IGCSE Physics / Stellar evolution
The Hertzsprung-Russell diagram revision notes
Edexcel International GCSE Physics (4PH1), chapter 27: Stellar evolution, specification point 8.12P. Free lesson.
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The Hertzsprung-Russell (HR) diagram groups stars by their absolute magnitude and surface temperature. Knowing its reversed axes and where the main star groups lie helps you read and draw the diagram, and follow a star’s path.
How to read the axes
Both axes run opposite to the way you might expect on a normal graph. Absolute magnitude is on the vertical axis: brighter stars are higher up, and negative numbers are at the top. Surface temperature is on the horizontal axis and falls as you move right. The vertical scale runs from about −10 at the top to +15 at the bottom. The horizontal scale runs from about 30 000 K on the left to 3000 K on the right. Blue stars are hottest and sit on the left. Red stars are coolest and sit on the right.
Where the main groups of stars are
Most stars lie in a band called the main sequence, running from the top left to the bottom right. The Sun is a main sequence star, roughly in the middle. Other groups sit in different parts of the diagram. Red giants are at the top right. They are cool but very bright because they are huge. Red supergiants are above the red giants, so they are even bigger and brighter. White dwarfs are at the bottom left. They are hot but dim because they are very small and give out little light in total. Hot and bright stars are at the top left. Cool and dim stars are at the bottom right.
How a star moves across the diagram
A star’s path can be shown on an HR diagram. The Sun stays on the main sequence for most of its life, then moves to different regions as it changes. The diagram itself does not change.
How to draw and label the diagram
Use a ruler to draw the axes, then place each group according to its temperature and brightness. Label every part clearly.
How to compare red giants and white dwarfs
Use the positions on the diagram to compare temperature. White dwarfs are on the hot side, while red giants are on the cool side, so white dwarfs have the higher surface temperature. Red giants: top right. White dwarfs: bottom left.