Luminosity and temperature as evolutionary markers
The Hertzsprung-Russell diagram plots stellar luminosity (brightness) on the vertical axis against surface temperature (or spectral class) on the horizontal axis. What makes this diagram so revealing is that it is not a random scatter. Most stars cluster along a diagonal band called the main sequence, where stellar age and mass determine position. Hotter, more massive stars sit in the upper left, burning hydrogen fuel rapidly and shining brightly. Cooler, less massive stars occupy the lower right, burning fuel slowly and glowing dimly. A star spends roughly 90 percent of its life on the main sequence, gradually shifting position as its core hydrogen depletes and its core temperature creeps upward.
Beyond the main sequence lie distinct regions: red giants (cool but luminous) and white dwarfs (hot but dim). A star's path across the HR diagram over billions of years is essentially a fossil record of its internal evolution.
Reading stellar populations and ages
Star clusters plotted on the HR diagram reveal their age at a glance. Young clusters show a well-populated main sequence that extends far into the hot, bright upper left. Older clusters show a main sequence that turns off at lower luminosities, because the most massive (and thus shortest-lived) stars have already exhausted their hydrogen and evolved into giants. By matching a cluster's turnoff point to stellar evolution models, astronomers can date the cluster. Globular clusters with main sequence turnoffs at faint, cool stars are known to be over 10 billion years old, whereas open clusters with hot, bright main-sequence stars are often only millions of years old. This technique has been essential for constraining the age of the Milky Way and the universe itself.