Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive stars, to trillions of years for the smallest Red Dwarfs, considerably more than the age of the Universe.
We cannot study stellar evolution by observing the life of a single star because most stellar changes occur far too slowly to be detected. Instead, we come to understand how stars evolve by observing numerous stars, each at a different point in their life cycle.
Only since astronomers can observe stars in all stages of evolution, in wavelengths from radio through to gamma rays are we beginning to understand the life cycles of stars. We now have fascinating views of stellar evolution, from just emerging proto stars, via hydrogen fusing main sequence stars, to the remnants of the final stage of the star’s life, such as white dwarfs and their planetary nebula and supernova remnants, neutron stars and stellar black holes.
Material that has gone through previous stellar life cycles has been enriched with heavier chemical elements beyond hydrogen and helium. It has formed gas and dust clouds in inter-stellar space that are ready to be recycled to form new stars. Our Solar System, including the stuff we humans are made of, is thought to have gone through two previous stellar evolution cycles, because of the abundance of heavier elements that we find on Earth and elsewhere in the Solar System.
So we are made of recycled stellar material, and indeed the Universe can be called a giant “recycle factory”.