Giant molecular clouds are dense opaque clumps of very cold gas and dust usually formed through the merging of smaller gas clouds and gravitational instability. A giant molecular cloud is a huge combination of dust left over from the formation of galaxies and interstellar gas.
Giant molecular clouds contain smaller, denser clumps of interstellar material. Cloud cores are at the center of many of these clumps. Scientists still do not fully understand what forms cloud cores, however, it is thought that turbulent motion of the gas in molecular clouds compresses some regions which become cloud cores. Gas ejected from another star in the cloud compresses the part of the molecular cloud that surrounds the star is another possibility.