{"id":690,"date":"2016-11-15T21:38:13","date_gmt":"2016-11-15T21:38:13","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/astronomy\/?post_type=chapter&#038;p=690"},"modified":"2018-01-22T16:14:01","modified_gmt":"2018-01-22T16:14:01","slug":"further-evolution-of-stars","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/chapter\/further-evolution-of-stars\/","title":{"raw":"22.4 Further Evolution of Stars","rendered":"22.4 Further Evolution of Stars"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Explain what happens in a star\u2019s core when all of the hydrogen has been used up<\/li>\r\n \t<li>Define \"planetary nebulae\" and discuss their origin<\/li>\r\n \t<li>Discuss the creation of new chemical elements during the late stages of <strong>stellar evolution<\/strong><\/li>\r\n<\/ul>\r\n<\/div>\r\nThe \"life story\" we have related so far applies to almost all stars: each starts as a contracting protostar, then lives most of its life as a stable main-sequence star, and eventually moves off the main sequence toward the red-giant region.\r\n\r\nAs we have seen, the pace at which each star goes through these stages depends on its mass, with more massive stars evolving more quickly. But after this point, the life stories of stars of different masses diverge, with a wider range of possible behavior according to their masses, their compositions, and the presence of any nearby companion stars.\r\n\r\nBecause we have written this book for students taking their first astronomy course, we will recount a simplified version of what happens to stars as they move toward the final stages in their lives. We will (perhaps to your heartfelt relief) not delve into all the possible ways aging stars can behave and the strange things that happen when a star is orbited by a second star in a binary system. Instead, we will focus only on the key stages in the evolution of single stars and show how the evolution of high-mass stars differs from that of low-mass stars (such as our Sun).\r\n<h2>Helium Fusion<\/h2>\r\nLet\u2019s begin by considering stars with composition like that of the Sun and whose <em>initial<\/em> masses are comparatively low\u2014no more than about twice the mass of our Sun. (Such mass may not seem too low, but stars with masses less than this all behave in a fairly similar fashion. We will see what happens to more massive stars in the next section.) Because there are much more low-mass stars than high-mass stars in the Milky Way, the vast majority of stars\u2014including our Sun\u2014follow the scenario we are about to relate. By the way, we carefully used the term <em>initial masses<\/em> of stars because, as we will see, stars can lose quite a bit of mass in the process of aging and dying.\r\n\r\nRemember that red giants start out with a helium core where no energy generation is taking place, surrounded by a shell where hydrogen is undergoing <strong>fusion<\/strong>. The core, having no source of energy to oppose the inward pull of gravity, is shrinking and growing hotter. As time goes on, the temperature in the core can rise to much hotter values than it had in its main-sequence days. Once it reaches a temperature of 100 million K (but not before such point), three helium atoms can begin to fuse to form a single carbon nucleus. This process is called the <strong>triple-alpha process<\/strong>, so named because physicists call the nucleus of the helium atom an alpha particle.\r\n\r\nWhen the triple-alpha process begins in low-mass (about 0.8 to 2.0 solar masses) stars, calculations show that the entire core is ignited in a quick burst of fusion called a <strong>helium flash<\/strong>. (More massive stars also ignite helium but more gradually and not with a flash.) As soon as the temperature at the center of the star becomes high enough to start the triple-alpha process, the extra energy released is transmitted quickly through the entire helium core, producing very rapid heating. The heating speeds up the nuclear reactions, which provide more heating, and which accelerates the nuclear reactions even more. We have runaway generation of energy, which reignites the entire helium core in a flash.\r\n\r\nYou might wonder why the next major step in nuclear fusion in stars involves three helium nuclei and not just two. Although it is a lot easier to get two helium nuclei to collide, the product of this collision is not stable and falls apart very quickly. It takes three helium nuclei coming together <em>simultaneously<\/em> to make a stable nuclear structure. Given that each helium nucleus has two positive protons and that such protons repel one another, you can begin to see the problem. It takes a temperature of 100 million K to slam three helium nuclei (six protons) together and make them stick. But when that happens, the star produces a carbon nucleus.\r\n<div class=\"textbox shaded\">\r\n<h3>Stars in Your Little Finger<\/h3>\r\nStop reading for a moment and look at your little finger. It\u2019s full of carbon atoms because carbon is a fundamental chemical building block for life on Earth. Each of those carbon atoms was once inside a red giant star and was fused from helium nuclei in the triple-alpha process. All the carbon on Earth\u2014in you, in the charcoal you use for barbecuing, and in the diamonds you might exchange with a loved one\u2014was \"cooked up\" by previous generations of stars. How the carbon atoms (and other elements) made their way from inside some of those stars to become part of Earth is something we will discuss in the next chapter. For now, we want to emphasize that our description of stellar evolution is, in a very real sense, the story of our own cosmic \"roots\"\u2014the history of how our own atoms originated among the stars. We are made of \"star-stuff.\"\r\n\r\n<\/div>\r\n<h2>Becoming a Giant Again<\/h2>\r\nAfter the helium flash, the star, having survived the \"energy crisis\" that followed the end of the main-sequence stage and the exhaustion of the hydrogen fuel at its center, finds its balance again. As the star readjusts to the release of energy from the triple-alpha process in its core, its internal structure changes once more: its surface temperature increases and its overall luminosity decreases. The point that represents the star on the H\u2013R diagram thus moves to a new position to the left of and somewhat below its place as a red giant (Figure 1). The star then continues to fuse the helium in its core for a while, returning to the kind of equilibrium between pressure and gravity that characterized the main-sequence stage. During this time, a newly formed carbon nucleus at the center of the star can sometimes be joined by another helium nucleus to produce a nucleus of oxygen\u2014another building block of life.\r\n\r\n&nbsp;\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"467\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170443\/OSC_Astro_22_04_Evaluation.jpg\" alt=\"Evolution of a Star like the Sun on an H\u2013R Diagram. In this plot the vertical axis is labeled \" width=\"467\" height=\"512\" \/> <strong>Figure 1. Evolution of a Star Like the Sun on an H\u2013R Diagram: <\/strong> Each stage in the star\u2019s life is labeled. (a) The star evolves from the main sequence to be a red giant, decreasing in surface temperature and increasing in luminosity. (b) A helium flash occurs, leading to a readjustment of the star\u2019s internal structure and to (c) a brief period of stability during which helium is fused to carbon and oxygen in the core (in the process the star becomes hotter and less luminous than it was as a red giant). (d) After the central helium is exhausted, the star becomes a giant again and moves to higher luminosity and lower temperature. By this time, however, the star has exhausted its inner resources and will soon begin to die. Where the evolutionary track becomes a dashed line, the changes are so rapid that they are difficult to model.