Answer:
Gravity causes an increase in pressure and temperature in the nebula.
Gravity causes the gas and dust in the nebula to contract.
Gravity makes a red giant shrink into a ball of hot gas and dust.
Step-by-step explanation:
A star is a large sphere of gas which is held together by the gravity of its own. The life of a star is a constant struggle of it against its own force of gravity which tries to collapse the stars. The gravity of the star causes the dusts and the gases to contract in the nebula. The gravity always pulls the star or the gases inwards. It causes increase in pressure and temperature in the nebula. It makes the red giant shrink and form a hot ball of gases and dusts.