The theory of stellar nucleosynthesis can naturally account for the observed differences in heavy-element abundance between the old globular cluster stars and stars now forming in our galaxy. Even though an evolved star continuously creates new heavy elements in its interior, the composition changes are largely confined to the core, and the star's spectrum gives little indication of events within. Convection may carry some reaction products (such as the technetium observed in many red giants) from the core into the envelope but, the outer layers largely retain the composition with which the star originally formed. Only at the end of the star's life are its newly created elements released and scattered into space.

Thus, the spectra of the youngest stars show the most heavy elements, because each new generation of stars increases the concentration of these elements in the interstellar clouds from which the next generation forms. The photosphere of a recently formed star contains a much greater abundance of heavy elements than that of a star that formed long ago. Knowledge of stellar evolution allows astronomers to estimate the ages of stars from spectroscopic studies, even when the stars are isolated and not members of any cluster. (Sec. 20.5)

In the past three chapters we have seen all the ingredients that make up the complete cycle of star formation and evolution in our Galaxy. Let us now briefly summarize that process, which is illustrated in Figure 21.18.

1. Stars form when part of an interstellar cloud is compressed beyond the point at which it can support itself against its own gravity. The cloud collapses and fragments, forming a cluster of stars. The hottest stars heat and ionize the surrounding gas, sending shock waves through the surrounding cloud, possibly triggering new rounds of star formation.
2. Within the cluster, stars evolve. The most massive stars evolve fastest, creating heavy elements in their cores and spewing them forth into the interstellar medium in supernova explosions. The lighter stars take longer to evolve, but they too can create heavy elements and may contribute to the "seeding" of interstellar space when they shed their envelopes as planetary nebulae.
3. The creation and dispersal of new heavy elements are accompanied by further shock waves. Their passage simultaneously enriches the interstellar medium and compresses it into further star formation.

In this way, although some material is used up in each cycle—turned into energy or locked up in low-mass stars—the Galaxy continuously recycles its matter. Each new round of formation creates stars with more heavy elements than the preceding generation had. From the old, metal-poor globular clusters to the young, metal-rich open clusters, we observe this enrichment process in action. Our Sun is the product of many such cycles. We ourselves are another. Without the heavy elements synthesized in the hearts of stars, life on Earth would not exist.

Figure 21.18 The cycle of star formation and evolution continuously replenishes the Galaxy with new heavy elements and provides the driving force for the creation of new generations of stars.