SUN-LIKE EVOLUTION

Hardly anything notable befalls a star during most of its lifetime. Provided the nuclear events at its core continue to offset the relentless onslaught of gravity, nothing spectacular happens to the star as a whole. Predictably, its core fuses hydrogen into helium, its surface erupts in flares and storms, and its atmosphere passess vast amounts of radiation. But, by and large, stars experience no sudden changes while in equilibrium, which is actually a kind of "dynamic steady state." They simply consume hydrogen during this, the longest phase in the history of all stars, a duration lasting ~99% of their total lifetimes.

Main-sequence Equilibrium Actually, stars maintain hydrostatic equilibrium, not thermodynamic equilibrium, as illustrated in Figure 3.22. The former pertains to the structural integrity of a normal star, noting again its delicate balance in the tug-of-war between gravity pulling in and heat pushing out. Technically, it’s not heat that pushes out as much as gas pressure, heat being a form of energy whereas pressure—the product of temperature and density—is more akin to a force. Hydrostatic equilibrium—as in a “compressible fluid,” which is the way stars are modeled—tends to stabilize a star at every point within the star, to keep it from collapsing or exploding, in either case catastrophically. By contrast, thermodynamic equilibrium occurs when temperatures are uniform throughout, a state most definitely not achieved by any star. In fact, stars have a clear and obvious temperature gradient, from their fiery cores to their cooler (but still hot) surfaces. What’s more, such gradients grow as stars age, driving them further from thermal equilibrium. It is this temperature variation that establishes decidedly non-equilibrium conditions, thermodynamically, in all stars.

FIGURE 3.22 FIGURE 3.22 — A steadily burning star on the main sequence has its inward pull of gravity counterbalanced by the outward pressure of its hot gases. This is true at any point within the star, guaranteeing its stability. (Prentice Hall)

Note also, to make another clarification, that even in hydrostatic equilibrium, a star like the Sun continues to change its luminosity—that is, its rate of energy flow—ever so slightly over the course of its lifetime. Specifically, for the Sun’s case, that amounts to an increase in brightness of ~1% every 100 million years. Although that seems minute, extrapolating back some 3-4 billion years means that the early Sun was probably ~1/3 less luminous than it is today—and that might pose a problem understanding the origin and maintenance of early life if planet Earth were at the time too cold for water to be liquefied. We shall return to discuss this “faint-Sun paradox” later in the seventh, CULTURAL EPOCH.

Stars, then, in their normal, balanced state, continue to produce energy indefinitely, pending some drastic change. The great struggle between heat and gravity remains under control, typically for billions of years. Eventually, however, something drastic does occur: All stars eventually exhaust their fuel.

Computer simulations are again our foremost guide to the specific changes experienced by any star near death. Identifying numerous physical and chemical factors and adjusting their values repeatedly, theoreticians have built models to describe the wide variety of stars seen in the real Universe. Let’s first detail the death plunge of a star like our Sun, after which we can extrapolate to all stars, large and small. Keep in mind, though, that all these fatal events occur within the last 1% of a star’s lifetime.

Hydrogen Depletion As the Sun ages, its hydrogen steadily depletes, at least within a small, central core having a few percent of the star’s full crossectional size. After nearly 10 billion years of slow and steady burning, little hydrogen will remain within the innermost fusion zone. The star literally runs out of gas. Much like an automobile cruising along a highway at a constant speed for many hours without a care in the world, its engine starts to cough and sputter as the gas gauge approaches empty. Unlike automobiles, though, stars aren’t easy to refuel.

Widespread exhaustion of hydrogen in the stellar core causes the nuclear reactions there to cease. Hydrogen combustion continues unabated in the star's intermediate layers, above its core though well below its surface. But the core itself normally provides the bulk of the support in any star, acting as a foundation and guaranteeing its stability. By contrast, the lack of core burning assures instability because, although the outward gas pressure weakens in the cooling core, the inward pull of gravity most assuredly does not. Gravity never lets up; it’s relentless. Once the outward push against gravity is relaxed—even a little—structural changes in the star become inevitable.

Generation of more heat could bring the aged star back into hydrostatic balance. If, for example, the helium at the core began fusing into some heavier element such as carbon, then all would be well once again, for energy would be recreated as a by-product to help reestablish the outward gas pressure. But the helium there cannot burn—not yet, anyway. Despite the phenomenal temperature of millions of kelvins, the core is just too “cold” for helium to fuse into any heavier elements.

