What causes the contraction of a gas cloud leading to star formation and planetary formation?

• A. Rotation
• B. Gravitational forces
• C. Fault
• D. Ocean Basin

Answer: (B)

Gravity is the force that pulls matter inward. In a diffuse molecular cloud, gas pressure pushes outward, but if the cloud’s mass is large enough, gravity overwhelms this pressure and the cloud begins to collapse. This inward pull increases density and temperature until a protostar forms at the center. As material falls in, angular momentum causes the remaining gas to spin, flattening into a rotating disk around the young star. That protoplanetary disk is where dust and gas clump to form planetesimals and eventually planets. So the contraction and subsequent star and planet formation are driven by gravitational forces. Rotation shapes the disk and the orbits of forming planets, but it isn’t the initial cause of the collapse. The other options don’t apply to this astrophysical process.