E.5.5—HR diagram regions

Read the axes

An HR diagram compares luminosity with surface temperature. Temperature usually decreases from left to right, so the hottest stars are on the left.

HR region	Surface temperature	Luminosity and size
Main sequence	Hot at upper left to cool at lower right	Luminosity and typical radius decrease along the sequence
Red giants / supergiants	Cool, right side	Luminous because their radii are very large
White dwarfs	Hot, lower left	Dim because their radii are very small
Instability strip	Narrow diagonal region crossing the diagram	Pulsating stars whose luminosity varies periodically

Use constant-radius lines

From $L=4\pi R^2\sigma T^4$ , a constant-radius line links luminosity and temperature. At the same temperature, greater luminosity means greater radius; at the same luminosity, the hotter star has the smaller radius.

Common trap

Do not read the temperature axis as increasing to the right, and do not identify a white dwarf from temperature alone; its low luminosity is also essential.

How it is assessed

Questions identify a white dwarf or compare temperatures, radii and luminosities of plotted stars.

Command terms

Identify / Compare

What earns marks

Read both axes with their directions and use the appropriate region or constant-radius relation.

Watch for

Reading the horizontal temperature direction incorrectly or using only one of luminosity and temperature.

E.5.5—HR diagram regions

Read the HR Diagram

E.5.5 Exam Analysis

Assessment in practice