The energy required to begin nuclear fusion originally came from a. the gravitational potential energy of the protostar. b. the kinetic energy of the protostar. c. the wind from nearby stars. d. the pressure from the interstellar medium.

Gravitational potential energy is a form of energy related to the position of objects within a gravitational field. For a protostar, this energy is incredibly important in the early stages of development. As a cloud of gas and dust collapses under its own gravity, this gravitational potential energy is converted into other forms of energy.
Imagine a ball at the top of a hill. When it rolls down, its potential energy converts into kinetic energy. Similarly, in a protostar, the gravitational potential energy converts largely into thermal energy, which is crucial for the next stages in a star’s life.

As a protostar forms, it undergoes a contraction process where it shrinks in size due to gravity pulling its material inward. This contraction causes the core of the protostar to heat up.
The shrinking process is what leads to the conversion of gravitational potential energy into thermal energy. As the protostar contracts, it gets denser and its temperature increases.
Imagine blowing air into a balloon and then letting it go. The contracting material behaves like the air rushing back in, creating pressure and heat. This heat is critical for making nuclear fusion possible in the core of the protostar.

Thermal energy conversion is the process where one type of energy is turned into heat. In protostars, the gravitational potential energy gets converted into thermal energy because of the core's increased pressure and density.
This thermal energy heats the protostar's core to the incredibly high temperatures needed for nuclear fusion. Think of it like rubbing your hands together on a cold day. The motion converts into heat, warming your hands. Similarly, the collapsing mass of a protostar essentially 'rubs' itself into generating significant heat through compression and contraction.
It’s that generated thermal energy which creates the right conditions for nuclear fusion to kickstart.

Nuclear fusion initiates when the core temperature of a protostar reaches a critical threshold where atomic nuclei can overcome their electrostatic repulsion and merge. This merging releases a tremendous amount of energy.
To achieve this, the core temperature must rise to millions of degrees, facilitated by converting gravitational potential energy into thermal energy. Once hot enough, hydrogen atoms in the protostar’s core fuse to form helium, unleashing energy that powers the star for most of its life.
Imagine lighting a match—once the match head reaches the ignition temperature, it burns brightly. Similarly, once the protostar's core temperature is high enough, nuclear fusion ignites, powering the star like a cosmic engine.