[\/caption]\r\n\r\nHowever, at a temperature of 100 million K, the inner core is converting its helium fuel or carbon (and a bit of oxygen) at a rapid rate. Thus, the new period of stability cannot last very long: it is far shorter than the main-sequence stage. Soon, all the helium hot enough for fusion will be used up, just like the hot hydrogen that was used up earlier in the star\u2019s evolution. Once again, the inner core will not be able to generate energy via fusion. Once more, gravity will take over, and the core will start to shrink again. We can think of stellar evolution as a story of a constant struggle against gravitational collapse. A star can avoid collapsing as long as it can tap energy sources, but once any particular fuel is used up, it starts to collapse again.\r\n\r\nThe star\u2019s situation is analogous to the end of the main-sequence stage (when the central hydrogen got used up), but the star now has a somewhat more complicated structure. Again, the star\u2019s core begins to collapse under its own weight. Heat released by the shrinking of the carbon and oxygen core flows into a shell of helium just above the core. This helium, which had not been hot enough for fusion into carbon earlier, is heated just enough for fusion to begin and to generate a new flow of energy.\r\n\r\nFarther out in the star, there is also a shell where fresh hydrogen has been heated enough to fuse helium. The star now has a multi-layered structure like an onion: a carbon-oxygen core, surrounded by a shell of helium fusion, a layer of helium, a shell of hydrogen fusion, and finally, the extended outer layers of the star (see Figure 2). As energy flows outward from the two fusion shells, once again the outer regions of the star begin to expand. Its brief period of stability is over; the star moves back to the red-giant domain on the H\u2013R diagram for a short time (see Figure 1). But this is a brief and final burst of glory.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170446\/OSC_Astro_22_04_Layers.jpg\" alt=\"Layers inside a Low-mass Star before Death. The layers within the core are shown as concentric circles of various colors. Starting at the center are: \" width=\"975\" height=\"413\" \/> <strong>Figure 2. Layers inside a Low-Mass Star before Death: <\/strong> Here we see the layers inside a star with an initial mass that is less than twice the mass of the Sun. These include, from the center outward, the carbon-oxygen core, a layer of helium hot enough to fuse, a layer of cooler helium, a layer of hydrogen hot enough to fuse, and then cooler hydrogen beyond.[\/caption]\r\n\r\nRecall that the last time the star was in this predicament, helium fusion came to its rescue. The temperature at the star\u2019s center eventually became hot enough for the <em>product<\/em> of the previous step of fusion (helium) to become the <em>fuel<\/em> for the next step (helium fusing into carbon). But the step after the fusion of helium nuclei requires a temperature so hot that the kinds of lower-mass stars (less than 2 solar masses) we are discussing simply cannot compress their cores to reach it. No further types of fusion are possible for such a star.\r\n\r\nIn a star with a mass similar to that of the Sun, the formation of a carbon-oxygen core thus marks the end of the generation of nuclear energy at the center of the star. The star must now confront the fact that its death is near. We will discuss how stars like this end their lives in <a class=\"target-chapter\" href=\".\/chapter\/introduction-to-the-death-of-stars\/\" target=\"_blank\" rel=\"noopener\">The Death of Stars<\/a>, but in the meantime, Table 1 summarizes the stages discussed so far in the life of a star with the same mass as that of the Sun. One thing that gives us confidence in our calculations of stellar evolution is that when we make H\u2013R diagrams of older clusters, we actually see stars in each of the stages that we have been discussing.\r\n<table id=\"fs-id1168048452256\" class=\"span-all\" summary=\"This table contains five columns and five rows. The first row is a header row, and it labels each column, \">\r\n<thead>\r\n<tr valign=\"top\">\r\n<th colspan=\"5\">Table 1. The Evolution of a Star with the Sun\u2019s Mass<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th>Stage<\/th>\r\n<th>Time in This Stage (years)<\/th>\r\n<th>Surface Temperature (K)<\/th>\r\n<th>Luminosity (<em>L<\/em><sub>Sun<\/sub>)<\/th>\r\n<th>Diameter\r\n\r\n(Sun = 1)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>Main sequence<\/td>\r\n<td>11 billion<\/td>\r\n<td>6000<\/td>\r\n<td>1<\/td>\r\n<td>1<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Becomes red giant<\/td>\r\n<td>1.3 billion<\/td>\r\n<td>3100 at minimum<\/td>\r\n<td>2300 at maximum<\/td>\r\n<td>165<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Helium fusion<\/td>\r\n<td>100 million<\/td>\r\n<td>4800<\/td>\r\n<td>50<\/td>\r\n<td>10<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Giant again<\/td>\r\n<td>20 million<\/td>\r\n<td>3100<\/td>\r\n<td>5200<\/td>\r\n<td>180<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h2>Mass Loss from Red-Giant Stars and the Formation of Planetary Nebulae<\/h2>\r\nWhen stars swell up to become red giants, they have very large radii and therefore a low escape velocity.<a href=\"#footnote1\" name=\"footnote-ref1\"><sup>[footnote]Recall that the force of gravity depends not only on the mass doing the pulling, but also on our distance from the center of gravity. As a red giant star gets a lot bigger, a point on the surface of the star is now farther from the center, and thus has less gravity. That\u2019s why the speed needed to escape the star goes down.[\/footnote]<\/sup><\/a> Radiation pressure, stellar pulsations, and violent events like the helium flash can all drive atoms in the outer atmosphere away from the star, and cause it to lose a substantial fraction of its mass into space. Astronomers estimate that by the time a star like the Sun reaches the point of the helium flash, for example, it will have lost as much as 25% of its mass. And it can lose still more mass when it ascends the red-giant branch for the second time. As a result, aging stars are surrounded by one or more expanding shells of gas, each containing as much as 10\u201320% of the Sun\u2019s mass (or 0.1\u20130.2 <em>M<\/em><sub>Sun<\/sub>).\r\n\r\nWhen nuclear energy generation in the carbon-oxygen core ceases, the star\u2019s core begins to shrink again and to heat up as it gets more and more compressed. (Remember that this compression will not be halted by another type of fusion in these low-mass stars.) The whole star follows along, shrinking and also becoming very hot\u2014reaching surface temperatures as high as 100,000 K. Such hot stars are very strong sources of stellar winds and ultraviolet radiation, which sweep outward into the shells of material ejected when the star was a red giant. The winds and the ultraviolet radiation heat the shells, ionize them, and set them aglow (just as ultraviolet radiation from hot, young stars produces H II regions; see <a class=\"target-chapter\" href=\".\/chapter\/introduction-to-between-the-stars-gas-and-dust-in-space\/\" target=\"_blank\" rel=\"noopener\">Between the Stars: Gas and Dust in Space<\/a>).