12.4: Further Evolution of Stars

Older Stars

So far, the life story 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. During this time, the star continues the process of nucleosynthesis, building heavy elements out of lighter ones. 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. Instead of reviewing all the possible ways aging stars can behave, 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.

Helium Fusion

This section will discuss the aging of stars with initial masses up to twice the mass of the Sun, or 0.8 to 2.0 M_Sun. We consider the initial masses of stars because, as we will see, stars can lose a lot of mass in the process of aging and dying. Because there are many more low-mass stars than high-mass stars in the Milky Way, the vast majority of stars, including our Sun, follow the scenario we are about to relate.

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 fusion. 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 100 million K. At this temperature, three helium atoms can begin to fuse to form a single carbon nucleus. This process is called the triple-alpha process, 3He→ C. It is named after the nucleus of the helium atom, also known as an alpha particle.

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. Each helium nucleus has two positive protons, and positive charges repel each other with a force that gets stronger with the number of particles. It takes a temperature of 100 million K to slam three helium nuclei, six protons, together and make them stick. When that happens, the star produces a carbon nucleus.

When the triple-alpha process begins in low-mass stars, calculations show that the entire core is ignited in a quick burst of fusion called a helium flash. 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.

Stop reading for a moment and look at your little finger. It’s 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 was fused together by previous generations of stars. Remember that our own atoms originated among the stars. We are all made of stardust.

Becoming a Giant Again

To review, a low-mass star, like the Sun, spends most of its lifetime on the main sequence, fusing hydrogen. Figure \(\PageIndex{1}\) is an H-R diagram with the evolutionary track, the black line with arrows, of a star with the same initial mass as the Sun. Each section of the track is labeled (a) through (d). The section of the track labeled (a) is when the star runs out of hydrogen in its core, it moves off of the Main sequence, which is represented by a red line, to the upper right of the chart. The star's surface temperature decreases and its luminosity increases as it becomes a red giant. When it is hot enough, it begins fusing helium into carbon. The section of the track labeled (b) represents the core helium flash in the upper right corner of the chart.

After the helium flash, the star readjusts to the release of energy from the triple-alpha process in its core. Its surface temperature increases and its overall luminosity decreases. This is represented by a dashed line that moves down and to the left of section (b) and stops at a point halfway between section (b) and the main sequence. The dashed line represents a section of the star's evolution where the changes are so rapid that they are difficult to model. 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. This is represented by the end of the dashed-line section, labeled (c). 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, another building block of life.

Figure \(\PageIndex{1}\): Evolutionary Path of the Sun. After leaving the main sequence, the Sun will go through several phases of increasing and decreasing luminosity and temperature. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{1}\).

However, at a temperature of 100 million K, the inner core is converting its helium fuel to carbon, and a bit of oxygen, at a rapid rate. The new period of stability is much 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’s 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. The shrinking of the core releases heat that makes the star decrease in temperature and grow in size and luminosity, becoming a giant again. The section of the track labeled (d) extends up and to the right from section (c) and ends close to section (b). Heat released by the shrinking of the carbon and oxygen core also 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. The end point of track section (d) is labeled Helium shell flashes.

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. Figure \(\PageIndex{2}\) is an illustration of a star at this stage, including 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. 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–R diagram for a short time. But this is a brief and final burst of glory.

Figure \(\PageIndex{2}\): Layers in an Old Low-Mass Star. A low-mass star near the end of its life has a layered interior structure separated by composition. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{2}\).

Recall that the last time the star was in this predicament, helium fusion came to its rescue. The temperature at the star’s center eventually became hot enough for the product of the previous step of fusion, helium, to become the fuel for the next step, helium fusing into carbon. But the step after the fusion of helium nuclei requires a temperature so hot that lower-mass stars cannot compress their cores to reach it. No further types of fusion are possible for such a star.

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. Table \(\PageIndex{1}\) 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–R diagrams of older clusters, we actually see stars in each of the stages that we have been discussing.

Table \(\PageIndex{1}\): The Evolution of a Star with the Sun’s Mass

Stage	Time in This Stage (years)	Surface Temperature (K)	Luminosity (LSun)	Diameter (Sun = 1)
Main sequence	11 billion	6000	1	1
Becomes red giant	1.3 billion	3100 at minimum	2300 at maximum	165
Helium fusion	100 million	4800	50	10
Giant again	20 million	3100	5200	180

Formation of Planetary Nebulae

When stars swell up to become red giants, they have very large radii and therefore a low escape velocity. The force of gravity depends not only on the mass doing the pulling, but also on the 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’s why the speed needed to escape the star goes down. 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 10–20% of the Sun’s mass, or 0.1–0.2 M_Sun.

When nuclear energy generation in the carbon-oxygen core ceases, the star’s core begins to shrink again and to heat up as it gets more and more compressed. 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, reaching 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.

The result is the creation of some of the most beautiful objects in the cosmos, planetary nebula. 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. Figure \(\PageIndex{3}\) includes four Hubble Space Telescope images of example planetary nebulae, labeled (a) through (d).

