Final answer:
According to Newton's law of universal gravitation, doubling the mass of one star while keeping the distance the same will result in the gravitational force between the two stars increasing by a factor of two.
Step-by-step explanation:
When considering the gravitational force between two stars, and if we increase the mass of one star by a factor of two, according to Newton's law of universal gravitation, the gravitational force between them would also increase by a factor of two.
The gravitational force is directly proportional to the product of the two masses involved and inversely proportional to the square of the distance between them. Therefore, doubling the mass of one star while keeping the other factors constant would increase the gravitational force by a factor of two. The correct answer is b) Gravitational force increases by a factor of two.