\r\n\r\nThe result is the creation of some of the most beautiful objects in the cosmos (see the gallery in Figure 3. These objects were given an extremely misleading name when first found in the eighteenth century: <strong>planetary nebulae<\/strong>. The name is derived from the fact that a few planetary nebulae, when viewed through a small telescope, have a round shape bearing a superficial resemblance to planets. Actually, they have nothing to do with planets, but once names are put into regular use in astronomy, it is extremely difficult to change them. There are tens of thousands of planetary nebulae in our own Galaxy, although many are hidden from view because their light is absorbed by interstellar dust.\r\n\r\n&nbsp;\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"731\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170449\/OSC_Astro_22_04_Gallery.jpg\" alt=\"Gallery of Planetary Nebulae. Panel (a), at the upper left corner of this image, shows M 57, a fairly symmetrical ring of glowing gas surrounding the faint central star. Panel (b), at the upper right corner of this image, shows M 2-9, which appears like an elongated butterfly. The central star being the body and the gaseous \" width=\"731\" height=\"645\" \/> <strong>Gallery of Planetary Nebulae. <\/strong>[\/caption]\r\n\r\nAs Figure 3 shows, sometimes a planetary nebula appears to be a simple ring. Others have faint shells surrounding the bright ring, which is evidence that there were multiple episodes of <strong>mass loss<\/strong> when the star was a red giant (see image (d) in Figure 3. In a few cases, we see two lobes of matter flowing in opposite directions. Many astronomers think that a considerable number of planetary nebulae basically consist of the same structure, but that the shape we see depends on the viewing angle (Figure 4). According to this idea, the dying star is surrounded by a very dense, doughnut-shaped disk of gas. (Theorists do not yet have a definite explanation for why the dying star should produce this ring, but many believe that binary stars, which are common, are involved.)\r\n\r\n&nbsp;\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"447\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170452\/OSC_Astro_22_04_Model.jpg\" alt=\"Diagram to Explain the Different Shapes of Planetary Nebulae. In the lower left-hand portion of this figure, a schematic representation of a planetary nebula is shown. A yellow ellipse, labeled \" width=\"447\" height=\"429\" \/> <strong>Figure 4. Model to Explain the Different Shapes of Planetary Nebulae:<\/strong> The range of different shapes that we see among planetary nebulae may, in many cases, arise from the same geometric shape, but seen from a variety of viewing directions. The basic shape is a hot central star surrounded by a thick torus (or doughnut-shaped disk) of gas. The star\u2019s wind cannot flow out into space very easily in the direction of the torus, but can escape more freely in the two directions perpendicular to it. If we view the nebula along the direction of the flow (Helix Nebula), it will appear nearly circular (like looking directly down into an empty ice-cream cone). If we look along the equator of the torus, we see both outflows and a very elongated shape (Hubble 5). Current research on planetary nebulae focuses on the reasons for having a torus around the star in the first place. Many astronomers suggest that the basic cause may be that many of the central stars are actually close binary stars, rather than single stars. (credit \"Hubble 5\": modification of work by Bruce Balick (University of Washington), Vincent Icke (Leiden University, The Netherlands), Garrelt Mellema (Stockholm University), and NASA\/ESA; credit \"Helix\": modification of work by NASA, ESA, C.R. O\u2019Dell (Vanderbilt University), and M. Meixner, P. McCullough)[\/caption]\r\n\r\nAs the star continues to lose mass, any less dense gas that leaves the star cannot penetrate the torus, but the gas <em>can<\/em> flow outward in directions perpendicular to the disk. If we look perpendicular to the direction of outflow, we see the disk and both of the outward flows. If we look \"down the barrel\" and into the flows, we see a ring. At intermediate angles, we may see wonderfully complex structures. Compare the viewpoints in Figure 4 with the images in Figure 3.\r\n\r\nPlanetary nebula shells usually expand at speeds of 20\u201330 km\/s, and a typical planetary nebula has a diameter of about 1 light-year. If we assume that the gas shell has expanded at a constant speed, we can calculate that the shells of all the planetary nebulae visible to us were ejected within the past 50,000 years at most. After this amount of time, the shells have expanded so much that they are too thin and tenuous to be seen. That\u2019s a pretty short time that each planetary nebula can be observed (when compared to the whole lifetime of the star). Given the number of such nebulae we nevertheless see, we must conclude that a large fraction of all stars evolve through the planetary nebula phase. Since we saw that low-mass stars are much more common than high-mass stars, this confirms our view of planetary nebulae as sort of \"last gasp\" of low-mass star evolution.\r\n<h2>Cosmic Recycling<\/h2>\r\nThe loss of mass by dying stars is a key step in the gigantic cosmic recycling scheme we discussed in <a class=\"target-chapter\" href=\".\/chapter\/introduction-to-between-the-stars-gas-and-dust-in-space\/\" target=\"_blank\" rel=\"noopener\">Between the Stars: Gas and Dust in Space<\/a>. Remember that stars form from vast clouds of gas and dust. As they end their lives, stars return part of their gas to the galactic reservoirs of raw material. Eventually, some of the expelled material from aging stars will participate in the formation of new star systems.\r\n\r\nHowever, the atoms returned to the Galaxy by an aging star are not necessarily the same ones it received initially. The star, after all, has fused hydrogen and helium to form new elements over the course of its life. And during the red-giant stage, material from the star\u2019s central regions is dredged up and mixed with its outer layers, which can cause further nuclear reactions and the creation of still more new elements. As a result, the winds that blow outward from such stars include atoms that were \"newly minted\" inside the stars\u2019 cores. (As we will see, this mechanism is even more effective for high-mass stars, but it does work for stars with masses like that of the Sun.) In this way, the raw material of the Galaxy is not only resupplied but also receives infusions of new elements. You might say this cosmic recycling plan allows the universe to get more \"interesting\" all the time.\r\n<div class=\"textbox shaded\">\r\n<h3>The Red Giant Sun and the Fate of Earth<\/h3>\r\nHow will the evolution of the Sun affect conditions on Earth in the future? Although the Sun has appeared reasonably steady in size and luminosity over recorded human history, that brief span means nothing compared with the timescales we have been discussing. Let\u2019s examine the long-term prospects for our planet.\r\n\r\nThe Sun took its place on the zero-age main sequence approximately 4.5 billion years ago. At that time, it emitted only about 70% of the energy that it radiates today. One might expect that Earth would have been a lot colder than it is now, with the oceans frozen solid. But if this were the case, it would be hard to explain why simple life forms existed when Earth was less than a billion years old. Scientists now think that the explanation may be that much more carbon dioxide was present in Earth\u2019s atmosphere when it was young, and that a much stronger greenhouse effect kept Earth warm. (In the greenhouse effect, gases like carbon dioxide or water vapor allow the Sun\u2019s light to come in but do not allow the infrared radiation from the ground to escape back into space, so the temperature near Earth\u2019s surface increases.)