Recall that a temperature of at least 107 K is needed to initiate the hydrogen --> helium fusion cycle. That’s what it takes for two colliding hydrogen nuclei (protons) to get up enough speed to ram each other violently and thus overwhelm the repulsive electromagnetic force between two like charges. Otherwise, the nuclei cannot penetrate the domain of the nuclear binding force and the fusion process simply doesn’t work. Even 107 K, however, is insufficient for helium fusion, since each helium nucleus (2 protons + 2 neutrons) has a net charge twice that of the hydrogen nucleus, making the repulsive electromagnetic force greater. To ensure successful fusion by means of a violent collision between helium nuclei, even higher temperatures are needed. How high? About a hundred million kelvins—108 K.

Lacking that degree of heat, the star’s core of helium “ash” doesn’t remain idle for long. Its hydrogen fuel spent, the core begins contracting. It has to; there’s not enough pressure to hold back gravity. However, this very shrinkage allows the gas density to increase, thereby creating more heat as gas particle collisions become ever more frequent. Once again, it’s gravity, in the guise of gravitational potential energy converting to frictional heat energy, that drives this process—indeed drives up the temperature.

The increasingly hot core continues to roil the overlying layers of this stellar furnace. It’s very much like a domestic thermostat that calls for more heat in our homes, thereby keeping the air temperature comfortably stable. In an aged star, Nature seeks more energy to restabilize events, and when the star generates enough of it, negative feedback terminates the contraction—at least for a while. (“Feedback” because, as in a central heating system, a change in the effect is fed back to modify its cause, and “negative” because the feedback loop controlling the process ensures that the effect doesn’t increase or decrease without limit.) But first, higher temperatures—at this stage, well >107 K—cause hydrogen nuclei in the star’s intermediate layers to fuse even more furiously than in the core before. All the while, helium ash continues to pile up around the core. Figure 3.23 depicts this rather peculiar condition where hydrogen is burning at a fantastic rate around the nonburning helium ash.

FIGURE 3.23 FIGURE 3.23 — As a star's core becomes progressively depleted of hydrogen, the hydrogen fusion reactions continue to burn in its intermediate layers, high above the nonburning helium ash. (Prentice Hall)

The aged star is really in a predicament now. Its days are numbered. The core is unbalanced and shrinking, on its way toward generating enough heat for helium fusion. The intermediate layers are also scrambling to maintain some semblance of poise, fusing hydrogen into helium at faster-than-normal rates. Alas, the gas pressure exerted by this enhanced hydrogen burning does build up, forcing the star’s outermost layers to expand; not even gravity can stop them. So, although the core is shrinking, the overlying layers are expanding! Clearly, the star’s structural stability is completely ruined.

Observational Consequences Two observable aspects of such a perverse star are interesting. To a viewer far away, this celestial object would seem gigantic, nearly 100 times larger than usual. Captured radiation would also imply that the star’s surface was a little cooler (~1000 K) than normal. This is not to say that the act of ballooning and chilling of an aged star could be observed directly during any one human lifetime. The transition from a normal, solar-mass star to an elderly giant still takes ~100 million years.

These large-scale changes in the disposition of an aged 1-solar-mass star can be traced on the HR diagram. Figure 3.24 shows the resulting path away from the main sequence. As illustrated, the luminosity of this giant star—again, R2T4—becomes ~100 times the current brightness of our Sun.

FIGURE 3.24 FIGURE 3.24 — As the core of helium ash shrinks and the intermediate stellar layers expand, the star leaves the main sequence. Labeled stage 8, it's on its way to becoming a red-giant star. (Lola Chaisson)

The second change—surface cooling—is a direct result of the first change—increased size. As the star expands, the sum total of its heat spreads throughout a much larger stellar volume. Hence, visible radiation emitted from such a cooling, yet still-hot, surface shifts in color. Like a white-hot piece of metal that turns red while cooling, the whole extended star displays a reddish tint. Over the course of time, again long by human though short by stellar standards, a star of normal size and yellow color slowly changes into one of giant size and red color. The bright normal star has evolved into a dim red-giant star.

Figure 3.25 compares the relative sizes of our Sun and a red-giant star. The typical giant star is huge, having swollen to ~100 times its main-sequence size. By contrast, the helium core is surprisingly small, probably ~1000 times smaller than the entire star. This makes the core only a few times larger than Earth.

