Chapter 19 - The Evolution of Stars
CHAPTER 19
THE EVOLUTION OF STARS
CHAPTER OUTLINE AND LECTURE NOTES
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Why Do Stars Evolve?
I sometimes do a demonstration to give the students a feeling for why nuclear fusion reactions only go on at
very high temperatures in the cores of stars. I use an airtrack with two sliding cars. Both of the cars have
springs on the ends closest to the ends of the airtrack and magnets (of the same polarity) at the ends facing the
center of the track. I tell the class that the cars represent nuclei and that the magnets simulate the electric
charges on the nuclei. The cars must touch in order for a nuclear reaction to occur. I send one car toward the
other at low speed and, of course, the two magnets repel one another before the cars can touch. I ask the class
what I can do to make the cars touch and someone will volunteer that I need to make one approach the other at
greater speed. I then ask if these were nuclei in a gas, how would I make them move faster. It usually doesn’t
take very long for someone to volunteer that I should make the gas hotter. I push one car toward the other at
greater speed and hear a click as the two cars bang into one another (before being pushed apart). I finish by
asking what would happen if the magnets were stronger, simulating nuclei of greater charge. This sounds
pretty simple-minded, but I find it helps students who have no feeling for electric charge.
Evolutionary Tracks and Star Clusters
Some students find it very hard to understand that when astronomers talk about a star “moving” on an
evolutionary track in an H-R diagram they aren’t talking about movement through space. I’ve tried to get
across the point that movement in the H-R diagram describes changing appearance by using the analogy of a
height-weight diagram (Figure 19.3) and the evolutionary track of a star (Figure 19.4).
Main Sequence Stars
Even though life on Earth will be imperiled as the Sun grows brighter (even on the main sequence), the outer
solar system will become more benign, maybe even habitable. By the time the Sun leaves the main sequence,
Mars will have grown warmer by almost 50° F, though it will still have an average temperature well below
freezing. When the Sun becomes a red giant, the temperature on Jupiter’s satellites will be about the same as
the Earth today. An optimist can imagine a wave of colonization moving outward through the solar system as
the Sun grows more luminous.
After the Main Sequence
I suspect that this will be one of the more difficult sections of the book for many students because there are so
many exotic topics in this section. AGB stars are what I study, so I have given them more attention than in
most other books.
KEY TERMS
alpha particle — The nucleus of a helium atom, consisting of two protons and two neutrons.
asymptotic giant branch (AGB) — The portion of the H-R diagram occupied by enormous, cool
stars with helium-burning shells.
carbon cycle — The series of reactions by means of which massive stars fuse hydrogen into
helium.
Cepheid variable — A member of a class of yellow pulsating stars that vary in brightness as
they expand and contract. The period of a Cepheid is related to its luminosity.
degenerate gas — A gas in which a type of particle (electrons or neutrons) are as tightly packed
as permitted by the Pauli exclusion principle. In a degenerate gas, temperature has
essentially no influence on pressure.
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Chapter 19 - The Evolution of Stars
equation of state — The relationship among pressure, density, and temperature for a gas or
fluid. The ideal gas law, for which pressure is proportional to the product of temperature
and density, is an example of an equation of state.
evolutionary track — The path in an H-R diagram followed by the point representing the
changing luminosity and temperature of a star as it evolves.
helium flash — The explosive consumption of helium in the core of a star when helium is
initiated in a degenerate gas in which pressure doesn’t rise as energy is produced and
temperature increases.
horizontal branch star — A star that is undergoing helium fusion in its core and hydrogen
fusion in a shell surrounding the core.
instability strip — A region of the H-R diagram occupied by pulsating stars, including Cepheid
variables and RR Lyrae stars.
isochrone — Lines in an H-R diagram occupied by stars of different masses but the same age.
main sequence lifetime — The length of time that a star spends as a main sequence star.
Pauli exclusion principle — A physical law that limits the number of particles of a particular
kind that can be placed in a given volume. A gas in which that limit is reached is
degenerate.
period-luminosity relationship — The relationship between the period of brightness variation
and the luminosity of a Cepheid variable star. The longer the period of a Cepheid, the more
luminous the Cepheid.
planetary nebula — A luminous shell surrounding a hot star. The gas in a planetary nebula was
ejected from the star while it was a red giant.
r-process — The process of building up massive nuclei in which neutrons are captured at a rate
faster than the newly produced nuclei can undergo radioactive decay.
RR Lyrae star — A member of a class of giant pulsating stars, all of which have pulsation
periods of about 1 day.
s-process — The process of building up massive nuclei in which neutrons are captured at a rate
slower than the newly produced nuclei can undergo radioactive decay.
thermal pulse — The rapid consumption of helium in a shell within an asymptotic giant branch
star.
triple  process — A pair of nuclear reactions through which three helium nuclei (alpha
particles) are transformed into a carbon nucleus.
Vogt-Russell theorem — The concept that the original mass and chemical composition of an
isolated star completely determine the course of its evolution.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
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The temperature is highest at the center, so nuclear reactions proceed most rapidly there.
The temperature required for the fusion of helium is about 10 times higher than that required for hydrogen
fusion. By the time the center of the star becomes hot enough for helium fusion, all of the hydrogen has been
fused into helium.
Their nuclei have greater positive charges than hydrogen, so a higher temperature is required for the nuclei to
have enough energy to overcome the repulsive electrical force between the nuclei.
If opacity is high, convection occurs.
The pressure doubles.
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Chapter 19 - The Evolution of Stars
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The pressure remains essentially unchanged.
Their evolution should be identical.
Nothing.
Because the stars in a cluster have nearly the same age, they fall on an isochrone in an H-R diagram.
Cluster 2 is younger because its less massive stars (which will become K and M main sequence stars) have not
yet had time to reach the main sequence. Cluster 1 is older because its K stars have had time to reach the main
sequence.
They have hydrogen fusion taking place in their cores.
The most massive main sequence stars are about 20 times as hot, 600 times as massive, 150 times as large,
and a billion times as luminous as the least massive main sequence stars.
Less massive stars become degenerate and stop heating up before hydrogen fusion can begin.
During the next 5 billion years the luminosity of the Sun will increase by about 60%. This will probably
increase the average temperature of the Earth to 125° F.
At the time core helium fusion begins, the core of a 1 solar mass main sequence stars is degenerate, so the
energy released by helium increases the temperature without increasing pressure and expanding the core.
Eventually, most of the energy produced in the helium flash in a 1 solar mass main sequence star is used to
expand the degenerate core of the star.
When the star is smallest, pressure dominates gravity so the star begins to expand. When the star reaches
average size, pressure and gravity balance, but inertia carries the surface outward until maximum size is
reached and gravity dominates pressure. The star begins to contract again until it reaches minimum size and
the cycle begins anew.
AGB stars have thick, cool dust shells around them that absorb their visible light and re-emit it in
the infrared.
The gas in the planetary nebula was shed from the star while it was an AGB star.
The star must be hot in order to produce ultraviolet radiation, which can ionize the hydrogen atoms in the
nebula and make the nebula luminous.
There would be no elements heavier than iron.
Problems
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About 40 of them (this assumes a typical person spends 1 hour per day eating)
The gravitational energy radiated away by the Sun is 5 times as large.
The two would be the same.
The lifetime of star B is twenty times as long.
The main sequence lifetime of B is 56 times as long.
Figure-based Questions
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1.7 × 108 years
103 solar luminosities, 6 solar masses, 4 × 107 years
7 × 107 years
30,000 L.
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