The process described is accretion, which is fundamental to the formation of celestial bodies. The force responsible is gravity. According to Newton's Law of Universal Gravitation, the force of attraction between two objects is directly proportional to the product of their masses. As small particles join together, their combined mass increases. This larger mass exerts a stronger gravitational pull on surrounding particles, causing them to be attracted and join the growing body. This cycle continues, allowing the object to accumulate more and more mass, eventually forming planets, stars, or other large celestial objects.

B. Electrical: This force depends on the electric charge of particles, not primarily their mass. While electrostatic forces can contribute to the initial clumping of tiny dust grains, gravity is the dominant force for large-scale accretion.

C. Frictional: Friction is a contact force that opposes motion between surfaces. It is not a long-range attractive force that pulls particles together in the vacuum of space.

D. Magnetic: This force acts on moving charges and magnetic materials. While magnetic fields play a role in stellar and planetary system dynamics, gravity is the primary force driving accretion based on mass.

1. OpenStax. (2016). Astronomy. Rice University. Chapter 14.2, "The Solar Nebula Hypothesis," Section: "Formation of the Protoplanets." The text states, "The largest protoplanets grew by gravitational attraction and were able to capture the lighter gases as well..."

2. Massachusetts Institute of Technology. (1999). 8.01 Physics I: Classical Mechanics, Fall 1999. MIT OpenCourseWare. Lecture 11: "Universal Law of Gravitation." This lecture explains that the gravitational force is proportional to the product of the masses (F ∝ m₁m₂), which is the principle described in the question.

3. Fraknoi, A., Morrison, D., & Wolff, S. C. (2022). Astronomy 2e. OpenStax. Chapter 7.1, "The Universal Law of Gravitation," Section: "Newton's Law of Universal Gravitation." This section details the equation F = G(M₁M₂/R²), explicitly showing the direct relationship between mass and gravitational force.