General shape: main sequence, dwarfs and giants.

The idea

The HR diagram plots luminosity (or absolute magnitude) against surface temperature, with temperature increasing right to left. Most stars, including the Sun, lie on the main sequence, a broad band running from hot, bright, blue stars at the top left to cool, dim, red stars at the bottom right. Above and right of the main sequence sit the giants and supergiants: cool but very luminous because of their huge radii. Below the main sequence lie the white dwarfs: hot but dim because they are tiny. A star's position therefore encodes its radius via L = 4πR²σT⁴, so the diagram reveals stellar evolution stages.

What earns the marks

• The main sequence is a diagonal band from top left (high luminosity, high temperature) to bottom right (low luminosity, low temperature), containing stars fusing hydrogen in their cores.
• Giants and supergiants occupy the upper right region: relatively cool surfaces but very high luminosity, which requires very large radii.
• White dwarfs occupy the lower left region: hot surfaces but very low luminosity, which requires very small radii.
• Position on the diagram links to the Stefan-Boltzmann law L = 4πR²σT⁴, so for a given temperature luminosity depends on radius squared.
• The Sun lies on the main sequence, near the middle, providing a reference point for luminosity and temperature.
• Evolutionary paths move stars off the main sequence towards the giant region and eventually to the white dwarf region for low- and medium-mass stars.

Common mistakes and how to put them right

• Thinking the main sequence runs from bottom left to top right.

  Put it right: The temperature axis increases to the left, so the correct band runs from top left to bottom right.

• Assuming all cool stars are dim.

  Put it right: Giants are cool yet highly luminous, so temperature alone does not determine luminosity.

• Believing white dwarfs are cold.

  Put it right: They are hot but faint because their surface area is very small.

• Confusing luminosity with apparent brightness.

  Put it right: The HR diagram uses intrinsic luminosity or absolute magnitude, not observed brightness.

Exam tips

• Sketch the axes with temperature decreasing to the right and label the main sequence, giants and white dwarfs before adding any specific star.
• When asked to classify a star, quote both its luminosity and its temperature, then justify the region using radius reasoning.
• Use the Sun as an anchor: state whether a star is more or less luminous and hotter or cooler than the Sun to locate it quickly.