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How can a star’s colour help astronomers estimate its absolute magnitude?

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Diagram illustrating how astronomers calculate the distance to a star using parallax: Earth is shown in two positions on opposite sides of its orbit around the Sun, with lines extending to a nearby star X and further to distant background stars. The apparent shift in star X's position, measured as angle A, is used to determine its distance, with the baseline labeled as 186,000,000 miles. — Parallax Calculating stellar distances.

A star’s colour is a direct indicator of its surface temperature, which allows astronomers to estimate its absolute magnitude by using the Hertzsprung-Russell (H-R) diagram.12

Because a star's colour is closely related to its temperature and spectral type, astronomers can plot these properties on a colour-magnitude diagram. On this graph, stars do not fall randomly; they cluster in specific regions based on their evolutionary stage. For example, most stars fall along the main sequence, a diagonal band where luminosity is predictable based on temperature.314

By determining a star's temperature from its colour—often calculated via the colour index derived from the difference between its brightness in blue (B) and visual (V) wavelengths—astronomers can identify where the star lies on the H-R diagram. If the star is identified as a main-sequence star, its position on the diagram provides a reliable estimate of its absolute magnitude, or intrinsic brightness.451