\r\n\r\nCarbon dioxide in Earth\u2019s atmosphere has steadily declined as the Sun has increased in luminosity. As the brighter Sun increases the temperature of Earth, rocks weather faster and react with carbon dioxide, removing it from the atmosphere. The warmer Sun and the weaker greenhouse effect have kept Earth at a nearly constant temperature for most of its life. This remarkable coincidence, which has resulted in fairly stable climatic conditions, has been the key in the development of complex life-forms on our planet.\r\n\r\nAs a result of changes caused by the buildup of helium in its core, the Sun will continue to increase in luminosity as it grows older, and more and more radiation will reach Earth. For a while, the amount of carbon dioxide will continue to decrease. (Note that this effect counteracts increases in carbon dioxide from human activities, but on a much-too-slow timescale to undo the changes in climate that are likely to occur in the next 100 years.)\r\n\r\nEventually, the heating of Earth will melt the polar caps and increase the evaporation of the oceans. Water vapor is also an efficient greenhouse gas and will more than compensate for the decrease in carbon dioxide. Sooner or later (atmospheric models are not yet good enough to say exactly when, but estimates range from 500 million to 2 billion years), the increased water vapor will cause a runaway greenhouse effect.\r\n\r\nAbout 1 billion years from now, Earth will lose its water vapor. In the upper atmosphere, sunlight will break down water vapor into hydrogen, and the fast-moving hydrogen atoms will escape into outer space. Like Humpty Dumpty, the water molecules cannot be put back together again. Earth will start to resemble the Venus of today, and temperatures will become much too high for life as we know it.\r\n\r\nAll of this will happen before the Sun even becomes a red giant. Then the bad news really starts. The Sun, as it expands, will swallow Mercury and Venus, and friction with our star\u2019s outer atmosphere will make these planets spiral inward until they are completely vaporized. It is not completely clear whether Earth will escape a similar fate. As described in this chapter, the Sun will lose some of its mass as it becomes a red giant. The gravitational pull of the Sun decreases when it loses mass. The result would be that the diameter of Earth\u2019s orbit would increase (remember Kepler\u2019s third law). However, recent calculations also show that forces due to the tides raised on the Sun by Earth will act in the opposite direction, causing Earth\u2019s orbit to shrink. Thus, many astrophysicists conclude that Earth will be vaporized along with Mercury and Venus. Whether or not this dire prediction is true, there is little doubt that all life on Earth will surely be incinerated. But don\u2019t lose any sleep over this\u2014we are talking about events that will occur billions of years from now.\r\n\r\nWhat then are the prospects for preserving Earth life as we know it? The first strategy you might think of would be to move humanity to a more distant and cooler planet. However, calculations indicate that there are long periods of time (several hundred million years) when no planet is habitable. For example, Earth becomes far too warm for life long before Mars warms up enough.\r\n\r\nA better alternative may be to move the entire Earth progressively farther from the Sun. The idea is to use gravity in the same way NASA has used it to send spacecraft to distant planets. When a spacecraft flies near a planet, the planet\u2019s motion can be used to speed up the spacecraft, slow it down, or redirect it. Calculations show that if we were to redirect an asteroid so that it follows just the right orbit between Earth and Jupiter, it could transfer orbital energy from Jupiter to Earth and move Earth slowly outward, pulling us away from the expanding Sun on each flyby. Since we have hundreds of millions of years to change Earth\u2019s orbit, the effect of each flyby need not be large. (Of course, the people directing the asteroid had better get the orbit exactly right and not cause the asteroid to hit Earth.)\r\n\r\nIt may seem crazy to think about projects to move an entire planet to a different orbit. But remember that we are talking about the distant future. If, by some miracle, human beings are able to get along for all that time and don\u2019t blow ourselves to bits, our technology is likely to be far more sophisticated than it is today. It may also be that if humans survive for hundreds of millions of years, we may spread to planets or habitats around other stars. Indeed, Earth, by then, might be a museum world to which youngsters from other planets return to learn about the origin of our species. It is also possible that evolution will by then have changed us in ways that allow us to survive in very different environments. Wouldn\u2019t it be exciting to see how the story of the story of the human race turns out after all those billions of years?\r\n\r\n<\/div>\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Key Concepts and Summary<\/h3>\r\nAfter stars become red giants, their cores eventually become hot enough to produce energy by fusing helium to form carbon (and sometimes a bit of oxygen.) The fusion of three helium nuclei produces carbon through the triple-alpha process. The rapid onset of helium fusion in the core of a low-mass star is called the helium flash. After this, the star becomes stable and reduces its luminosity and size briefly. In stars with masses about twice the mass of the Sun or less, fusion stops after the helium in the core has been exhausted. Fusion of hydrogen and helium in shells around the contracting core makes the star a bright red giant again, but only temporarily. When the star is a red giant, it can shed its outer layers and thereby expose hot inner layers. Planetary nebulae (which have nothing to do with planets) are shells of gas ejected by such stars, set glowing by the ultraviolet radiation of the dying central star.\r\n\r\n<\/div>\r\n<h3>Glossary<\/h3>\r\n<strong>helium flash: <\/strong>\r\n\r\na nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star\r\n\r\n<strong>planetary nebula: <\/strong>\r\n\r\na shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the central star)\r\n\r\n<strong>triple-alpha process: <\/strong>\r\n\r\na nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Explain what happens in a star\u2019s core when all of the hydrogen has been used up<\/li>\n<li>Define &#8220;planetary nebulae&#8221; and discuss their origin<\/li>\n<li>Discuss the creation of new chemical elements during the late stages of <strong>stellar evolution<\/strong><\/li>\n<\/ul>\n<\/div>\n<p>The &#8220;life story&#8221; we have related so far applies to almost all stars: each starts as a contracting protostar, then lives most of its life as a stable main-sequence star, and eventually moves off the main sequence toward the red-giant region.<\/p>\n<p>As we have seen, the pace at which each star goes through these stages depends on its mass, with more massive stars evolving more quickly. But after this point, the life stories of stars of different masses diverge, with a wider range of possible behavior according to their masses, their compositions, and the presence of any nearby companion stars.