FIGURE 3.25 FIGURE 3.25 — Schematic diagram of the relative sizes (to scale) of a normal G-type star when on the main sequence, and after it has ballooned to become a red-giant star. The difference in size is ~70 times.

The density in the core is now huge. Continued shrinkage of the red giant's core has compacted its helium gas to ~105 g/cm3. This value may be contrasted with ~10-6 g/cm3 in the outermost layers of the red-giant star, with ~5 g/cm3 average Earth density, or with ~150 g/cm3 in the core of the present Sun. Owing to this greatly compressed helium state, ~25% of the mass of the entire star is packed into its small core.

To recapitulate these momentous events and give them some local relevamce, once the Sun exhausts its hydrogen fuel supply at its core, instability is sure to set in. Its core will shrink as its overlying layers swell, all the while equilibrium is shot. As such, the Sun is destined to become a bloated sphere hundreds of times its normal size, perhaps large enough to engulf many of the planets, including Mercury and Venus, and maybe even Earth and Mars as well.

Humans need not panic, not yet at any rate. Provided the theory of stellar evolution is reasonably correct as described here, we can be sure that our Sun will not swell to this red-giant stage for another 5 billion years. Whether life can remain viable on Earth that long is debatable; there are two competing arguments: First, owing to the faint-Sun paradox noted a few pages earlier, the future Sun seems likely to increase its luminosity by ~10% in a “mere” billion years, possibly rendering our planet unsuitable for life well before the Sun itself expires. Planet Earth will eventually get quite steamy regardless of any global pollution caused by humankind. Second, countering that long-term heating is a natural cooling forecast by the expected outward migration of the planets’ orbits as the Sun loses mass and lessens its gravitational grip. Hard to believe, the Sun is shedding its own matter (in a “solar wind”) at the prodigious rate of about a million tons each second, yet even in a billion years will have lost <0.1% of its total mass, which might not be enough for the planets to drift away much. Whether the resulting cooling trend caused by the receding planets can offset the heating trend caused by increased sunlight is an unsolved problem—a rare astronomical enigma with life-and-death terrestrial implications. Whichever, life’s days on Earth are surely numbered, its oceans destined to evaporate and its atmosphere dissipate, our planet eventually resembling a ceramic-encrusted Mercury. Not to worry, such a hell-on-Earth will not commence for nearly another trillion or so days.

Red-giant stars are not the fiction of some theoretician’s mind. They really do exist, scattered in numerous places across the sky. Even the naked eye can perceive the most famous of all red giants—the bright star Betelgeuse, that swollen, elderly, distinctly reddish member of the constellation Orion—a prominent beacon in the northern hemisphere’s winter sky. This star is so luminous, it can be seen even through the smog and light pollution of our biggest cities. Look up!

Helium Fusion Should the inherent imbalance of a red-giant star be maintained unabated, the core would eventually implode, while the rest of the star drifts into space. Various forces and pressures at work inside such a decrepit star would literally, though slowly, pull it apart. Fortunately for the stellar veteran, this tortuous shrinkage-expansion doesn't continue indefinitely. Within 100 million years after the star first begins to panic for lack of hydrogen fuel, something else happens—helium ignites in thermonuclear burning. Accordingly, the star's natural thermostat shuts off the flow of additional heat as the core stabilizes once more. Though this seems like a whole new lease on life, it amounts to only a brief reprieve.

Deep down inside a red-giant star, the density increases as the interior pressure builds. Once the matter in the star’s core becomes ~1000 times denser than that of a normal star (i.e., ~105 g/cm3), collisions among the gas particles are violent and frequent enough to generate sufficient heat, via friction, to reach the 108 K temperature needed for helium fusion. Helium nuclei henceforth collide, trigger the central fires once again, and begin transforming into carbon. Thereafter for a period of a few hours, the helium burns ferociously, like an uncontrolled bomb. This onset of helium burning is such a sudden and rapid event in the history of a star that astronomers give it a special name—“helium flash.” It’s remarkable that the star doesn’t explode.

Despite their brevity, these renewed nuclear events release an enormous flood of new energy. The energy is potent enough to etherealize the core matter somewhat, thereby lowering its density and relieving some of the pent-up pressure among the charged nuclei. This small expansive adjustment of the core halts the gravitational contraction of the star, reestablishing an equilibrium of sorts—in this case, a balance occurring at the quantum level among the densely packed electrons whose tiny point-like spheres are essentially touching one another, thereby physically holding up the aged star against gravity.

