What causes nebula to begin to contract?

Nebulae begin to contract primarily due to gravitational forces. A disturbance, such as shock waves from nearby supernovae or collisions with other gas clouds, can trigger this contraction. As the gas and dust within the nebula clump together, gravitational attraction increases, leading to further collapse and the eventual formation of stars and planetary systems. Additionally, the cooling of the gas can enhance the process by allowing particles to come closer together.