1. Gravity: This is the most important force, acting as the "glue" that holds the star together. Gravity pulls all the matter in the star towards its center, creating immense pressure.

2. Nuclear Fusion: This is the energy source of a star. In the core, under immense pressure and heat, hydrogen atoms fuse together to form helium, releasing a tremendous amount of energy in the process. This energy provides the outward pressure that counteracts gravity.

3. Radiation Pressure: The energy released by nuclear fusion creates radiation, which exerts an outward pressure on the star's layers.

4. Gas Pressure: The hot, dense gas inside the star also exerts an outward pressure.

5. Magnetic Fields: Stars have magnetic fields that can play a role in shaping their behavior, especially during their later stages of evolution.

Balance: A star is in a delicate balance between these forces. Gravity wants to collapse the star inward, while nuclear fusion and the other forces create outward pressure. This balance determines the star's size, temperature, and lifetime.

Here's a simple analogy: Imagine a balloon. The air inside the balloon is like the pressure created by nuclear fusion and gas pressure. The balloon's rubber is like the force of gravity, trying to pull the balloon inwards. The balance between these forces determines the shape and size of the balloon.

It's worth noting that these forces are constantly interacting and changing throughout a star's lifetime. As a star ages, its fuel supply changes, impacting the balance of forces and ultimately determining its fate.