Plot every star by its colour and its brightness and they do not scatter — they fall into a great diagonal river, the main sequence, with red giants marooned above and white dwarfs sunk below. A star's place on this chart is its whole life story. Drop one in by mass and read its fate.
A real main-sequence relation: luminosity L ≈ (M/M☉)³·⁵ L☉ and temperature rising with mass, so heavier stars sit up-and-left (hot, bright, blue) and lighter ones down-and-right (cool, dim, red). Spectral class (O B A F G K M) is read from the temperature, and lifetime ∝ M/L ∝ M⁻²·⁵ — massive stars blaze and die fast. Every placed star lands on the drawn main sequence at its computed (T, L); the axes and the M–L–T relations are the real astrophysics.
The diagram shows the main sequence only (the H-burning phase); the giant and white-dwarf regions are sketched as context, not populated by evolving your star. The M–L exponent (~3.5) is an average that varies along the sequence, and 'fate' is the textbook endpoint by mass — a compact summary, not a stellar-evolution simulation.