Final answer:
The Sun remains in equilibrium due to the balance between gravitational forces attempting to collapse it and the thermal pressure from the high temperatures at its core necessary for nuclear fusion.
Step-by-step explanation:
The force that holds the gas of the Sun together is the mutual gravitational attraction among its various regions, which generates forces that tend to make the Sun collapse toward its center. However, this gravitational force is counterbalanced by thermal pressure of the gases within the Sun. To prevent gravitational collapse, the temperature at the Sun's center must be approximately 15 million Kelvin, allowing for the fusion of hydrogen into helium, which produces the outward pressure necessary to resist gravitational pull.
The Sun is constituted mostly of plasma, an ionized gas where atoms have lost electrons due to extremely high temperatures, making the environment electrically charged. The rapid motion and collisions of these particles create gas pressure. This plasma state facilitates the necessary conditions for nuclear fusion, which not only contributes to the outward pressure balancing gravity but is also the process by which the Sun has maintained its energy output over billions of years.
In summary, the Sun achieves equilibrium through the continuous nuclear fusion at its core, where hydrogen atoms collide and fuse, generating the energy needed for the Sun to resist gravitational collapse and maintain its structure.