To make yet another clarifying technical comment, note that, in actuality, the nuclear reaction that changes helium into carbon occurs in two steps known as the “triple-alpha process.” First, two helium nuclei (which are also termed alpha particles) combine to form beryllium, which is a very unstable nucleus that would normally break right back down (in less than a microsecond) into two helium nuclei—causing the process to be stuck in an endless cycle that yields nothing heavier than helium. However (and this is the second step), the huge densities in the helium ash guarantee that a third helium nucleus sometimes collides with newly made beryllium before it has a chance to decay. This is not a miracle, or some sort of “anthropic principle,” implying that a supernatural being designed it that way to permit heavy elements and therefore life. Rather, given the very high densities in a red-giant’s core, the time scale for collision and then fusion among three helium nuclei (hence the name, "triple") is naturally shorter than for the fusion and breakdown of beryllium. The result is carbon, the nucleus of a vitally important element in the later CHEMICAL EPOCH of our cosmic-evolutionary story.

Once the helium --> carbon fusion reactions commence, thus stabilizing the core, the hydrogen --> helium fusion reactions churning in the layers above subside (but do not stop). Theoretical computations implies that the star expands its outer layers a bit too rapidly, overshooting the distance at which it reestablishes a relaxed structural balance. The entire star is then able to shrink a little, losing some of its swollen appearance. This slight shrinkage of the outer layers causes the luminosity to decrease and the surface temperature to increase, reversing the star's evolutionary path once again, as shown in Figure 3.26. Like all the other evolutionary changes in the early or late phases of a star, this slight size adjustment onto the "horizontal branch" is made quickly—at least by cosmic standards—namely, in ~100,000 years.

FIGURE 3.26 FIGURE 3.26 — After a large increase in luminosity, a red-giant star finally settles down into another equilibrium state at stage 10, on the so-called horizontal branch. (Lola Chaisson)

Though the time scales for marked stellar change are deemed rapid for a star’s birth in gas and dust as well as its thrust toward an end-fate, all these transient durations are still long compared to human lifespans. Observers have little hope of watching a given star move through all, or even some, of the evolutionary paces underway in the STELLAR EPOCH. Instead, much as before, astronomers search the Galaxy for evidence of diverse cosmic objects at different stages of their evolutionary cycles, trying to position them like puzzle pieces into a self-consistent picture. Or, to use another metaphor, like social behaviorists charged with the task of unraveling the population dynamics of animals, astronomers are finding that the deeper they peer into galactic lairs, the more instructive the menagerie of stellar inhabitants becomes. In the end, we always rely on mathematical modeling to match (and adjust) the theory of stellar evolution with the observations of the many varied stages in the birth and death of stars.

Table 3-2 summarizes a computer calculation done for a 1-solar-mass object. It's a continuation of the previous compilation listed in Table 3-1, except that the density units have been switched from particles/cm3 to g/cm3, to reflect the growing densities (there being about 1024 atomic particles in 1 gram of matter). The previous table ended with stage 7, a main-sequence object fusing hydrogen into helium over the course of ~10 billion years. The new table here begins with stage 8, the evolutionary path away from the main sequence. Stage 9 describes an established red-giant star fusing helium into carbon at its core.

As for the physical quantities listed in Table 3-1, those describing each of the stages of Table 3-2 cannot be specified with high accuracy. The temperature, density, size, and luminosity, as well as the precise evolutionary path, are not completely understood at this time. Each of these quantities depends on the initial conditions used for the mass and composition of a star, as well as on the rate of nuclear burning deep inside.

Solar Neutrinos This reliance on computer modeling is exactly what made the results of an important experiment so disturbing—until recently. The one experiment that bears directly on the physical events inside stars didn’t jibe well with the predictions for a star like our Sun. For decades, scientists were puzzled by the number of neutrino elementary particles found in the solar radiation reaching Earth. Derived from an Italian word meaning “little neutral one,” neutrinos are known from experiments on Earth to be virtually massless and chargeless, and to travel at (or very close to) the velocity of light. Interacting with almost nothing, neutrinos are ghost-like particles endowed with an ability to pass freely through several light-years of lead! Hence, they should be able to escape unhesitatingly from the solar core, where they are created in copious amounts as by-products of nuclear reactions. Ordinary radiation scatters around (or “random walks”) in the solar interior for about a million years before being emitted from the Sun’s surface into space, but neutrinos should pierce the solar surface in 2 seconds and arrive at Earth a mere 8 minutes after being made at the core. They thus comprise the only direct test of the nuclear events responsible for powering the Sun.

