Stellar evolution tracks (blue lines) for the pre-main-sequence. The nearly-horizontal curves are called Henyey tracks.
High-mass stars have nearly horizontal evolution tracks from their birth until they arrive on the main sequence. Lighter stars, first follow the nearly-vertical Hayashi track
before bending left into the Henyey track.
The end (leftmost point) of every track is labeled with the star's mass in solar masses, and represents its position on the main sequence. The red curves labeled in years are isochrones at the given ages. In other words, stars
years old lie along the curve labeled
, and similarly for the other 3 isochrones.