Stellar evolution well explains how myriad stars in the celestial sky form, mature, and die. It enables us to date numerous types of stellar objects scattered across the Galaxy, ordering each of them into a consistent temporal sequence along the arrow of time. And it shows how all those many varied objects—from galactic clouds and protostars to red giants and white dwarfs, and including nebulae, pulsars, and supernovae—fit into an overall framework of understanding based on the unifying concept of change. These varying interrelationships among the many diverse components of stellar and interstellar matter in our Milky Way comprise nothing less than a “galactic ecosystem,” an evolutionary posture nearly as intricate and delicate as life in a tidal pool or a tropical forest.
Without a theory of stellar evolution, we would seemingly witness a huge and confusing zoo of unrelated objects strewn throughout space. Astronomers would resemble stamp collectors, with lots of data but not much insight into how one cosmic object relates to any other. With stellar evolution, we enjoy a more powerful intellectual stance. We can place each type of cosmic object into a comprehensive evolutionary perspective, thereby deciphering both the bigger picture of all the stars in toto as well as their detailed relationships among odd and varied kinds of stars. Indeed, we now know how to follow indirectly the evolutionary tracks of stars, as one type of celestial object changes into another.
In contrast to our poor grasp of galactic evolution, astronomers know much about stellar evolution. The way stars change, especially elemental evolution whose theory matches the observed cosmic abundances remarkably well, is one of the best developed and well understood aspects of cosmic evolution. The subject of stellar evolution can naturally account for the observed differences in element abundance between the old, globular-cluster stars that are perhaps as ancient as the Milky Way itself, and the young, open-cluster stars that more recently formed in our Galaxy. The older stars have few heavy elements, the youngest ones the most heavies; a definite trend tends toward a buildup of greater complexity over billions of years. All things considered, our knowledge of stellar aging and elemental creation is surprisingly robust, given that the stars are so far and foreign.
Though some of the evolutionary events described here in the STELLAR EPOCH are cyclical, they actually lead slowly and surely, over generations of star cycles, toward increased energy flows and ordered material structures. Operating at countless localized sites within the vastly larger galaxies, the ongoing process of star birth, maturity, and death constantly enrich and fertilize interstellar space with heavy elements that sow the seeds for later-generation stars—as well as planets. And much as elsewhere in our story, with time comes change, and with change rising complexity.
On and on, stellar cycles do churn. Build up, break down, change. Dust to dust, and to dust some more—in this case stardust engaged in a kind of cosmic reincarnation. From the ashes of dead stars come the origins of new ones. Indeed, without the heavy elements made inside stars, both life on Earth and Earth itself would not exist. That we are made of stardust sounds poetic, but its validity is one of the greatest success stories in all of science.
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