Stars are glowing balls of gas, tenuous and hot on the outside, dense and hotter on the inside. Sized midway between the smallest and largest of all known objects, stars are bigger than atoms by roughly the same factor of a billion billion (1018) by which they’re dwarfed by galaxy clusters.
Except for their shape, stars do not resemble hard, rocky planets in any way whatever. Normal stars are immensely larger and tremendously hotter than planets, and they experience changes in a completely different manner. They have no real surface, let alone any hard, solid matter like that on Earth. Stars are simply composed of loose gas held intact by the relentless pull of their own gravity, which, at their cores, manages to compact that gas enough to trigger thermonuclear fusion.
This same gravity forces the gas to take on an austere geometrical configuration—a sphere. Wherever gravity dominates, it compels matter to adopt a round shape, which is Nature’s minimum energy configuration for objects of sufficiently large mass. All the known stars, planets, and moons are spheres, or very nearly so.
Gravity is not the only force operating in stars. Otherwise, the inward pull of this all-pervasive force would shrink stars to such a small size that, as black holes, they could not radiate any heat and light. Competing against gravity in a star is the pressure of its heated gas, which, pushing outward, tries to disperse the star into space. The result is a structural balance, or stable condition: gravity in, pressure out. That’s the simple prescription for a star—any star.
The star we know best is the Sun. Ole Sol is an average star, whose properties lie in the midst of the observed ranges of mass, size, brightness, and composition for all known stars. Its very mediocrity is what makes the Sun so interesting to astronomers—it’s “typical.” Its proximity to us is also useful, allowing us to see this star “up close.” Only 8 light-minutes away, the Sun is ~300,000 times closer than our next nearest neighbor, the Alpha Centauri star system ~4 light-years away. Accordingly, we know far more about the Sun than about any of the other distant points of light in the Universe. Our parent star is a benchmark against which we compare many other objects in the cosmos.
Dual Importance Stars are fascinating for many reasons, though two prevail above all. First and foremost, stars play essential roles in the heating and lighting of any nearby planets. For us, the energy of our Sun is critically important not only for the origin of life on Earth, but also for the continued maintenance and further development of that life. Without a nearby star, Earth would be a frozen, barren wasteland—a boulder so unimaginably hostile that life as we know it couldn’t possibly exist.
Second, stars are the furnaces where heavy elements are forged. Colliding viciously, light nuclei of hydrogen and helium fuse into the more complex nuclei at the hearts of >100 chemical elements, such as carbon, nitrogen, oxygen, silicon, and iron. “Better living through chemistry” is surely a theme (and not just an industrial slogan) prominent later in our cosmic-evolutionary scenario, and the building blocks of chemistry began in the stars. Without the heavies, nothing around us—not the ground, not the air, not much of Earth itself—would exist.
Stars, then, are key in the evolution of both matter and life; they might be absolute prerequisites for any kind of life. Stars themselves participate in the great, ongoing process of change in the Universe—so-called stellar evolution involving aspects of adaptation and selection, though not as dramatically so as for the biological evolution of life forms later in the sixth, BIOLOGICAL EPOCH. “Generations” of billions of stars were “born,” have “lived,” and have “died” since our Galaxy originated ~12 billion years ago. By forming in galaxies everywhere and by providing heat, light, and heavy elements for planets and life to follow, stars of this third, STELLAR EPOCH segue nicely from the previous GALACTIC EPOCH to the next PLANETARY EPOCH. Stars comprise a pivotally important, integral part of the cosmic-evolutionary story.
A Strength of Cosmic Evolution During the second half of the 20th century, astrophysicists learned a great deal about how stars pass through phases of youth, maturity, and aging—through intricate developmental and evolutionary paces. Although they appear immutable in the nighttime sky—secure in their remote and steady brilliance night after night—stars do actually change their appearance over great durations of time. The giant red star Betelgeuse in the constellation Orion and the dwarf white companion to Sirius the Dog Star, among myriad yellowish stars like our Sun, aren’t really different types of stars. Rather, each is at a distinctly different stage in the changing “life cycle” of nearly all stars.
As with most aspects of evolution broadly considered, change is central, albeit among the stars woefully slow. Some stars are old and bloated, some young and luminous. Others are long gone, having literally run out of fuel and perished eons ago. Still others are only now emerging from the interstellar hodgepodge of surging gas and dust—those vast and dark regions in and amongst the stars of our nighttime sky. Much of this change is unobvious because stellar life cycles are astronomically longer than human life spans—often billions of years compared to hardly a hundred, or millions of millennia compared to less than a century. Throughout all of evolution, we usually see change in snapshots—here and there in Nature, quick and sometimes dirty—each nonetheless helping us depict that broad and big picture of natural history writ large.
The subject of stellar evolution brings to mind a forest of trees. Some are young, some old, many deciduous, others evergreen; a few trees here and there are peculiar to say the least. Our study of the changes among stars resembles the study of a changing forest. By examining each of the various types of trees and stars, not only do we learn some things about those individual objects, but also, and perhaps more importantly, we explore the grander spectacle of the forest of stars in the Universe beyond.
The learning goals for this epoch are:
- to appreciate how stellar modeling is key to our knowledge of stellar evolution
- to understand the changes in density and temperature as stars form in interstellar space
- to become familiar with the wealth of observational evidence supporting the theory of star formation and evolution
- to know the evolutionary events that led to the birth of our Sun, and those that will eventually cause its death
- to realize that the many different star-like objects seen in the sky are basically the same type of object at different stages of stellar evolution
- to understand the reasons for the gentle deaths of low-mass stars and the violent deaths of their high-mass counterparts
- to understand the specific nuclear events that synthesize heavy elements in the cores of stars.