Star Formation Regions
It is in these large molecular clouds that star birth can occur. This process is generally thought to begin with the gravitational collapse of the interstellar clouds and dust. This can occur as in the core of the giant molecular clouds the density is high. However as the mass for instability is significantly greater than the typical stellar mass for typical cloud densities then the magnetic fields will oppose this collapse. This observation suggests that a cloud collapse is probably triggered, by something like a supernova shockwave.
We can observe star formation as stars that are in the "free fall" gravitational collapse phase emit strongly in the infrared. This collapse continues until the temperature at the core has reached 107K at which stage hydrogen burning begins and the star begins it's main sequence lifetime. The stars formed by this process will generally occur in cluster and are strong ultraviolet emitters. This will ionize the surrounding gas clouds and so as they combine hydrogen spectral lines will be observed.