What is the primary source of energy for stars, including the sun?

The primary source of energy for stars, including the Sun, is nuclear fusion. This process occurs in the core of a star, where temperatures and pressures are incredibly high, allowing hydrogen nuclei to overcome their repulsive forces and collide to form helium. During this fusion process, a tremendous amount of energy is released in the form of light and heat, which is what we perceive as the star's brightness and heat.

This energy generation is essential for maintaining a star's stability against gravitational collapse. The balance between the energy produced from nuclear fusion and the gravitational forces pulling inward allows a star to remain in a steady state for billions of years. In the Sun's case, approximately 4.6 billion years into its lifecycle, it primarily fuses hydrogen into helium, which continues to sustain it.

Other processes such as nuclear fission, which involves splitting heavy atomic nuclei to release energy, are not the main energy source for stars. Solar wind refers to the stream of charged particles ejected from the Sun's atmosphere, while gravitational contraction—though a phase of star formation—does not serve as the ongoing source of energy during the star's main sequence phase like fusion does. Thus, the correct answer reflects the fundamental process that powers stars throughout their lifecycle.