The Hertzsprung-Russell Diagram

The Hertzsprung-Russell (HR) diagram is one of the most important tools in astrophysics. Independently developed by Ejnar Hertzsprung and Henry Norris Russell in the early 20th century, it plots stars according to their luminosity and surface temperature. The resulting pattern reveals fundamental relationships between stellar properties and provides a framework for understanding stellar evolution.

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The Axes of the HR Diagram

The HR diagram has two axes:

• Vertical axis: Luminosity (or absolute magnitude). Luminosity increases upward and is usually plotted on a logarithmic scale, covering a range from about 10⁻⁴ L☉ to 10⁶ L☉.

• Horizontal axis: Surface temperature (or spectral class). Temperature increases to the left — this is the convention, and it catches many students out. The hottest stars (O and B type, ~40,000 K) are on the left; the coolest (M type, ~3,000 K) are on the right.

The axes can equivalently be labelled with spectral class (O, B, A, F, G, K, M from left to right) instead of temperature, or with absolute magnitude instead of luminosity. The diagram is the same regardless of which labels are used.

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The Main Sequence