IT is several years since the first discovery of stellar clusters which appeared to be so young that a number of stars in them were still in the process of contracting onto the main sequence1–4. The first theoretical model covering the evolution of stars, on the assumption that their self gravitational energy is the only energy source available, was produced by Henyey, LeLevier and Levee5. The results of these computations have been used to produce a theoretical Hertzsprung–Russell diagram for young stellar clusters6,7. In this theory, the evolution of the stars is governed by the equations and where L, R, T and M denote the luminosity, radius, effective temperature and mass, respectively, all in solar units, and in which τ denotes the age in units of the Helmholtz–Kelvin contraction time, α and β are constants with approximate values given by Huang6 as α = 5.4 and β=0.79.