Evolution of Stars

The evolution of a star refers to the changes in its properties over time, leading it through various stages of life. Stars evolve very slowly for most of their lifetime. Stars continuously emit energy, which causes their energy reserves to decrease over time. To remain stable, the temperature inside the star must stay constant, requiring continuous energy generation. 

Evolution of stars based on their mass and their end stages

Energy is generated by the burning of fuel (nuclear fusion) at the centre of the star.

• Hydrogen fusion: Hydrogen nuclei combine to form helium, releasing energy. When hydrogen is depleted, helium fusion starts.
• The number of fuels a star can burn depends on its mass. Larger stars can burn more types of fuels, such as carbon and oxygen.

Once the fuel in the centre of the star is exhausted:

• Energy generation stops, and the temperature begins to decrease.
• Gas pressure decreases, disrupting the balance between gravitational force (pulling inward) and gas pressure (pushing outward).
• Gravity becomes dominant, causing the star to contract.

As the star contracts, the temperature rises again, igniting another fuel (if available). As stars evolve:

• They may contract (shrink due to gravity) or expand (due to energy from fusion).
• Different processes occur depending on the type of fuel being burnt and the star's mass.

When all possible fuels are exhausted:

• Energy generation permanently stops.
• The star cools down, and its temperature decreases over time.

The end stage of a star depends on its mass, and stars follow different paths based on their size and fuel-burning capability:

1. Low-Mass Stars (Like the Sun): After all fuels are used, the star contracts and becomes a white dwarf. In a white dwarf, new types of pressures (called electron degeneracy pressure) prevent further collapse. The star remains stable as a white dwarf, slowly cooling over billions of years.
2. Medium-Mass Stars: Medium-mass stars can go through more intense fusion stages and may end their lives as neutron stars. Neutron stars are extremely dense remnants where gravitational collapse is halted by neutron degeneracy pressure.
3. High-Mass Stars: High-mass stars burn through their fuel very quickly and may end in a spectacular supernova explosion.

After the explosion, they either:

• Collapse into a neutron star.
• Form a black hole if the mass is extremely large, where gravity becomes so strong that nothing, not even light, can escape.

For stars like white dwarfs and neutron stars:

• Even after energy generation stops completely, new types of pressures keep the star stable.
• These pressures are not dependent on temperature, allowing the star to maintain stability forever.

For black holes:

• There is no stable end stage, as they continue to collapse under their own gravitational pull.