Question

What do you understand by the term "evolution of a star"?

Answer 1

Evolution of a star means change in its properties with time resulting in its passing through different stages. Stars evolve very slowly for most of their lifetime. As stars are continuously emitting energy, their energy is constantly decreasing.

• For their stability to remain intact i.e. for maintaining a balance between the gas pressure and the gravitational force, it is necessary that the temperature remains constant.
• For the temperature to remain constant, energy must be generated inside the star. This generation of energy occurs because of burning of fuel at the centre of the star. The reason for the evolution of stars is the burning of fuel and therefore, the decrease in the amount of fuel in their centre.
• When the fuel in the centre finishes, the energy generation stops. As a result, the temperature of the star starts decreasing. Due to the decrease in temperature, the gas pressure decreases and the balance between gas pressure and gravitational force cannot be maintained. As the gravitational force is now higher than the gas pressure, the star starts contracting.
• When hydrogen at the centre is finished, helium starts undergoing fusion and energy generation starts again. This cause increase in gas pressure and the star expands.

Hence, the burning of the fuel at the centre of the star and a gradual decrease in its amount is the main reason for the evolution of a star.