1. Chapter 20 Stellar Evolution
Chapter 20 opener. This striking composite image combines visible light (colored red and purple) acquired by the Hubble telescope and X-ray radiation (blue) by the Chandra telescope. Known as NGC 6543, or informally as the Cat’s Eye, this complex object is a planetary nebula—an old star (at center) shedding its outer layers over light-year dimensions as it ends its life. (STScI/CXC)

2. Units of Chapter 20 20.1 Leaving the Main Sequence
20.2 Evolution of a Sun-Like Star
20.3 The Death of a Low-Mass Star
Learning Astronomy from History
20.4 Evolution of Stars More Massive than the Sun
Mass Loss from Giant Stars
20.5 Observing Stellar Evolution in Star Clusters
20.6 The Evolution of Binary-Star Systems

3. 20.1 Leaving the Main Sequence
We cannot observe a single star going through its whole life cycle; even short-lived stars live too long for that.
Observation of stars in star clusters gives us a look at stars in all stages of evolution; this allows us to construct a complete picture.

4. During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium

5. 20.1 Leaving the Main Sequence
Eventually, as hydrogen in the core is consumed, the star begins to leave the Main Sequence
Its evolution from then on depends very much on the mass of the star:
Low-mass stars go quietly
High-mass stars go out with a bang!

6. 20.2 Evolution of a Sun-like Star
Even while on the Main Sequence, the composition of a star’s core is changing
Figure Solar Composition Change Theoretical estimates of the changes in a Sun-like star’s composition. Hydrogen (yellow) and helium (blue) abundances are shown (a) at birth, just as the star arrives on the zero-age main sequence; (b) after 5 billion years; and (c) after 10 billion years. At stage (b) only a few percent of the star’s total mass has been converted from hydrogen into helium. This change speeds up as the nuclear burning rate increases with time.

7. Hydrogen begins to fuse outside the core:
As the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse.
Hydrogen begins to fuse outside the core:
Figure Hydrogen- Shell Burning As a star’s core converts more and more of its hydrogen into helium, the hydrogen in the shell surrounding the nonburning helium “ash” burns ever more violently. By the time shown here (a little after stage 8 in Table 20.1), the core has shrunk to a few tens of thousands of kilometers in diameter, whereas the star’s photosphere is ten times the star’s original size.

8. Stages of a star leaving the Main Sequence:

9. Stage 9: The Red-Giant Branch
As the core continues to shrink, the outer layers of the star expand and cool.
It is now a red giant, extending out as far as the orbit of Mercury.
Despite its cooler temperature, its luminosity increases enormously due to its large size.

10. The red giant stage on the H-R diagram:
Figure Red Giant on the H–R Diagram As its helium core shrinks and its outer envelope expands, the star leaves the main sequence (stage 7). At stage 8, the star is well on its way to becoming a red giant. The star continues to brighten and grow as it ascends the red-giant branch to stage 9. As noted in Chapter 17, the dashed diagonal lines are lines of constant radius, allowing us to gauge the changes in the size of the star.

11. Stage 10: Helium fusion
Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse, through a three-alpha process:
4He + 4He → 8Be + energy
8Be + 4He → 12C + energy
The 8Be nucleus is highly unstable and will decay in about 10–12 s unless an alpha particle fuses with it first. This is why high temperatures and densities are necessary.

12. The helium flash:
The pressure within the helium core is almost totally due to “electron degeneracy”—two electrons cannot be in the same quantum state, so the core cannot contract beyond a certain point.
This pressure is almost independent of temperature—when the helium starts fusing, the pressure cannot adjust.

13. Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium
Figure Horizontal Branch A large increase in luminosity occurs as a star ascends the red-giant branch, ending in the helium flash. The star then settles down into another equilibrium state at stage 10, on the horizontal branch.

14. Stage 11: Back to the giant branch
As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster.
The star is now similar to its condition just as it left the Main Sequence, except now there are two shells:
Figure Helium-Shell Burning Within a few million years after the onset of helium burning (stage 9), carbon ash accumulates in the star’s inner core. Above this core, hydrogen and helium are still burning in concentric shells.

