Extreme (1, 2, or 3) in the center of nebulae causes hydrogen atoms to shed their electrons and collide with one another. the (4, 5, or 6) of the collisions cause the smaller, bare nuclei to fuse into large helium nuclei. 1. expansion 2. heat 3. density 4. size and luminosity 5. force and speed 6. mass and temperature

2. heat

5. force and speed

Explanation:

Nuclear fusion in stars causes the hydrogen atoms to collide with each other and form the heavier helium atom. The fusion reaction is therefore:

₁H²  +  ₁H² →  ₂He⁴

Extreme heat in the center of nebulae causes hydrogen atoms to shed their electrons and collide with one another. The force and speed of the collisions cause the smaller, bare nuclei to fuse into large helium nuclei.