Solar neutrinos nonchalantly penetrate Earth all the time. Some 5 million neutrinos pepper every cm2 of our bodies each second, though we are neither aware of nor harmed by them. Despite their elusiveness, however, the effects of neutrinos can be studied with carefully built instruments made of rare materials. One of those materials is a chemical with the tongue-twisting name of tetrachloroethylene. As toxic as it sounds, C2Cl4 is a safe fluid often used in the dry-cleaning industry. So a “neutrino telescope” was built in the 1970s at the bottom of a South Dakota gold mine by filling a large tank with 400,000 liters (~100,000 gallons) of this stuff. In that way, some of the solar neutrinos arriving at Earth can be counted and analyzed, though actually only 1 is detected for every 1015 of them streaming through the tank. The underground location of this laboratory and its unique telescope is essential to shield the experiment from interference due to cosmic rays and other elementary particles hailing from non-solar sources such as ancient supernovae. Although the equipment seems to have worked properly for decades, the rate of neutrino detection has often been consistently less than theory predicts; they are seen about twice per week, rather than once per day—about a three-fold discrepancy.

Astrophysicists have wrestled with these puzzling results for many years. Both theorists and experimentalists are reluctant to blame any underabundance of solar neutrinos on conceptual errors in the theory of stellar evolution. No one wants to discard what seems like a good understanding of solar fusion, all other aspects of which agree so well with observations. Some researchers (mostly theorists) suspect the experimental gear; perhaps it wasn’t quite tuned properly, and in any case a factor of ~3 isn’t usually a large issue in astronomy. Others (mostly experimenters) are leery about the computer models; if the Sun’s core were only 10% cooler than theory maintains, the predicted number of solar neutrinos would be less. Still others argue that we don’t yet know enough about the odd neutrino particle itself; the physical properties of the neutrinos might make them the culprit, especially if they turn out to have even minute amounts of mass.

More recently, in the first decade of the 21st century, this factor-of-three discrepancy seems to have been resolved during experiments in new underground laboratories located in Japan and Canada, the latter using a 1000-ton sphere of ultrapure water suspended more than a kilometer beneath the surface and surrounded by 10,000 neutrino sensors. The new results do indicate that neutrinos have minute amounts of mass—roughly a millionth (10-6) the mass of an electron, which is itself nearly 2000 times lighter than a proton. However, even this ultra-tiny mass is enough to cause the apparently schizophrenic neutrinos to change their properties, even to transform them into other particles, during their 8-minute journey from the Sun to the Earth. And that’s what most astronomers now think is happening: Neutrinos are likely produced in the Sun at the rate predicted by theory, but some of them change into something else—probably morphing into other types of neutrinos—while en route to Earth. The original experiments were insensitive to these changes, but the newer experiments are detecting evidence of them. At issue now is the need to fix up the standard model of particle physics, in which neutrinos are expected to have precisely zero mass—or to begin a whole new search to solve another potential contradiction between quantum theory and delicate experimentation.

Assuming these latest results are correct—namely, that neutrinos have both intrinsic mass and mutable properties—we once again wonder if the neutrinos could be the solution to the elusive dark-matter quandary. Given the tremendous number of neutrinos likely flooding our Galaxy—both leftovers from particle interactions in the early Universe as well as new ones created in all the stars of the Milky Way—it still seems doubtful. Although neutrinos are surely part of the cosmic mix, their total accumulation likely amounts to <1% of the overall mass of the Galaxy.

In any event, few researchers regard the surprising solution to the solar-neutrino problem as a threat to our understanding of the way that stars shine. This decades-old neutrino dilemma now seems to have been more of a problem with the physics of the particle than with the astronomy of the Sun. By checking and double-checking both theory and experiment, all the while continuing to address the issue with reason and skepticism—which is exactly the way science progresses—what once loomed as a serious misunderstanding of stellar fusion has apparently now been resolved.

