These clouds are called nebulae . A typical nebula is many light-years across and contains enough mass to make several thousand stars the size of our sun. These heavier atoms are remnants of older stars, which have exploded in an event we call a supernova . The source of the organic molecules is still a mystery. Irregularities in the density of the gas causes a net gravitational force that pulls the gas molecules closer together.
Some astronomers think that a gravitational or magnetic disturbance causes the nebula to collapse .
As the gases collect, they lose potential energy, which results in an increase in temperature. As the collapse continues , the temperature increases. The collapsing cloud separates into many smaller clouds, each of which may eventually become a star. The core of the cloud collapses faster than the outer parts, and the cloud begins to rotate faster and faster to conserve angular momentum . When the core reaches a temperature of about 2,000 degrees Kelvin, the molecules of hydrogen gas break apart into hydrogen atoms.