to classify it as a main-sequence star, giant, supergiant or white dwarf.
Reading the Axes
An HR diagram plots stars according to luminosity and surface temperature. Luminosity normally increases upward and may be shown in watts or relative to the Sun, while temperature decreases from left to right.
| Feature | How to read it |
|---|---|
| Vertical axis | Higher position means greater luminosity, or total power emitted by the star. |
| Horizontal axis | Left means hotter; right means cooler. Temperature is measured in kelvin. |
| Upper left | Hot and very luminous stars. |
| Lower right | Cool and low-luminosity stars. |
The reversed temperature scale is a common source of error. Do not assume that temperature increases toward the right as it does on most graphs.
Identifying Stellar Regions
| Region | Position and properties |
|---|---|
| Main sequence | Diagonal band from the hot, luminous upper left to the cool, dim lower right. Stars spend most of their lifetimes here. |
| Red giants and supergiants | Upper right: cool surfaces but high luminosities, implying very large radii. |
| White dwarfs | Lower left: hot surfaces but low luminosities, implying very small radii. |
Radius can be inferred using the Stefan-Boltzmann relationship . For two stars with the same temperature, the more luminous star has the larger radius. This explains why a cool red giant can emit much more power than a hotter white dwarf.
A common misconception is that luminosity means how bright a star appears from Earth. Apparent brightness also depends on distance, whereas luminosity is the star's total emitted power.
Exam Technique
For syllabus topic E.5 Fusion and stars, always state both the star's location and the physical reasoning. Examiners may ask you to identify a region, compare two stars, or deduce relative radius from luminosity and temperature. Remember: temperature decreases to the right.