Nagging Issues Uncertainties limit our understanding of every epoch of cosmic evolution. Here in the STELLAR EPOCH, as elsewhere, we seem able to identify the broad outlines of many possible events, but the fine details aren’t always in hand. What causes flaring on our Sun, resulting in huge prominences of matter and radiation that escape our star and impact our planet? How does the ~11-year solar cycle work, turning surface sunspots off and on at nearly decade intervals? Can we explain satisfactorily the million-degree corona, or outer atmosphere of the Sun, when its surface is only 6000 K? What’s the role of magnetic fields in the origin, maintenance, and demise of all stars?

Even the brightest star in the nighttime sky seems a little puzzling, at least as regards the historical record. Sirius A, only 9 light-years away, appears twice as luminous as any other visible star (excluding the Sun) and has been prominently observed by many ancient civilizations. Cuneiform texts of the Babylonians refer to this star as far back as 1000 B.C., and historians know that the star strongly influenced the agriculture and religion of the Egyptians of about 3000 B.C. So, given the lengthy record of observations of Sirius, here’s an object for which we might have a chance to study slight evolutionary changes, despite the long time scales usually needed to produce such changes. Yet herein lies the puzzle.

Sirius A does seem to have changed its appearance over the ages; written historical records clearly imply it. But the naked-eye observations of the ancients are confusing. Every piece of information about Sirius recorded between the years 100 B.C. and A.D. 200 claims that this star was red. In contrast, modern observations now show it to be white or bluish white, but definitely not red. Accordingly to the theory of stellar evolution, no star should be able to change its color from red to blue-white so dramatically in such a short time—even over thousands of years. Any change of this sort should take at least ~100,000 years, and in any case would more likely change from blue to red.

Astronomers have offered many explanations for the rather sudden change in Sirius A. These include the possibility that some ancient observers were wrong and other scribes copied them. Or perhaps a galactic dust cloud passed between Sirius and Earth ~2000 years ago, reddening the star much as Earth’s dusty atmosphere often does for our Sun at dusk. Or maybe a companion to Sirius A, namely Sirius B, was a red giant and dominant star of this double-star system 2000 years ago and has since expelled its outer envelope to reveal the small (white-dwarf) star that we now observe as Sirius B.

None of these explanations seems plausible, however. How could the color of the sky’s brightest star be incorrectly recorded for hundreds of years? Where’s the intervening galactic cloud now? Where’s the shell of the former red giant? We are left with the uneasy feeling that the night’s brightest star doesn’t seem to fit well into the currently accepted scenario of stellar evolution.

As if that were not enough, our resolute navigational beacon, Polaris the North Star, is also a bit of a conundrum. Despite Shakespeare's classic line for Julius Ceasar, "But I am constant as the Northern Star," the light from Polaris is not so steady, yet the sky is replete with variable stars so that is perhaps alright. Alas, the extent of its variability is also changing and quickly too, and that is indeed puzzling. Greek astronomers of 2000 years ago claimed that Polaris' average brightness was 3 times dimmer than now, a rate of change, if real, much greater than that predicted by current models of stellar evolution. That not all the loose ends are yet tied up isn’t meant to imply major cracks in our understanding of stars; rather that plenty of work remains to be done regarding those picky little details that often serve to fine-tune that understanding.

These subtle yet bothersome issues aside, stellar evolution is judged one of the great success stories of modern astrophysics. Theory and observation have advanced hand in hand over the last many decades, refining our knowledge of stars as they proceed from cradle to grave. Today, the subject of stellar evolution is a cornerstone of the cosmic-evolutionary narrative, a key part of that broadest view of the biggest picture that we’ve come to know rather well.

Carbon Core Nuclear reactions in an old star’s helium core churn on, but not for long. Whatever helium exists in the core is rapidly consumed. The helium --> carbon fusion cycle, like the hydrogen --> helium cycle before it, runs at a rate proportional to the temperature; the greater the core heat, the faster the reactions proceed. Under these very high temperatures, helium fuel simply doesn’t last long—probably less than a few million years after its initial “flash.”

