The formation of a star begins with a collapse. Perhaps a supernova shockwave or the wind from a massive star triggers it off, but once it starts the collapse proceeds until something stops it. When a clump of material in the cloud begins to contract, the first stage of stellar life begins. As the particles in the clump get closer together the force of gravity between them increases. This in turn makes the star contract faster, pulling the particles closer together still, thus increasing gravity's pull further. A feedback loop is produced creating a self sustaining gravitational contraction.
At this stage the star is only a few tens of thousands of years old.
Most of the material that will eventually make up the star is still
quite far from the core and is very cool (10's of degrees Kelvin),
producing emission only at millimeter wavelengths. This phase is called
infrared Class 0.
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