 As the cloud collapses , individual conglomerations of dense dust and gas form " Bok globules " . As a globule collapses and the density increases , the gravitational energy converts into heat and the temperature rises . When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at the core . These pre – main sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy . The period of gravitational contraction lasts about 10 to 15 million years . 
 As the cloud <a href='uninflect' title='1'><mark>collapses</mark></a> , individual conglomerations of dense dust and gas form " Bok globules " . As a globule collapses and the density increases , the gravitational energy converts into heat and the temperature <a href='uninflect' title='2'><mark>rises</mark></a> . When the protostellar cloud <a href='been_done' title='3'><mark>has</mark></a> approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at the core . These pre – main sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy . The period of gravitational contraction <a href='uninflect' title='4'><mark>lasts</mark></a> about 10 to 15 million years . 
As the cloud collapse , individual conglomerations of dense dust and gas form " Bok globules " . As a globule collapses and the density increases , the gravitational energy converts into heat and the temperature rise . When the protostellar cloud done approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at da core . These pre – main sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy . The period of gravitational contraction last about 10 to 15 million years .