Stars undergo various stages of evolution throughout their lifetime, and while their light output may appear relatively constant over certain timescales, it is not truly constant. The process by which stars eventually die and, in some cases, go nova or supernova is a result of complex interactions and transformations within the star.
Stars generate energy through nuclear fusion, specifically in their cores where hydrogen is converted into helium. This fusion process releases an immense amount of energy in the form of light and heat, which radiates outwards from the star. This energy sustains the star's equilibrium against the inward force of gravity.
However, as a star consumes its hydrogen fuel, it begins to change. In the case of larger stars, when the hydrogen in the core is depleted, the core contracts and heats up, causing the outer layers of the star to expand. This expansion leads to an increase in the star's overall luminosity, and it enters a new phase known as a red giant or supergiant.
During this phase, the star may undergo further fusion reactions involving heavier elements, such as helium fusing into carbon and oxygen. Eventually, depending on the star's mass, it may continue fusing heavier elements, such as carbon into neon, oxygen into silicon, and so on. Each fusion reaction releases energy, but it also generates a progressively denser core.
When a massive star reaches the fusion of iron in its core, it can no longer sustain fusion reactions that release energy. Iron fusion actually absorbs energy, leading to a buildup of an inert iron core. This core becomes unstable, and the star's outer layers, now extremely massive, collapse inward due to gravity. The sudden collapse results in an intense explosion known as a supernova.
During a supernova, the star releases an enormous amount of energy in the form of light, heat, and various subatomic particles. This explosive event can outshine an entire galaxy for a brief period. The remnants of the star may form a neutron star or, in the case of extremely massive stars, collapse further to form a black hole.
In summary, while a star's light output may appear constant over certain timescales, it is not truly constant. The evolution of a star involves a sequence of internal changes, fusion reactions, and eventual instability that can lead to explosive events like supernovae.