Hertzsprung-Russell Diagram

Definition

A scatter graph of stars showing the relationship between each star’s absolute magnitude and its estimated surface temperature, or between optical and perceptual proxies for these quantities.

Absolute Magnitude and Temperature Scales

The absolute magnitude and temperature can be described as follows:

A star’s absolute magnitude is the attenuation (in factors 10^−0.4) of the star Vega’s power (as received at 10 pc viewing distance [32.6 light-years]) to equal that of the star (also corrected to 10 pc). The convention that dimmer stars have higher magnitude is a historical precedent that dates from Hipparchus (c. 190 BCE–c. 120 BCE), whose system of stellar magnitudes was based on visual assessment.

A star’s surface temperature is estimated in one of three ways: by the observed color (an old way), by comparison of two sensor outputs such as blue and violet (a newer way), or by model prediction of the temperature of a black-body radiator with the same radiation power per unit...