15. The star has become a red giant for the second time
Figure Reascending the Red-Giant Branch A carbon-core star reenters the giant region of the H–R diagram—this time on a track called the asymptotic-giant branch (stage 11)—for the same reason it evolved there the first time around: Lack of nuclear fusion at the center causes the core to contract and the overlying layers to expand.

16. 20.3 The Death of a Low-Mass Star
This graphic shows the entire evolution of a Sun-like star.
Such stars never become hot enough for fusion past carbon to take place.
Figure G-Type Star Evolution Artist’s conception of the relative sizes and changing colors of a normal G-type star (such as our Sun) during its formative stages, on the main sequence, and while passing through the red-giant and white-dwarf stages. At maximum swelling, the red giant is approximately 70 times the size of its main-sequence parent; the core of the giant is about 1/15th the main-sequence size and would be barely discernible if this figure were drawn exactly to scale. The duration of time spent in the various stages—protostar, main-sequence star, red giant, and white dwarf—is roughly proportional to the lengths shown in this imaginary trek through space. The star’s brief stay on the horizontal and asymptotic-giant branches are not shown.

17. The outer layers become unstable and are eventually ejected.
There is no more outward fusion pressure being generated in the core, which continues to contract.
The outer layers become unstable and are eventually ejected.
Figure Red-Giant Instability Buffeted by helium-shell flashes from within, and subject to the destabilizing influence of recombination, the outer layers of a red giant become unstable and enter into a series of growing pulsations. Eventually, the envelope is ejected and forms a planetary nebula.

18. The ejected envelope expands into interstellar space, forming a planetary nebula.
Figure Ejected Envelope A planetary nebula is an extended region of glowing gas surrounding an intensely hot central star (marked with an arrow here). The small, dense star is the core of a former red giant. The gas is what remains of the giant’s envelope, now ejected into space. (a) Abell 39, some 2100 pc away, is a classic planetary nebula shedding a spherical shell of gas about 1.5 pc across. (b) The brightened appearance around the edge of Abell 39 is caused by the thinness of the shell of glowing gas around the central core. Very little gas exists along the line of sight between the observer and the central star (path A), so that part of the shell is invisible. Near the edge of the shell, however, more gas exists along the line of sight (paths B and C), so the observer sees a glowing ring. (c) Ring Nebula, perhaps the most famous of all planetary nebulae at 1500 pc away and 0.5 pc across, is too small and dim to be seen with the naked eye. Astronomers once thought its appearance could be explained in much the same way as that of Abell 39. However, it now seems that the Ring really is ring shaped! Researchers are still unsure as to why a spherical star should eject a ring of material during its final days. (AURA; NASA)

19. The star now has two parts:
A small, extremely dense carbon core
An envelope about the size of our solar system.
The envelope is called a planetary nebula, even though it has nothing to do with planets—early astronomers viewing the fuzzy envelope thought it resembled a planetary system.

20. Planetary nebulae can have many shapes:
As the dead core of the star cools, the nebula continues to expand and dissipates into the surroundings.
Figure Planetary Nebulae (a) The Eskimo Nebula clearly shows several “bubbles” (or shells) of material being blown into space from this planetary nebula, which resides some 1500 pc away in the constellation Gemini. (b) The Cat’s Eye Nebula, about 1000 pc away and 0.1 pc across, is an example of a much more complex planetary nebula, possibly produced by a pair of binary stars (unresolved at center) that have both shed envelopes. (c) M2-9, some 600 pc away and 0.5 pc end-to-end, shows surprising twin lobes (or jets) of glowing gas emanating from a central, dying star and racing out at speeds of about 300 km/s. (AURA; NASA)

21. Stages 13 and 14: White and black dwarfs
Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small.
It is luminous only due to its high temperature.
Figure White Dwarf on the H–R Diagram A star’s passage from the horizontal branch (stage 10) to the white-dwarf stage (stage 13) by way of the asymptotic-giant branch creates an evolutionary path that cuts across the entire H–R diagram.

