Predicted critical mass (in Earth masses, logarithmic scale) as a function of the critical mass obtained by solving the internal structure equations (blue), and using the IL04 formula (red; see Eq. (2), with exponents of the formulas equal to 0.25). Left: using the ISM opacity from BL94. The lower end of each line corresponds to the result obtained for an opacity of 1 cm² g⁻¹, the higher end to the result for an opacity of 10 cm² g⁻¹. Right: reducing the BL94 opacity by a factor 100. The lower end of each line corresponds to the result obtained for an opacity of 0.01 cm² g⁻¹, the higher end to the result for an opacity of 0.1 cm² g⁻¹.