English: Isochrones and evolutionary tracks for pre-main-sequence stars. The curves labeled in years are isochrones at the given ages. In other words, stars 1 x 10⁵ years old lie along the curve labeled 1 x 10⁵, and similarly for the other 3 isochrones. The darker curves, cutting across the isochrones, are evolutionary tracks. At the leftmost side, the tracks are labeled with the mass of the star (in solar masses) that they represent. A star of that mass follows its evolutionary track from 1 x 10⁵ years, shortly after its birth, to 1 x 10⁸ years, when it is on the main sequence.