What to watch for
Drag the mass up and watch the starting point march to the upper-left, bright and blue, while the predicted main-sequence lifetime in the corner collapses from gigayears to mere megayears. That trade-off is the whole story of stellar demographics: the showy blue stars are rare and short-lived, while the dim red ones outlive the present age of the universe many times over.
Follow a full track and notice the right-angle: stars cross the diagram nearly horizontally (cooling at almost constant luminosity) on their way to the giant branch, then drop almost vertically to the remnant. The remnant marker changes character with mass: a fading white-dwarf trail for Sun-like stars, a flag at the hot-faint corner for the neutron star, and an event-horizon ring (no photosphere, so nothing to plot) for the black hole.
Knobs
- Initial mass: the star's birth mass in solar masses (0.3–40). It sets everything: where the track starts, how luminous and hot the star is, how long it lives, and which remnant it leaves.
- Metallicity: the abundance of elements heavier than helium, relative to the Sun (0.1–2.5 ). Metal-poor stars are slightly hotter and fainter at fixed mass, nudging the track a touch to the left.
- Animate track: toggle the evolution. On, the star walks its track over a few seconds; off, the whole path is drawn at once so you can compare shapes across masses.