Central density of an isothermal self-gravitating molecular cylinder (black) and sphere (grey), for fixed values of the ambient pressure. p_ext/k = 2    ×    10⁴   K   cm^-3 and cloud temperature T = 10   K. The dotted line is the value achieved by a critical stable sphere. This same central density (or overpressure) is reached in a cylinder with f_cyl = 0.733 (filled black circle).