22. The small star Sirius B is a white-dwarf companion of the much larger and brighter Sirius A:
Figure Sirius Binary System Sirius B (the speck of light to the right of the much larger and brighter star Sirius A) is a white dwarf star, a companion to Sirius A. The “spikes” on the image of Sirius A are not real; they are caused by the support struts of the telescope. (Palomar Observatory)

23. The Hubble Space Telescope has detected white dwarf stars in globular clusters:
Figure Distant White Dwarfs (a) The globular cluster M4, as seen through a large ground-based telescope at Kitt Peak National Observatory in Arizona (see also Figure 18.14). At 1700 pc away, M4 is the closest globular cluster to us; it spans some 16 pc. (b) A peek at M4’s “suburbs” by the Hubble Space Telescope shows nearly a hundred white dwarfs within a small 0.2 square-parsec region. (AURA; NASA)

24. As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to glow.

25. This outline of stellar formation and extinction can be compared to observations of star clusters. Here a globular cluster:
Figure Globular Cluster H–R Diagram (a) The globular cluster M80, some 8 kpc away. (b) Combined H–R diagram, based on ground- and space-based observations, for several globular clusters similar in overall composition to M80. The various evolutionary stages predicted by theory and depicted schematically in Figure are clearly visible. Note also the blue stragglers—main-sequence stars that appear to have been “left behind” as other stars evolved into giants. They are probably the result of merging binary systems or actual collisions between stars of lower mass in this remarkably dense stellar system. (See also Figure ) The inset shows the H–R diagram of another globular cluster, NGC 2808, revealing that the main sequence is actually made up of three distinct sequences, increasing in helium content from right to left, and suggesting multiple generations of star formation shortly after the cluster formed.(NASA; data courtesy W.E. Harris)

26. The “blue stragglers” in the previous H-R diagram are not exceptions to our model; they are stars that have formed much more recently, probably from the merger of smaller stars.

27. 20.4 Evolution of Stars More Massive than the Sun
It can be seen from this H-R diagram that stars more massive than the Sun follow very different paths when leaving the Main Sequence
Figure High-Mass Evolutionary Tracks Evolutionary tracks for stars of 1, 4, and 10 solar masses (shown only up to helium ignition in the low-mass case). Stars with masses comparable to that of the Sun ascend the giant branch almost vertically, whereas higher-mass stars move roughly horizontally across the H–R diagram from the main sequence into the red-giant region. The most massive stars experience smooth transitions into each new burning stage. No helium flash occurs for stars more massive than about 2.5 solar masses. Some points are labeled with the element that has just started to fuse in the inner core.

28. High-mass stars, like all stars, leave the Main Sequence when there is no more hydrogen fuel in their cores.
The first few events are similar to those in lower-mass stars—first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells.

29. Stars with masses more than 2
Stars with masses more than 2.5 solar masses do not experience a helium flash—helium burning starts gradually.
A 4-solar-mass star makes no sharp moves on the H-R diagram—it moves smoothly back and forth.

30. A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate.
Its path across the H-R diagram is essentially a straight line—it stays at just about the same luminosity as it cools off.
Eventually the star dies in a violent explosion called a supernova.

31. In summary:

32. Discovery 20-2: Mass Loss from Giant Stars
All stars lose mass via some form of stellar wind. The most massive stars have the strongest winds; O- and B-type stars can lose a tenth of their total mass this way in only a million years.
These stellar winds hollow out cavities in the interstellar medium surrounding giant stars.

33. Discovery 20-2: Mass Loss from Giant Stars
The sequence below, of actual Hubble images, shows a very unstable red giant star as it emits a burst of light, illuminating the dust around it:

34. 20.5 Observing Stellar Evolution in Star Clusters
The following series of H-R diagrams shows how stars of the same age, but different masses, appear as the whole cluster ages.
After 10 million years, the most massive stars have already left the Main Sequence, while many of the least massive have not even reached it yet.
Figure Cluster Evolution on the H–R Diagram The changing H–R diagram of a hypothetical star cluster. (a) Initially, stars on the upper main sequence are already burning steadily while the lower main sequence is still forming. (b) At 107 years, O-type stars have already left the main sequence (as indicated by the arrows), and a few red giants are visible.

