1. What is the source of energy that enables a mainsequence star to shine? a. Friction between its atoms, b. Fusion of hydrogen in a shell that surrounds the core, c. Fusion of helium in its core, d....

1. What is the source of energy that enables a mainsequence star to shine? a. Friction between its atoms, b. Fusion of hydrogen in a shell that surrounds the core, c. Fusion of helium in its core, d. Fusion of hydrogen in its core e. Burning of gases on its surface

2. What are giant molecular clouds, and what role do they play in star formation?