<\/p>\n<p>Because we have written this book for students taking their first astronomy course, we will recount a simplified version of what happens to stars as they move toward the final stages in their lives. We will (perhaps to your heartfelt relief) not delve into all the possible ways aging stars can behave and the strange things that happen when a star is orbited by a second star in a binary system. Instead, we will focus only on the key stages in the evolution of single stars and show how the evolution of high-mass stars differs from that of low-mass stars (such as our Sun).<\/p>\n<h2>Helium Fusion<\/h2>\n<p>Let\u2019s begin by considering stars with composition like that of the Sun and whose <em>initial<\/em> masses are comparatively low\u2014no more than about twice the mass of our Sun. (Such mass may not seem too low, but stars with masses less than this all behave in a fairly similar fashion. We will see what happens to more massive stars in the next section.) Because there are much more low-mass stars than high-mass stars in the Milky Way, the vast majority of stars\u2014including our Sun\u2014follow the scenario we are about to relate. By the way, we carefully used the term <em>initial masses<\/em> of stars because, as we will see, stars can lose quite a bit of mass in the process of aging and dying.<\/p>\n<p>Remember that red giants start out with a helium core where no energy generation is taking place, surrounded by a shell where hydrogen is undergoing <strong>fusion<\/strong>. The core, having no source of energy to oppose the inward pull of gravity, is shrinking and growing hotter. As time goes on, the temperature in the core can rise to much hotter values than it had in its main-sequence days. Once it reaches a temperature of 100 million K (but not before such point), three helium atoms can begin to fuse to form a single carbon nucleus. This process is called the <strong>triple-alpha process<\/strong>, so named because physicists call the nucleus of the helium atom an alpha particle.<\/p>\n<p>When the triple-alpha process begins in low-mass (about 0.8 to 2.0 solar masses) stars, calculations show that the entire core is ignited in a quick burst of fusion called a <strong>helium flash<\/strong>. (More massive stars also ignite helium but more gradually and not with a flash.) As soon as the temperature at the center of the star becomes high enough to start the triple-alpha process, the extra energy released is transmitted quickly through the entire helium core, producing very rapid heating. The heating speeds up the nuclear reactions, which provide more heating, and which accelerates the nuclear reactions even more. We have runaway generation of energy, which reignites the entire helium core in a flash.<\/p>\n<p>You might wonder why the next major step in nuclear fusion in stars involves three helium nuclei and not just two. Although it is a lot easier to get two helium nuclei to collide, the product of this collision is not stable and falls apart very quickly. It takes three helium nuclei coming together <em>simultaneously<\/em> to make a stable nuclear structure. Given that each helium nucleus has two positive protons and that such protons repel one another, you can begin to see the problem. It takes a temperature of 100 million K to slam three helium nuclei (six protons) together and make them stick. But when that happens, the star produces a carbon nucleus.<\/p>\n<div class=\"textbox shaded\">\n<h3>Stars in Your Little Finger<\/h3>\n<p>Stop reading for a moment and look at your little finger. It\u2019s full of carbon atoms because carbon is a fundamental chemical building block for life on Earth. Each of those carbon atoms was once inside a red giant star and was fused from helium nuclei in the triple-alpha process. All the carbon on Earth\u2014in you, in the charcoal you use for barbecuing, and in the diamonds you might exchange with a loved one\u2014was &#8220;cooked up&#8221; by previous generations of stars. How the carbon atoms (and other elements) made their way from inside some of those stars to become part of Earth is something we will discuss in the next chapter. For now, we want to emphasize that our description of stellar evolution is, in a very real sense, the story of our own cosmic &#8220;roots&#8221;\u2014the history of how our own atoms originated among the stars. We are made of &#8220;star-stuff.&#8221;<\/p>\n<\/div>\n<h2>Becoming a Giant Again<\/h2>\n<p>After the helium flash, the star, having survived the &#8220;energy crisis&#8221; that followed the end of the main-sequence stage and the exhaustion of the hydrogen fuel at its center, finds its balance again. As the star readjusts to the release of energy from the triple-alpha process in its core, its internal structure changes once more: its surface temperature increases and its overall luminosity decreases. The point that represents the star on the H\u2013R diagram thus moves to a new position to the left of and somewhat below its place as a red giant (Figure 1). The star then continues to fuse the helium in its core for a while, returning to the kind of equilibrium between pressure and gravity that characterized the main-sequence stage. During this time, a newly formed carbon nucleus at the center of the star can sometimes be joined by another helium nucleus to produce a nucleus of oxygen\u2014another building block of life.<\/p>\n<p>&nbsp;<\/p>\n<div style=\"width: 477px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170443\/OSC_Astro_22_04_Evaluation.jpg\" alt=\"Evolution of a Star like the Sun on an H\u2013R Diagram. In this plot the vertical axis is labeled\" width=\"467\" height=\"512\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 1. Evolution of a Star Like the Sun on an H\u2013R Diagram: <\/strong> Each stage in the star\u2019s life is labeled. (a) The star evolves from the main sequence to be a red giant, decreasing in surface temperature and increasing in luminosity. (b) A helium flash occurs, leading to a readjustment of the star\u2019s internal structure and to (c) a brief period of stability during which helium is fused to carbon and oxygen in the core (in the process the star becomes hotter and less luminous than it was as a red giant). (d) After the central helium is exhausted, the star becomes a giant again and moves to higher luminosity and lower temperature. By this time, however, the star has exhausted its inner resources and will soon begin to die. Where the evolutionary track becomes a dashed line, the changes are so rapid that they are difficult to model.<\/p>\n<\/div>\n<p>However, at a temperature of 100 million K, the inner core is converting its helium fuel or carbon (and a bit of oxygen) at a rapid rate. Thus, the new period of stability cannot last very long: it is far shorter than the main-sequence stage. Soon, all the helium hot enough for fusion will be used up, just like the hot hydrogen that was used up earlier in the star\u2019s evolution. Once again, the inner core will not be able to generate energy via fusion. Once more, gravity will take over, and the core will start to shrink again. We can think of stellar evolution as a story of a constant struggle against gravitational collapse. A star can avoid collapsing as long as it can tap energy sources, but once any particular fuel is used up, it starts to collapse again.