Image (a) is the Ring Nebula (M57). It is located about 2000 light-years away in the constellation of Lyra. The ring is about 1 light-year in diameter, and the central star has a temperature of about 120,000 K. Careful study of this image has shown scientists that, instead of looking at a spherical shell around this dying star, we may be looking down the barrel of a tube or cone. The blue region shows emission from very hot helium, which is located very close to the star. The red region isolates emission from ionized nitrogen, which is radiated by the coolest gas farthest from the star. The green region represents oxygen emission, which is produced at intermediate temperatures and is at an intermediate distance from the star.

Image (b) is M2-9, an example of a butterfly nebula. The central star, which is part of a binary system, has ejected mass preferentially in two opposite directions. In other images, a disk, perpendicular to the two long streams of gas, can be seen around the two stars in the middle. The stellar outburst that resulted in the expulsion of matter occurred about 1200 years ago. Neutral oxygen is shown in red, once-ionized nitrogen in green, and twice-ionized oxygen in blue. The planetary nebula is about 2100 light-years away in the constellation of Ophiuchus.

Image (c) includes NGC 6751. The blue regions mark the hottest gas, which forms a ring around the central star. The orange and red regions show the locations of cooler gas. The origin of these cool streamers is not known, but their shapes indicate that they are affected by radiation and stellar winds from the hot star at the center. The temperature of the star is about 140,000 K. The diameter of the nebula is about 600 times larger than the diameter of the Solar System. The nebula is about 6500 light-years away in the constellation of Aquila.

Image (d) is NGC 7027 and demonstrates several stages of mass loss. The faint blue concentric shells surrounding the central region identify the mass that was shed slowly from the surface of the star when it became a red giant. Somewhat later, the remaining outer layers were ejected but not in a spherically symmetric way. The dense clouds formed by this late ejection produce the bright inner regions. The hot central star can be seen faintly near the center of the object. NGC 7027 is about 3000 light-years away in the direction of the constellation of Cygnus.

Figure \(\PageIndex{3}\): Planetary Nebulae. Four images of planetary nebulae (a) M57, (b) M2-9, (c) NGC 6751, and (d) NGC 7027 all have a star surrounded by gas, but differ in the shape of the gas. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\) demonstrates that planetary nebulae can be many different shapes. 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 mass loss when the star was a red giant, such as Image (d) in Figure \(\PageIndex{3}\). 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. 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 think that the structure is caused by binary stars.

Figure \(\PageIndex{4}\) is a schematic of a planetary nebula and examples of how it would appear when viewed from different locations. The basic shape is a hot central star surrounded by a thick torus, or doughnut-shaped disk of gas. The star’s 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, as we may be observing the Helix Nebula, it will appear nearly circular. If we look along the equator of the torus, we see both outflows and a very elongated shape. Hubble 5 may be an example of this viewing geometry.

Figure \(\PageIndex{4}\): Observation of Planetary Nebulae. This diagram demonstrates how planetary nebulae look different depending on the position of the observer, with examples: the Helix Nebula, and Hubble 5. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{4}\).

Planetary nebula shells usually expand at speeds of 20–30 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’s 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 the end of low-mass star evolution.

Cosmic Recycling

The loss of mass by dying stars is a key step in a gigantic cosmic recycling scheme. 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. However, the atoms returned 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. During the red-giant stage, material from the star’s 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 created inside the stars’ cores. In this way, the raw material of the Milky Way Galaxy is not only resupplied but also receives infusions of new elements.

The Fate of the Sun and Earth

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’s examine the long-term prospects for our planet.

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’s atmosphere when it was young, and that a much stronger greenhouse effect kept Earth warm. The greenhouse effect is when gases like carbon dioxide or water vapor allow the Sun’s light to reach Earth's surface. However, they absorb the infrared radiation from the ground preventing its escape back into space, and increasing the temperature near Earth’s surface.

Carbon dioxide in Earth’s atmosphere has steadily declined as the Sun has increased in luminosity. As the brighter Sun increases the temperature of Earth, the warmer ocean absorbs carbon dioxide and converts it to minerals that settle on the bottom of the ocean. Plant life also absorbs carbon dioxide and releases oxygen, which is not a greenhouse gas. 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.

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 light will reach Earth. For a while, the amount of carbon dioxide will continue to decrease. This effect counteracts increases in carbon dioxide from human activities, but on a much slower timescale. It will be unable to undo the changes in climate that are likely to occur in the next 100 years.

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. 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.

About 1 billion years from now, Earth will lose its water. The higher temperatures will bake the carbon dioxide out of the empty ocean beds, creating a thick, hot atmosphere. In the upper atmosphere, sunlight will break down water vapor into hydrogen, and the fast-moving hydrogen atoms will escape into outer space. Earth will start to resemble the Venus of today, and temperatures will become much too high for life as we know it.

All of this will happen before the Sun even becomes a red giant. The Sun, as it expands, will swallow Mercury and Venus, and friction with our star’s outer atmosphere will make these planets spiral inward until they are completely destroyed. 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’s orbit would increase. 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’s orbit to shrink. Thus, many astrophysicists conclude that Earth will be destroyed 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’t worry about this since we are talking about events that will occur billions of years from now.

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, up to several hundred million years, when no planet is habitable. For example, Earth becomes far too warm for life long before Mars warms up enough.

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’s 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’s orbit, the effect of each flyby need not be large.

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 when 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’t it be exciting to see how the story of the human race turns out after all those billions of years?