Our Sun is considered a stable star because it is in a state of equilibrium between the forces that act upon it. The stability of a star is determined by a delicate balance between gravity, which tries to compress the star, and the pressure generated by nuclear fusion in its core, which pushes outward.
The main factor that contributes to the Sun's stability is its mass. The Sun is classified as a main-sequence star, and its mass falls within a range (about 0.08 to 150 times the mass of the Sun) where it can maintain a stable state for a long time. Stars that are significantly more massive or less massive than the Sun have different characteristics and may exhibit more unstable behavior.
In the core of the Sun, hydrogen nuclei undergo nuclear fusion, specifically a process called the proton-proton chain reaction. This fusion process releases an enormous amount of energy in the form of light and heat. The outward pressure generated by this energy production counteracts the inward force of gravity, maintaining a stable equilibrium.
The Sun's stable state is maintained by the delicate balance between the gravitational force, which tends to collapse the star, and the pressure created by the energy released through fusion, which opposes gravity and maintains the star's structure.
While the Sun experiences occasional fluctuations and activity such as solar flares or sunspots, these are relatively minor compared to the more dramatic behavior observed in some other types of stars, such as massive stars that can undergo violent supernova explosions or unstable variable stars that exhibit irregular changes in brightness.
It's important to note that stars come in a wide range of sizes, masses, and compositions, resulting in different lifetimes and behaviors. The stability or instability of a star depends on various factors, including its mass, composition, and stage of evolution.