Which of the following forces opposes gravity, keeping main sequence stars in equilibrium?

The force that opposes gravity and helps maintain the equilibrium of main sequence stars is known as thermal pressure. This pressure arises due to the nuclear fusion occurring in the core of stars, where hydrogen is converted into helium.

1. Nuclear Fusion: In the core of a main sequence star, the temperature and pressure are extremely high, allowing hydrogen nuclei (protons) to fuse together. This process releases a tremendous amount of energy in the form of thermal energy and radiation.

2. Thermal Pressure: The energy produced through nuclear fusion generates thermal pressure that exerts an outward force against the gravitational pull trying to collapse the star. This balance between gravitational force pulling inward and thermal pressure pushing outward is what defines stability in a main sequence star.

3. Hydrostatic Equilibrium: The condition where these two forces are in balance is referred to as hydrostatic equilibrium. It ensures that the star remains stable over long periods, allowing it to maintain its structure and luminosity.

In essence, while gravity tries to pull the star matter inward, thermal pressure from nuclear fusion keeps it from collapsing, allowing main sequence stars to shine steadily for billions of years.