Final answer:
The end of a star's main-sequence life is marked by the end of hydrogen fusion in the core, resulting in a carbon-oxygen core and the star transitioning to later stages of stellar evolution such as becoming a white dwarf.
Step-by-step explanation:
The event that marks the end of every star's main-sequence life is the cessation of hydrogen fusion in the core, leading to the formation of a carbon-oxygen core. This signifies that the star has exhausted its nuclear fuel and is on the brink of death. For a star like the Sun, this transition point is characterized by a shift from the stable main-sequence stage of stellar evolution to the red giant phase or other complex stages, depending on the initial mass of the star. This change occurs rapidly, especially in more massive stars, as their fuel consumption increases while their fuel supply dwindles.
Ultimately, stars leave the main sequence once fusion reactions cease, often turning into white dwarfs where nuclear fusion has completely stopped. This main-sequence turnoff point can be seen in H-R diagrams of star clusters, where stars begin to move toward the red giant region, indicating their advanced age and the end of their time on the main sequence.