Molecular Clouds

Molecular clouds are regions of gas and dust from which stars and planets form. They are generally colder regions, so are darker than the surrounding areas. They can only be seen when they are in front of a brighter region and appear as "holes" in space.

A good example of a molecular cloud is the Horsehead Nebula in Orion.

Credit and Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

These clouds are also called dark nebula because they don't emit radiation in the visible spectrum. These are the regions of space where stars and planets form. The cloud will start to collapse because of an outside force, like the wavefront of a supernova. The collapse will also create rotation on the cloud. From the conservation of angular momentum, as the cloud collapses, the rotation also speeds up. At the core of the cloud, the density increases and the temperature increases. This is where the protostar is formed. The outer sections of the cloud are where planets, dwarf planets, asteroids, comets, and other bodies in a planetary system will form.