35. After 1 billion years, the main-sequence turnoff is much clearer.
After 100 million years, a distinct main-sequence turnoff begins to develop. This shows the highest-mass stars that are still on the Main Sequence.
After 1 billion years, the main-sequence turnoff is much clearer.
Figure Cluster Evolution on the H–R Diagram The changing H–R diagram of a hypothetical star cluster. (c) By 108 years, stars of spectral type B have evolved off the main sequence. More red giants are visible, and the lower main sequence is almost fully formed. (d) At 109 years, the main sequence is cut off at about spectral type A. The subgiant and red-giant branches are just becoming evident, and the formation of the lower main sequence is complete. A few white dwarfs may be present.

36. After 10 billion years, a number of features are evident:
The red-giant, subgiant, asymptotic giant, and horizontal branches are all clearly populated.
Figure Cluster Evolution on the H–R Diagram The changing H–R diagram of a hypothetical star cluster. (e) At 1010 years, only stars less massive than the Sun still remain on the main sequence. The cluster’s subgiant, red-giant, horizontal, and asymptotic-giant branches are all discernible. Many white dwarfs have now formed.
White dwarfs, indicating that solar-mass stars are in their last phases, also appear.

37. This double cluster, h and chi Persei, must be quite young—its H-R diagram is that of a newborn cluster. Its age cannot be more than about 10 million years.
Figure Newborn Cluster H–R Diagram (a) The “double cluster” h and chi Persei, two open clusters that apparently formed at the same time, possibly even orbiting one another. (b) The H–R diagram of the pair indicates that the stars are very young—probably only about 10 million years old. Even so, the most massive stars have already left the main sequence. (AURA)

38. The Hyades cluster, shown here, is also rather young; its main-sequence turnoff indicates an age of about 600 million years.
Figure Young Cluster H–R Diagram (a) The Hyades cluster, a relatively young group of stars visible to the naked eye, is found 46 pc away in the constellation Taurus. (b) The H–R diagram for this cluster is cut off at about spectral type A, implying an age of about 600 million years. A few massive stars have already become white dwarfs. (AURA)

39. This globular cluster, 47 Tucanae, is about 10–12 billion years old, much older than the previous examples:
Figure Old Cluster H–R Diagram (a) The southern globular cluster 47 Tucanae. (b) Fitting its main-sequence turnoff and its giant and horizontal branches to theoretical models gives 47 Tucanae an age of between 12 and 14 billion years, making it one of the oldest-known objects in the Milky Way Galaxy. The inset is a high-resolution ultraviolet image of 47 Tucanae’s core region, taken with the Hubble Space Telescope and showing many blue stragglers—massive stars lying on the main sequence above the turnoff point, resulting perhaps from the merging of binary-star systems. (See also Figure ) The points representing white dwarfs, some red dwarfs, and blue stragglers have been added to the original data set, based on Hubble observations of this and other clusters. The white-dwarf data are for the cluster M4 (Figure 20.13). Data on the faintest main-sequence stars shown were obtained from ground-based observations. The thickness of the lower main sequence is due almost entirely to observational limitations, which make it difficult to determine accurately the apparent brightnesses and colors of low-luminosity stars. (ESO; NASA)

40. 20.6 The Evolution of Binary-Star Systems
If the stars in a binary-star system are relatively widely separated, their evolution proceeds much as it would have if they were not companions.
If they are closer, it is possible for material to transfer from one star to another, leading to unusual evolutionary paths.

41. The Lagrangian point is where the gravitational forces are equal.
Each star is surrounded by its own Roche lobe; particles inside the lobe belong to the central star.
The Lagrangian point is where the gravitational forces are equal.
Figure Stellar Roche Lobes Each star in a binary system can be pictured as being surrounded by a “zone of influence,” or Roche lobe, inside of which matter may be thought of as being “part” of that star. The two teardrop-shaped Roche lobes meet at the Lagrangian point between the two stars. Outside the Roche lobes, matter may flow onto either star with relative ease.