Buildup of carbon ash in the inner core causes a series of physical events similar to those in the earlier helium core. Helium first becomes depleted at the star’s very center, after which fusion there ceases, the temperature being too low for carbon detonation. The carbon core then shrinks and heats a little, as Nature’s thermostat kicks in again while searching for more energy from renewed gravitational infall. This, in turn, causes the hydrogen and helium burning cycles to ramp up in the middle layers of the star. Such an aged star begins to resemble a huge onion, with different shells of progressively heavier elements toward its center. All this additional heating causes its outer envelope ultimately to expand, much as it did earlier, making the star once again a swollen red giant. Figures 3.27 and 3.28 depict the star's interior and the evolutionary path followed during these latest events.

FIGURE 3.27 FIGURE 3.27 — Within a few million years after the onset of helium burning, carbon ash accumulates in the inner core of a star, above which hydrogen and helium are still burning. (Prentice Hall)

FIGURE 3.28 FIGURE 3.28 — A carbon-core star eventually heads back toward higher luminosities—technically along an "asymptotic giant branch"—for the same reason it evolved there in the first place: lack of nuclear fusion at the core, causing contraction of the core and expansion of the overlying layers. (Lola Chaisson)

Provided the core temperature does become high enough for the fusion of two carbon nuclei, or more likely a union of carbon and helium nuclei, even heavier products can be synthesized. Newly generated energy supports the star at each stage in the nuclear chain, returning the star to its accustomed hydrostatic equilibrium. Again, this isn’t a thermodynamic equilibrium, for such decrepit old stars develop steep thermal and elemental gradients from core to surface. For this reason, such aged stars are decidedly more complex than their younger counterparts. Ironically, as the fusion process advances, old stars continue getting brighter, all the while they are dying.

This contracting-heating-fusing-cycle is generally the way that many of the heavy elements are fashioned within the last gasps of stellar cores. All elements heavier than carbon are created within the final 1% of some stars’ lifetimes. Our Sun, however, is not one of them; it's too small.

Mass Loss Stars of all spectral types are known to be active and to have stellar winds, much as the active Sun displays most evidently every ~11 years during its periods of increased sunspots, flares, and prominences. Consider the highly luminous, hot, blue stars (O- and B-types) that have by far the strongest winds. Observations of their ultraviolet spectra with telescopes on rockets and satellites have shown that their wind speeds (or gales!) often reach 3000 km/s (or several million mph). The corresponding mass-loss rates approach and sometimes exceed 10-5 solar mass per year; this is equivalent to an entire solar mass (yet typically only about a tenth of the total mass in these bigger stars) being carried off into space in the relatively short span of 100,000 years.

Observations made by the International Ultraviolet Explorer satellite operating in Earth orbit during the 1980s proved that to produce such great winds, the pressure of hot coronal gases (which drive the solar wind) does not suffice. Instead, the winds of the luminous hot stars must be driven directly by the pressure of the ultraviolet radiation emitted by these stars. The same mechanism has been theorized to eject gas from the cores of some particularly active galaxies, a subject touched on briefly in the previous GALACTIC EPOCH .

Such powerful stellar winds hollow out vast cavities in the interstellar medium, pushing outward expanding shells of galactic matter resembling those generated by exploded stars, as discussed both at the end of the GALACTIC EPOCH and in the next section of this STELLAR EPOCH. Aside from the well-known fact that copious quantities of ultraviolet radiation are available from luminous hot stars to drive such stellar winds, the details of the process are not well understood. Whatever is going on, it’s surely convoluted, for the ultraviolet spectra of the stars tend to vary with time, implying that the wind is unsteady. Apparently, stellar instabilities of some kind or another are at the heart of the issue.

Observations made more recently with radio and infrared as well as optical telescopes prove that luminous cool stars (e.g., K- and M-type giants) lose mass at rates comparable to those of the luminous hot stars; their wind velocities, however, are much lower, averaging 30 km/s (or "merely" 70,000 mph). Because luminous red stars are inherently cool objects (~3000 K surface temperature), they emit no detectable ultraviolet radiation, so the mechanism driving the winds probably differs from that in luminous hot stars. We can only surmise that gas turbulence and/or magnetic forces in the atmospheres of these stars are somehow responsible. Unlike the hot stars, winds from these cool stars are rich in dust particles and molecules. Since nearly all stars more massive than the Sun eventually evolve into such red giants, these winds, pouring forth from vast numbers of stars, provide a major source of new gas and dust in interstellar space. Thus, the recently discovered stellar winds provide a vital link in the cycle of star formation and galactic evolution. As with the hot stars, astronomers are unsure what affect these winds and mass losses have on the subsequent evolution of the stars themselves.


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