We have re-examined the classical problem of the macroscopic equation of state for a hydrostatic isothermal self-gravitating gas cloud bounded by an external medium at constant pressure .
We have obtained analytical conditions for its equilibrium and stability without imposing any specific shape and symmetry to the cloud density distribution .
The equilibrium condition can be stated in the form of an upper limit to the cloud mass ; this is found to be inversely proportional to the power 3 / 2 of a form factor \mu characterizing the shape of the cloud .
In this respect , the spherical solution , associated with the maximum value of the form factor , \mu = 1 , turns out to correspond to the shape that is most difficult to realize .
Surprisingly , the condition that defines the onset of the Bonnor instability ( or gravothermal catastrophe ) can be cast in the form of an upper limit to the density contrast within the cloud that is independent of the cloud shape .
We have then carried out a similar analysis in the two-dimensional case of infinite cylinders , without assuming axisymmetry .
The results obtained in this paper generalize well-known results available for spherical or axisymmetric cylindrical isothermal clouds that have had wide astrophysical applications , especially in the study of the interstellar medium .