Final answer:
The gravitational force between two objects like a star and a planet increases when they are brought closer together and decreases as they move farther apart. The force varies inversely with the square of the distance between them. Additionally, a shrinking star will exert a stronger gravitational pull on its surface, while an expanding star will have a weaker surface gravity.
Step-by-step explanation:
The force of gravity between a star and a planet, or any two objects, is governed by Newton's law of universal gravitation. This law states that the force of gravity decreases with the inverse square of the distance between the two objects. If two objects, like a star and a planet, are brought closer together, the gravitational force between them will increase. Conversely, as the distance increases, for instance if a planet is moved farther from a star, the gravitational force weakens significantly. For example, if the distance between the objects is doubled, the gravitational force becomes one-fourth as strong; if the distance is tripled, the force drops to one-ninth.
Furthermore, as a star shrinks, an object on its surface gets closer to the star's center of mass, thereby experiencing a stronger gravitational pull. Conversely, if the star expands, the surface moves away from the center and the gravitational force at the surface decreases. This implies that for red giant stars, which have expanded significantly, the gravity on their surfaces is less than what it was when they were smaller.