Stars do not have constant light output throughout their lifetimes. While the light from a star may appear relatively constant to us over human timescales, stars undergo various processes and changes throughout their existence, leading to fluctuations in their luminosity and eventual death.
Stars generate energy through nuclear fusion in their cores, where hydrogen atoms combine to form helium, releasing a tremendous amount of energy in the process. This fusion process provides the primary source of the star's light and heat.
However, the balance between the inward force of gravity and the outward force generated by fusion reactions determines the stability of a star. As a star exhausts its hydrogen fuel, its core contracts under gravity, causing the temperature and pressure to rise. This increase in pressure enables the fusion of helium into heavier elements like carbon and oxygen.
Different stages of a star's life, such as the main sequence, red giant, and supergiant phases, can result in variations in luminosity. For example, during the red giant phase, a star's outer layers expand significantly, increasing its surface area and causing it to emit more light. In contrast, when a massive star nears the end of its life, it can undergo a supernova explosion, where its luminosity dramatically increases for a brief period before eventually fading away.
During a supernova, the star undergoes a catastrophic collapse and explosion, releasing an immense amount of energy. This explosion can briefly outshine an entire galaxy and result in the creation of heavy elements. After the supernova, the remnant can take different forms, such as a neutron star or a black hole, depending on its mass.
So, stars do not have a constant light output over their lifetimes. Their luminosity can change due to internal processes, and they can experience explosive events like supernovae that mark the end of their lives.