2 conditions to form a star - all atoms in the gas cloud must be gravitationally bound to each other (so the virial theorem applies) - the cloud must permanently lose overall energy E=-GMm/2R therefore R=-GMm/2E, so for R to become smaller and the energy to also become smaller the energy needs to be negative. Besides this reason, why would the energy be increasingly negative? Is the energy of the cloud negative to begin with? Also, Unstable gas clouds do not become star as they cannot continually lose energy, does anyone know why this is? Why can't unstable gas clouds continually lose energy what makes them different to stable gas clouds? Cooling time determines wether a cloud goes into free fall without pressure support or if it will heat the nebula adiabatically (which is where no heat enters or leaves the system). Does anyone know the criteria which distinguishes the two? Wether it'll be one or the other? Sorry for all the questions. Thanks in advance.