What causes a nebula of gas and dust to condense?

• A. Magnetic fields do not cause condensation.
• B. Gravitational forces nearby supernovae cause the nebula to condense and form stars.
• C. Nuclear fusion begins in the nebula.
• D. The Sun's radiation condenses the nebula.

Answer: (B)

Gravity is the main force that lets a nebula condense. The gas and dust need gravity to pull it inward enough to overcome internal pressure and form denser clumps. A nearby supernova can provide the trigger by sending a powerful shock wave into the cloud, compressing it and raising the density. That compression helps gravity take over, causing the cloud to collapse and fragment into protostars.

Nuclear fusion cannot start in a nebula itself; fusion only begins in a collapsing core once temperatures and pressures become extreme enough. The Sun’s radiation tends to heat and push gas away rather than condense it. Magnetic fields can influence the collapse, sometimes delaying it or guiding flows, but they don’t by themselves cause the nebula to condense. The key idea is that external compression sets the stage, and gravity drives the subsequent collapse into stars.