<\/p>\n<p>The star\u2019s situation is analogous to the end of the main-sequence stage (when the central hydrogen got used up), but the star now has a somewhat more complicated structure. Again, the star\u2019s core begins to collapse under its own weight. Heat released by the shrinking of the carbon and oxygen core flows into a shell of helium just above the core. This helium, which had not been hot enough for fusion into carbon earlier, is heated just enough for fusion to begin and to generate a new flow of energy.<\/p>\n<p>Farther out in the star, there is also a shell where fresh hydrogen has been heated enough to fuse helium. The star now has a multi-layered structure like an onion: a carbon-oxygen core, surrounded by a shell of helium fusion, a layer of helium, a shell of hydrogen fusion, and finally, the extended outer layers of the star (see Figure 2). As energy flows outward from the two fusion shells, once again the outer regions of the star begin to expand. Its brief period of stability is over; the star moves back to the red-giant domain on the H\u2013R diagram for a short time (see Figure 1). But this is a brief and final burst of glory.<\/p>\n<div style=\"width: 985px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170446\/OSC_Astro_22_04_Layers.jpg\" alt=\"Layers inside a Low-mass Star before Death. The layers within the core are shown as concentric circles of various colors. Starting at the center are:\" width=\"975\" height=\"413\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 2. Layers inside a Low-Mass Star before Death: <\/strong> Here we see the layers inside a star with an initial mass that is less than twice the mass of the Sun. These include, from the center outward, the carbon-oxygen core, a layer of helium hot enough to fuse, a layer of cooler helium, a layer of hydrogen hot enough to fuse, and then cooler hydrogen beyond.<\/p>\n<\/div>\n<p>Recall that the last time the star was in this predicament, helium fusion came to its rescue. The temperature at the star\u2019s center eventually became hot enough for the <em>product<\/em> of the previous step of fusion (helium) to become the <em>fuel<\/em> for the next step (helium fusing into carbon). But the step after the fusion of helium nuclei requires a temperature so hot that the kinds of lower-mass stars (less than 2 solar masses) we are discussing simply cannot compress their cores to reach it. No further types of fusion are possible for such a star.<\/p>\n<p>In a star with a mass similar to that of the Sun, the formation of a carbon-oxygen core thus marks the end of the generation of nuclear energy at the center of the star. The star must now confront the fact that its death is near. We will discuss how stars like this end their lives in <a class=\"target-chapter\" href=\".\/chapter\/introduction-to-the-death-of-stars\/\" target=\"_blank\" rel=\"noopener\">The Death of Stars<\/a>, but in the meantime, Table 1 summarizes the stages discussed so far in the life of a star with the same mass as that of the Sun. One thing that gives us confidence in our calculations of stellar evolution is that when we make H\u2013R diagrams of older clusters, we actually see stars in each of the stages that we have been discussing.<\/p>\n<table id=\"fs-id1168048452256\" class=\"span-all\" summary=\"This table contains five columns and five rows. The first row is a header row, and it labels each column,\">\n<thead>\n<tr valign=\"top\">\n<th colspan=\"5\">Table 1. The Evolution of a Star with the Sun\u2019s Mass<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th>Stage<\/th>\n<th>Time in This Stage (years)<\/th>\n<th>Surface Temperature (K)<\/th>\n<th>Luminosity (<em>L<\/em><sub>Sun<\/sub>)<\/th>\n<th>Diameter<\/p>\n<p>(Sun = 1)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>Main sequence<\/td>\n<td>11 billion<\/td>\n<td>6000<\/td>\n<td>1<\/td>\n<td>1<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Becomes red giant<\/td>\n<td>1.3 billion<\/td>\n<td>3100 at minimum<\/td>\n<td>2300 at maximum<\/td>\n<td>165<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Helium fusion<\/td>\n<td>100 million<\/td>\n<td>4800<\/td>\n<td>50<\/td>\n<td>10<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Giant again<\/td>\n<td>20 million<\/td>\n<td>3100<\/td>\n<td>5200<\/td>\n<td>180<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2>Mass Loss from Red-Giant Stars and the Formation of Planetary Nebulae<\/h2>\n<p>When stars swell up to become red giants, they have very large radii and therefore a low escape velocity.<a href=\"#footnote1\" name=\"footnote-ref1\"><sup><a class=\"footnote\" title=\"Recall that the force of gravity depends not only on the mass doing the pulling, but also on our distance from the center of gravity. As a red giant star gets a lot bigger, a point on the surface of the star is now farther from the center, and thus has less gravity. That\u2019s why the speed needed to escape the star goes down.\" id=\"return-footnote-690-1\" href=\"#footnote-690-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a><\/sup><\/a> Radiation pressure, stellar pulsations, and violent events like the helium flash can all drive atoms in the outer atmosphere away from the star, and cause it to lose a substantial fraction of its mass into space. Astronomers estimate that by the time a star like the Sun reaches the point of the helium flash, for example, it will have lost as much as 25% of its mass. And it can lose still more mass when it ascends the red-giant branch for the second time. As a result, aging stars are surrounded by one or more expanding shells of gas, each containing as much as 10\u201320% of the Sun\u2019s mass (or 0.1\u20130.2 <em>M<\/em><sub>Sun<\/sub>).<\/p>\n<p>When nuclear energy generation in the carbon-oxygen core ceases, the star\u2019s core begins to shrink again and to heat up as it gets more and more compressed. (Remember that this compression will not be halted by another type of fusion in these low-mass stars.) The whole star follows along, shrinking and also becoming very hot\u2014reaching surface temperatures as high as 100,000 K. Such hot stars are very strong sources of stellar winds and ultraviolet radiation, which sweep outward into the shells of material ejected when the star was a red giant. The winds and the ultraviolet radiation heat the shells, ionize them, and set them aglow (just as ultraviolet radiation from hot, young stars produces H II regions; see <a class=\"target-chapter\" href=\".\/chapter\/introduction-to-between-the-stars-gas-and-dust-in-space\/\" target=\"_blank\" rel=\"noopener\">Between the Stars: Gas and Dust in Space<\/a>).<\/p>\n<p>The result is the creation of some of the most beautiful objects in the cosmos (see the gallery in Figure 3. These objects were given an extremely misleading name when first found in the eighteenth century: <strong>planetary nebulae<\/strong>. The name is derived from the fact that a few planetary nebulae, when viewed through a small telescope, have a round shape bearing a superficial resemblance to planets. Actually, they have nothing to do with planets, but once names are put into regular use in astronomy, it is extremely difficult to change them. There are tens of thousands of planetary nebulae in our own Galaxy, although many are hidden from view because their light is absorbed by interstellar dust.