42. There are different types of binary-star systems, depending on how close the stars are.
In a detached binary, each star has its own Roche lobe.
In a semidetached binary, one star can transfer mass to the other.
In a contact binary, much of the mass is shared between the two stars.

43. It is thought to have begun as a detached binary
As the stars evolve, their binary system type can evolve as well. This is the Algol system:
It is thought to have begun as a detached binary
Figure Algol Evolution The evolution of the binary star Algol. (a) Initially, Algol was probably a detached binary made up of two main-sequence stars: a relatively massive blue giant and a less massive companion similar to the Sun.

44. As the blue-giant star entered its red-giant phase, it expanded to the point where mass transfer occurred (b).
Eventually enough mass accreted onto the smaller star that it became a blue giant, leaving the other star as a red subgiant (c).
Figure Algol Evolution The evolution of the binary star Algol. (b) As the more massive component (star 1) left the main sequence, it expanded to fill, and eventually overflow, its Roche lobe, transferring large amounts of matter onto its smaller companion (star 2). (c) Today, star 2 is the more massive of the two, but it is on the main sequence. Star 1 is still in the subgiant phase and fills its Roche lobe, causing a steady stream of matter to pour onto its companion.

45. Which of the following is not a necessary ingredient in the construction of a theoretical stellar model?
A balance between gravity and gas pressure.
A knowledge of the star's position and motion in space.
A knowledge of the star's mass and chemical composition.
A balance between the star's luminosity and the amount of energy generated. 

46. As the sun ages, the chemical composition of its core changes so that it contains a lower percentage of ______ and a greater percentage of ______.
helium, hydrogen
hydrogen, helium
uranium, lead
oxygen, carbon 

47. Which of the following is not true of red giants?
Their average density is very low.
Molecules are prominent in their spectra.
Most are variable stars.
Most are pre-main sequence stars.

48. When the sun goes from the main sequence to the red giant stage
the core gets hotter and the surface gets hotter.
the core gets hotter and the surface gets cooler.
the core gets cooler and the surface gets hotter.
the core gets cooler and the surface gets cooler. 

49. Our knowledge of the stellar evolutionary paths on the H-R diagram is derived from
observation of changes in stellar brightnesses with time.
calculations of models on high-speed electronic computers.
observations of the main sequence.
experiments with hydrogen, helium, and carbon fusion reactions conducted on Earth. 

50. After a star's core runs out of fuel, how does the core get to a high enough temperature to ignite the next stage of fusion reactions?
By chemical reactions.
By other fusion reactions.
By gravitational contraction.
None of these; the fusion reactions stop. 

51. The longest phase of a star's life is spent as a
proto star.
main sequence star.
cepheid variable star.
red giant star. 

52. Which of the following stars will spend the most time on the main sequence?
A 3 solar mass star.
A 10 solar mass star.
The sun.
A 0.5 solar mass star. 

53. Which of the following are old stars with no current nuclear reactions?
red giants
main sequence stars
white dwarfs
proto stars 

54. The main sequence phase of a star's life ends when
helium reactions begin.
hydrogen is exhausted in the center of the star.
all nuclear reactions cease permanently in the star.
hydrogen is exhausted throughout the star. 

55. Summary of Chapter 20
Stars spend most of their life on the Main Sequence
When fusion ceases in the core, it begins to collapse and heat. Hydrogen fusion starts in the shell surrounding the core.
The helium core begins to heat up; as long as the star is at least 0.25 solar masses, the helium will get hot enough that fusion (to carbon) will start.
As the core collapses, the outer layers of the star expand and cool.

56. Summary of Chapter 20 (cont.)
The star develops a nonburning carbon core, surrounded by shells burning helium and hydrogen.
The shell expands into a planetary nebula, and the core is visible as a white dwarf.
The nebula dissipates, and the white dwarf gradually cools off.
In Sun-like stars, the helium burning starts with a helium flash before the star is once again in equilibrium.

57. Summary of Chapter 20 (cont.)
High-mass stars become red supergiants, and end explosively.
The description of stars’ birth and death can be tested by looking at star clusters, whose stars are all the same age but have different masses.
Stars in binary systems can evolve quite differently due to interactions with each other.