What outward force must balance the inward pull of gravity to maintain a gas cloud's stability?
Answer
Internal pressure or external radiation pressure
The continued existence of a gas cloud against the universal tendency toward dispersal is dependent upon a delicate, dynamic equilibrium between opposing forces. Gravity exerts a persistent, inward pull, attempting to concentrate all the mass into a collapsed state. Resisting this compression is the outward pressure generated by the continuous motion of the gas particles themselves (internal thermal pressure). In some cases, external radiation pressure, perhaps from nearby hot stars, can also contribute to this outward support. Only when these outward pressures are nearly equal to the inward gravitational pull can the cloud maintain a stable configuration temporarily.
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