What happens to old stars after they get old?

Answer 1

Stars like our Sun live about 10 billion years.

Eventually the star expands to become a red giant star. That's the "old star" stage.

Finally the star can no longer produce energy by nuclear fusion. Gravity takes over and collapses the star into a white dwarf star, not much larger than the Earth.

Much more massive stars (with at least 8 times the Sun's mass) become supergiant stars when they get old. Finally all their fuel is used and the core can no longer stand up against gravity. It collapses then the star explodes in a supernova. The remnant is a neutron star or a black hole, depending on the mass of the original star.

All stars eventually reach a point where they cannot continue fusing nuclei to produce energy. Supernovas are what create elements heavier than iron, including all radioactive elements. Some extraordinarily large stars may undergo particular processes that either form a so-called quark star, or a pair-instability supernova that does not leave any residual stellar object.

Answer 2

Old stars go through various stages of evolution depending on their size. Small stars like our Sun will expand into red giants, shed their outer layers in a planetary nebula, and eventually cool down into a white dwarf. Larger stars can go supernova, where they explode and release a lot of energy into space, leaving behind a dense core like a neutron star or a black hole.