 As the cloud collapses , individual <unk> of dense dust and gas form " Bok <unk> " . As a <unk> collapses and the density increases , the gravitational energy converts into heat and the temperature rises . When the <unk> cloud has approximately reached the stable condition of hydrostatic equilibrium , a <unk> forms at the core . These pre – main sequence stars are often surrounded by a <unk> disk and powered mainly by the conversion of gravitational energy . The period of gravitational contraction lasts about 10 to 15 million years . 