<\/p>\n<p>&nbsp;<\/p>\n<div style=\"width: 741px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170449\/OSC_Astro_22_04_Gallery.jpg\" alt=\"Gallery of Planetary Nebulae. Panel (a), at the upper left corner of this image, shows M 57, a fairly symmetrical ring of glowing gas surrounding the faint central star. Panel (b), at the upper right corner of this image, shows M 2-9, which appears like an elongated butterfly. The central star being the body and the gaseous\" width=\"731\" height=\"645\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Gallery of Planetary Nebulae. <\/strong><\/p>\n<\/div>\n<p>As Figure 3 shows, sometimes a planetary nebula appears to be a simple ring. Others have faint shells surrounding the bright ring, which is evidence that there were multiple episodes of <strong>mass loss<\/strong> when the star was a red giant (see image (d) in Figure 3. In a few cases, we see two lobes of matter flowing in opposite directions. Many astronomers think that a considerable number of planetary nebulae basically consist of the same structure, but that the shape we see depends on the viewing angle (Figure 4). According to this idea, the dying star is surrounded by a very dense, doughnut-shaped disk of gas. (Theorists do not yet have a definite explanation for why the dying star should produce this ring, but many believe that binary stars, which are common, are involved.)<\/p>\n<p>&nbsp;<\/p>\n<div style=\"width: 457px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03170452\/OSC_Astro_22_04_Model.jpg\" alt=\"Diagram to Explain the Different Shapes of Planetary Nebulae. In the lower left-hand portion of this figure, a schematic representation of a planetary nebula is shown. A yellow ellipse, labeled\" width=\"447\" height=\"429\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 4. Model to Explain the Different Shapes of Planetary Nebulae:<\/strong> The range of different shapes that we see among planetary nebulae may, in many cases, arise from the same geometric shape, but seen from a variety of viewing directions. The basic shape is a hot central star surrounded by a thick torus (or doughnut-shaped disk) of gas. The star\u2019s wind cannot flow out into space very easily in the direction of the torus, but can escape more freely in the two directions perpendicular to it. If we view the nebula along the direction of the flow (Helix Nebula), it will appear nearly circular (like looking directly down into an empty ice-cream cone). If we look along the equator of the torus, we see both outflows and a very elongated shape (Hubble 5). Current research on planetary nebulae focuses on the reasons for having a torus around the star in the first place. Many astronomers suggest that the basic cause may be that many of the central stars are actually close binary stars, rather than single stars. (credit &#8220;Hubble 5&#8221;: modification of work by Bruce Balick (University of Washington), Vincent Icke (Leiden University, The Netherlands), Garrelt Mellema (Stockholm University), and NASA\/ESA; credit &#8220;Helix&#8221;: modification of work by NASA, ESA, C.R. O\u2019Dell (Vanderbilt University), and M. Meixner, P. McCullough)<\/p>\n<\/div>\n<p>As the star continues to lose mass, any less dense gas that leaves the star cannot penetrate the torus, but the gas <em>can<\/em> flow outward in directions perpendicular to the disk. If we look perpendicular to the direction of outflow, we see the disk and both of the outward flows. If we look &#8220;down the barrel&#8221; and into the flows, we see a ring. At intermediate angles, we may see wonderfully complex structures. Compare the viewpoints in Figure 4 with the images in Figure 3.<\/p>\n<p>Planetary nebula shells usually expand at speeds of 20\u201330 km\/s, and a typical planetary nebula has a diameter of about 1 light-year. If we assume that the gas shell has expanded at a constant speed, we can calculate that the shells of all the planetary nebulae visible to us were ejected within the past 50,000 years at most. After this amount of time, the shells have expanded so much that they are too thin and tenuous to be seen. That\u2019s a pretty short time that each planetary nebula can be observed (when compared to the whole lifetime of the star). Given the number of such nebulae we nevertheless see, we must conclude that a large fraction of all stars evolve through the planetary nebula phase. Since we saw that low-mass stars are much more common than high-mass stars, this confirms our view of planetary nebulae as sort of &#8220;last gasp&#8221; of low-mass star evolution.<\/p>\n<h2>Cosmic Recycling<\/h2>\n<p>The loss of mass by dying stars is a key step in the gigantic cosmic recycling scheme we discussed in <a class=\"target-chapter\" href=\".\/chapter\/introduction-to-between-the-stars-gas-and-dust-in-space\/\" target=\"_blank\" rel=\"noopener\">Between the Stars: Gas and Dust in Space<\/a>. Remember that stars form from vast clouds of gas and dust. As they end their lives, stars return part of their gas to the galactic reservoirs of raw material. Eventually, some of the expelled material from aging stars will participate in the formation of new star systems.<\/p>\n<p>However, the atoms returned to the Galaxy by an aging star are not necessarily the same ones it received initially. The star, after all, has fused hydrogen and helium to form new elements over the course of its life. And during the red-giant stage, material from the star\u2019s central regions is dredged up and mixed with its outer layers, which can cause further nuclear reactions and the creation of still more new elements. As a result, the winds that blow outward from such stars include atoms that were &#8220;newly minted&#8221; inside the stars\u2019 cores. (As we will see, this mechanism is even more effective for high-mass stars, but it does work for stars with masses like that of the Sun.) In this way, the raw material of the Galaxy is not only resupplied but also receives infusions of new elements. You might say this cosmic recycling plan allows the universe to get more &#8220;interesting&#8221; all the time.<\/p>\n<div class=\"textbox shaded\">\n<h3>The Red Giant Sun and the Fate of Earth<\/h3>\n<p>How will the evolution of the Sun affect conditions on Earth in the future? Although the Sun has appeared reasonably steady in size and luminosity over recorded human history, that brief span means nothing compared with the timescales we have been discussing. Let\u2019s examine the long-term prospects for our planet.<\/p>\n<p>The Sun took its place on the zero-age main sequence approximately 4.5 billion years ago. At that time, it emitted only about 70% of the energy that it radiates today. One might expect that Earth would have been a lot colder than it is now, with the oceans frozen solid. But if this were the case, it would be hard to explain why simple life forms existed when Earth was less than a billion years old. Scientists now think that the explanation may be that much more carbon dioxide was present in Earth\u2019s atmosphere when it was young, and that a much stronger greenhouse effect kept Earth warm. (In the greenhouse effect, gases like carbon dioxide or water vapor allow the Sun\u2019s light to come in but do not allow the infrared radiation from the ground to escape back into space, so the temperature near Earth\u2019s surface increases.)<\/p>\n<p>Carbon dioxide in Earth\u2019s atmosphere has steadily declined as the Sun has increased in luminosity. As the brighter Sun increases the temperature of Earth, rocks weather faster and react with carbon dioxide, removing it from the atmosphere. The warmer Sun and the weaker greenhouse effect have kept Earth at a nearly constant temperature for most of its life. This remarkable coincidence, which has resulted in fairly stable climatic conditions, has been the key in the development of complex life-forms on our planet.