To form a nebula, pulls matter together or causes an .

Question

First one. Gravity, heat, pressure.
Second choice an explosion, infrared light emission, pressure changes

Answer 1

First one: Gravity
Second choice: Explosion

Answer 2

Astronomers think that most galaxies are centered by a Select Answer , which exerts Select Answer that binds the galaxy together.
First one large planets, nebula, massive star, black hole.
Second choice Gravitational pull, centrifugal force, centripetal force, magnetic pull

Answer 3

First one: Black hole
Second choice: Gravitational pull