Edexcel A Level (IAL) Physics-5.43 The Hertzsprung–Russell Diagram- Study Notes- New Syllabus

Hertzsprung–Russell Diagram: Luminosity and Surface Temperature

The Hertzsprung–Russell (H–R) diagram is a graphical representation that shows how stars are related by their luminosity and surface temperature.

 Axes of the H–R Diagram

• The vertical axis shows luminosity (or absolute magnitude).
• Luminosity increases upwards.
• The horizontal axis shows surface temperature (K).
• Temperature decreases from left to right.

Important:

• Hot stars are on the left.
• Cool stars are on the right.

 Main Regions of the H–R Diagram

Main Sequence

• Diagonal band from top-left to bottom-right.
• Stars spend most of their lifetime here.
• Includes the Sun.

Giants and Supergiants

• Located above the main sequence.
• High luminosity but relatively low surface temperature.
• Very large radius.

White Dwarfs

• Located below the main sequence.
• Low luminosity but high surface temperature.
• Very small radius.

Interpreting Stellar Properties

• Stars of the same temperature can have different luminosities.
• Higher luminosity at same temperature → larger radius.
• Lower luminosity at same temperature → smaller radius.

This follows from the Stefan–Boltzmann law:

\( L = 4\pi r^2 \sigma T^4 \)

Sketching a Simple H–R Diagram

• Label vertical axis: Luminosity (increasing upwards).
• Label horizontal axis: Surface temperature (decreasing to the right).
• Draw a diagonal main sequence band.
• Label giants above and white dwarfs below.

Exam tip: Always show temperature decreasing left to right.

Stellar Evolution on the H–R Diagram

• Stars begin on the main sequence.
• As fuel runs out, stars move to the giant region.
• Low-mass stars eventually become white dwarfs.

Exam Note

• Do not confuse brightness with luminosity.
• State temperature direction clearly.
• Link position to size and energy output.
• Use terms: main sequence, giant, white dwarf.