<\/p>\n<p>As a result of changes caused by the buildup of helium in its core, the Sun will continue to increase in luminosity as it grows older, and more and more radiation will reach Earth. For a while, the amount of carbon dioxide will continue to decrease. (Note that this effect counteracts increases in carbon dioxide from human activities, but on a much-too-slow timescale to undo the changes in climate that are likely to occur in the next 100 years.)<\/p>\n<p>Eventually, the heating of Earth will melt the polar caps and increase the evaporation of the oceans. Water vapor is also an efficient greenhouse gas and will more than compensate for the decrease in carbon dioxide. Sooner or later (atmospheric models are not yet good enough to say exactly when, but estimates range from 500 million to 2 billion years), the increased water vapor will cause a runaway greenhouse effect.<\/p>\n<p>About 1 billion years from now, Earth will lose its water vapor. In the upper atmosphere, sunlight will break down water vapor into hydrogen, and the fast-moving hydrogen atoms will escape into outer space. Like Humpty Dumpty, the water molecules cannot be put back together again. Earth will start to resemble the Venus of today, and temperatures will become much too high for life as we know it.<\/p>\n<p>All of this will happen before the Sun even becomes a red giant. Then the bad news really starts. The Sun, as it expands, will swallow Mercury and Venus, and friction with our star\u2019s outer atmosphere will make these planets spiral inward until they are completely vaporized. It is not completely clear whether Earth will escape a similar fate. As described in this chapter, the Sun will lose some of its mass as it becomes a red giant. The gravitational pull of the Sun decreases when it loses mass. The result would be that the diameter of Earth\u2019s orbit would increase (remember Kepler\u2019s third law). However, recent calculations also show that forces due to the tides raised on the Sun by Earth will act in the opposite direction, causing Earth\u2019s orbit to shrink. Thus, many astrophysicists conclude that Earth will be vaporized along with Mercury and Venus. Whether or not this dire prediction is true, there is little doubt that all life on Earth will surely be incinerated. But don\u2019t lose any sleep over this\u2014we are talking about events that will occur billions of years from now.<\/p>\n<p>What then are the prospects for preserving Earth life as we know it? The first strategy you might think of would be to move humanity to a more distant and cooler planet. However, calculations indicate that there are long periods of time (several hundred million years) when no planet is habitable. For example, Earth becomes far too warm for life long before Mars warms up enough.<\/p>\n<p>A better alternative may be to move the entire Earth progressively farther from the Sun. The idea is to use gravity in the same way NASA has used it to send spacecraft to distant planets. When a spacecraft flies near a planet, the planet\u2019s motion can be used to speed up the spacecraft, slow it down, or redirect it. Calculations show that if we were to redirect an asteroid so that it follows just the right orbit between Earth and Jupiter, it could transfer orbital energy from Jupiter to Earth and move Earth slowly outward, pulling us away from the expanding Sun on each flyby. Since we have hundreds of millions of years to change Earth\u2019s orbit, the effect of each flyby need not be large. (Of course, the people directing the asteroid had better get the orbit exactly right and not cause the asteroid to hit Earth.)<\/p>\n<p>It may seem crazy to think about projects to move an entire planet to a different orbit. But remember that we are talking about the distant future. If, by some miracle, human beings are able to get along for all that time and don\u2019t blow ourselves to bits, our technology is likely to be far more sophisticated than it is today. It may also be that if humans survive for hundreds of millions of years, we may spread to planets or habitats around other stars. Indeed, Earth, by then, might be a museum world to which youngsters from other planets return to learn about the origin of our species. It is also possible that evolution will by then have changed us in ways that allow us to survive in very different environments. Wouldn\u2019t it be exciting to see how the story of the story of the human race turns out after all those billions of years?<\/p>\n<\/div>\n<div class=\"textbox learning-objectives\">\n<h3>Key Concepts and Summary<\/h3>\n<p>After stars become red giants, their cores eventually become hot enough to produce energy by fusing helium to form carbon (and sometimes a bit of oxygen.) The fusion of three helium nuclei produces carbon through the triple-alpha process. The rapid onset of helium fusion in the core of a low-mass star is called the helium flash. After this, the star becomes stable and reduces its luminosity and size briefly. In stars with masses about twice the mass of the Sun or less, fusion stops after the helium in the core has been exhausted. Fusion of hydrogen and helium in shells around the contracting core makes the star a bright red giant again, but only temporarily. When the star is a red giant, it can shed its outer layers and thereby expose hot inner layers. Planetary nebulae (which have nothing to do with planets) are shells of gas ejected by such stars, set glowing by the ultraviolet radiation of the dying central star.<\/p>\n<\/div>\n<h3>Glossary<\/h3>\n<p><strong>helium flash: <\/strong><\/p>\n<p>a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star<\/p>\n<p><strong>planetary nebula: <\/strong><\/p>\n<p>a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the central star)<\/p>\n<p><strong>triple-alpha process: <\/strong><\/p>\n<p>a nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus<\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-690\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Astronomy. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1\">http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1.<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-690-1\">Recall that the force of gravity depends not only on the mass doing the pulling, but also on our distance from the center of gravity. As a red giant star gets a lot bigger, a point on the surface of the star is now farther from the center, and thus has less gravity. That\u2019s why the speed needed to escape the star goes down. <a href=\"#return-footnote-690-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":5,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Astronomy\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1.\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-690","chapter","type-chapter","status-publish","hentry"],"part":666,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/690","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":6,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/690\/revisions"}],"predecessor-version":[{"id":2372,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/690\/revisions\/2372"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/parts\/666"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/690\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/media?parent=690"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapter-type?post=690"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/contributor?post=690"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/license?post=690"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}