The HR diagram is the most important chart in stellar physics. Plotting luminosity vs. temperature, stars fall in clear groups: the main sequence (stable H-fusion), the giant branch (exhausted H core) and white dwarfs (cooling remnants). The simulator contains 16 real stars clickable for details, with animated evolution tracks from ZAMS through giant branch to final fate. Cepheids in the instability strip provide cosmic distance ladders.
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