The Hertzsprung–Russell Diagram

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The Hertzsprung–Russell (HR) diagram plots stars' luminosity against surface temperature (or spectral type), with hot stars on the left. Most stars lie along the diagonal main sequence, fusing hydrogen. Giants and supergiants sit top right (cool but luminous), while white dwarfs sit bottom left (hot but dim).

A star's position shows its mass, age and stage in evolution, and its path across the diagram traces its life. Clusters' HR diagrams are used to measure their ages.

To read the diagram, look at the axes: temperature runs backward, from hot on the left to cool on the right, and luminosity increases upward. A star like the Sun sits mid-way along the main sequence. A red giant like Aldebaran is cool but very luminous because it is huge, and Sirius B, a white dwarf, is hot but very faint because it is tiny. This follows from the idea that luminosity depends on surface area and temperature to the fourth power.

Astronomers use the HR diagram to estimate stellar radii, to identify distance by comparing apparent and expected brightness (main sequence fitting), and to date star clusters. In a young cluster the main sequence extends to hot blue stars, but in an old cluster the upper part has peeled away because massive stars died first; the turn-off point reveals the age. A common mistake is to think the main sequence is a path stars follow during their lives; a star stays at nearly the same spot for most of its life, and only leaves it when core hydrogen runs out. Another is forgetting that the temperature axis is reversed.