A cloud of gas and dust that is contracting with an amount of mass great enough to form a star is called a protostar. A protostar is the first stage of a stars' life. A protostar takes near 100,000 years to reach the main sequence part of its life process. Which seems like a huge amount of time to us, but for a star it isn't. Adding atoms to the center of a protostar is a process astronomers call accretion. Because numerous reactions occur within the mass of forming star material, a protostar is extremely unstable. In order to achieve life as a star, the protostar will need to get and maintain equilibrium. If you don't know what equilibrium is, it is a balance, in this case a balance between gravity pulling atoms toward the center of the protostar and gas pressure pushing heat and light away from the center of the protostar. Achieving and keeping this balance is as hard thing to do. When a star can no longer maintain the balance known as equilibrium, it dies. If something were to disturb a protostar it would disrupt its equilibrium, and that would cause future problems for the star. An example of one of these disturbances would be a shockwave from a supernova. If a critical temperature in the core of a protostar is reached, then nuclear fusion begins. We identify the birth of a star as the moment that it begins fusing hydrogen in the core into helium.