Silicate mass fraction (color) as a function of interior layers (y axis) and time (x axis). Silicate mass fraction ranges between zero (gas only) in blue and pure silicate in brown. The gradual distribution of silicate from formation converges into a core-envelope structure after about 4.25 Gyr. The solid lines signify Z = 0.98 and Z = 0.02 enrichment levels. Versions of this figure for radius and pressure layers instead of mass are presented in Appendix B.