The Hertzsprung-Russell (HR) diagram is astronomy's most powerful stellar classification tool, plotting stars according to their luminosity (brightness) versus surface temperature. Created independently by Ejnar Hertzsprung and Henry Norris Russell in the early 1900s, this revolutionary diagram reveals that stars aren't randomly distributed but fall into distinct patterns that tell the story of stellar evolution.</p><p>The diagram's main sequence—a diagonal band containing about 90% of visible stars—runs from hot, blue giants (30,000K, 100,000 times the Sun's luminosity) in the upper left to cool, red dwarfs (3,000K, 0.01 solar luminosities) in the lower right. Our Sun sits midway along this sequence at 5,778K. Above the main sequence lie red giants and supergiants—stars that have exhausted their core hydrogen fuel and expanded dramatically. Below stretch the white dwarfs, Earth-sized stellar remnants with surface temperatures around 25,000K but very low luminosities.</p><p>The HR diagram functions like a stellar DNA test, allowing astronomers to determine a star's age, mass, and evolutionary stage from just two measurements. For example, comparing star clusters of different ages reveals how stellar populations evolve over millions of years. This fundamental tool continues driving discoveries about stellar lifecycles, galaxy formation, and the universe's chemical evolution.