Which condition in a nebula would prevent nuclear fusion

Decreasing concentration of hydrogen in nebula will prevent nuclear fusion.

Explanation:

Large clouds of dust and gases are called nebulae.The gas that comprises most part of nebula is hydrogen.Hydrogen and dust particles are pulled together due to gravity and the mass starts rotating.When hydrogen atoms collide with each other, they produce heat this process continues and the core of the cloud gets tremendously heated resulting in nuclear fusion.Nuclear fusion is the phenomenon in which two lighter nuclei fuse to form a heavier nuclei. In a nebula , two hydrogen nuclei fuse to form helium.Thus if the concentration of hydrogen is decreased in nebula then nuclear fusion will stop.