Transcript Chapter12 (with interactive links)

Lecture Slides
CHAPTER 12: Evolution of Low-Mass Stars
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
Evolution of Low-Mass Stars
 Understand the role of
stellar mass in the
evolution of a star.The
goal is to understand
this process.
 Explain the future
evolution of the Sun.
 Utilize the H-R diagram
to determine the
evolution of stars.
Role of Stellar Mass: Star’s Life
 Main sequence stars generate energy by converting
hydrogen to helium in their cores.
 Eventually the fusion sources change, then halt.
 A star’s life depends primarily on mass and
composition (to a lesser extent).
 Low-mass stars and high-mass stars evolve differently.
 Low-mass stars: M < 8 M
Role of Stellar Mass: Main-Sequence Lifetimes
 Higher mass leads to higher temperature and
pressure in the core.
 Higher core temperature means faster nuclear
fusion => Stars with higher masses burn their fuel
more quickly.
Role of Stellar Mass: Main Sequence Star
 Recall that a protostar
becomes a star when
nuclear fusion begins.
=> That is when it
becomes a main
sequence star!
 Mass establishes a star’s
evolutionary track.
Role of Stellar Mass: Changes in Structure
 The star’s structure will
change as it uses fuel.
=> Must maintain
balance between
pressure and gravity.
Fusion Reactions
 Main-sequence stars fuse hydrogen to helium in
their cores.
 Eventually, much of the hydrogen in the core is
converted to helium.
 A core of non-fusing helium builds up.
Fusion Reactions (Cont.)
Fusion Reactions (Cont.)
Fusion Reactions (Cont.)
Class Question
Prediction: When hydrogen fusion in the core stops,
its temperature ______ and its thermal pressure
_______ so the size of the core ______.
A.
B.
C.
D.
Decreases, decreases, decreases.
Increases, increases, increases.
Decreases, increases, decreases.
Increases, decreases, increases.
Fusion Reactions: Electron-Degenerate
 At this point, hydrogen fusion only takes place in
a shell around the helium core.
 Because the helium is not fusing, gravity begins
to win over the pressure, causing the helium core
to shrink.
 The core becomes more dense, and becomes
electron-degenerate.
• This means pressure is due to a quantum
mechanical effect: there’s a limit to how tightly
electrons can be packed together.
Red Giant: Hydrogen Shell Burning
 When the core shrinks, its
gravitational pull gets stronger.
 Stronger gravity => higher
pressure => faster nuclear
reactions in the hydrogen
burning shell => more energy
produced!
Red Giant: Hydrogen Shell Burning (Cont.)
Red Giant: Hydrogen Shell Burning (Cont.)
Red Giant: A Larger Star
 Increase in pressure and
energy production results in
larger size and lower surface
temperature.
 Star: larger, more luminous,
cooler, redder => Red Giant!
Red Giant: A Larger Star (Cont.)
Red Giant: A Larger Star (Cont.)
Class Question
What causes a low-mass star, like the Sun, to evolve
away from the main sequence?
A. When hydrogen is exhausted in the core.
B. When all of the hydrogen becomes helium.
C. When carbon fusion begins.
Red Giant: Branch
Red Giant: Branch (Cont.)
Red Giant: Branch (Cont.)
Helium Fusion
 As the helium core
shrinks, its density and
temperature increase.
 When hot and dense
enough, helium fusion
begins.
 Helium fuses to carbon
via the triple-alpha
process.
Helium Fusion: The Helium Flash
 Within seconds of helium
ignition, the thermal
pressure to the point that
the helium core literally
explodes.
 This explosion is called the
helium flash.
Helium Fusion: Horizontal Branch Star
 After the helium flash, the
stars are on the horizontal
branch of the H-R diagram.
 Helium fuses to carbon in
the core, while hydrogen
fuses to helium in a shell
around the core.
Helium Fusion: Asymptotic Giant Branch
 After helium is used up in
the core, hydrogen and
helium fusion continues in
shells around non-fusing
carbon core.
 Outer layers expand and
cool => asymptotic giant
branch of the H-R diagram.
Planetary Nebula
 As the star expands,
some of its material
leaves as a stellar
wind.
 This mass loss
means the star
cannot hold onto the
outer layers easily.
 Eventually the outer
layers are ejected
into space.
Planetary Nebula: The Ejected Material
 The ejected material creates
a planetary nebula.
• Planetary nebulae having
nothing to do with planets!
 The remaining star shrinks
and becomes hotter, moving
rapidly from right to left
across the H-R diagram.
Planetary Nebula: Lifespan
 The star ionizes the gas
in the expanding outer
layers, causing the
planetary nebula that we
can observe.
 Planetary nebulae do not
last forever – eventually
the gas disperses.
Planetary Nebula: Facts
 The first observed planetary nebulae had circular
appearances (hence the name), but now we observe
examples with much more complex structure.
White Dwarf
 Leftover core of star
remains as white dwarf.
 Masses 0.6–1.4 M, size
like Earth.
 They are hot, but not
very luminous due to
small size.
 White dwarfs cool off
because no nuclear
fusion is occurring.
•Our Sun’s
evolution
Star Clusters
 Star clusters are bound groups of stars, all made at the
same time.
Star Clusters (Cont.)
Star Clusters (Cont.)
Star Clusters (Cont.)
Star Clusters (Cont.)
Star Clusters (Cont.)
Star Clusters (Cont.)
Star Clusters: Young and Old
 Young clusters still have
high-mass stars on
main-sequence.
 In older clusters, high-mass
stars have already died.
 Location of main-sequence
turnoff gives cluster age.
Star Clusters: Models of Stellar Evolution
 We compare our
models to the observed
H-R diagrams of
star clusters.
 Agreement shows that
our models of stellar
evolution are on the
right track!
Evolution in Close Binary Systems
 Recall that most stars
are in binary systems.
 In each pair of low-mass
stars, the more massive
star evolves first.
 It can only expand so
much before it begins to
lose material.
Evolution in Close Binary Systems (Cont.)
Evolution in Close Binary Systems (Cont.)
Evolution in Close Binary Systems: Mass Transfer
 Material can flow from
the giant star to the
companion. This is
called mass transfer.
 The giant becomes a
white dwarf.
 When the second star
is a giant, it can dump
material onto the
white dwarf.
Evolution in Close Binary Systems: Mass Transfer (Cont.)
Evolution in Close Binary Systems: Mass Transfer (Cont.)
Evolution in Close Binary Systems: Evolution of Nova
 As hydrogen collects on the white
dwarf, nuclear reactions can start
on the surface => gets much
brighter temporarily => nova.
 For a few hours, it can be a halfmillion times more luminous than
the Sun.
Evolution in Close Binary Systems: Evolution of Nova (Cont.)
Evolution in Close Binary Systems: Evolution of Nova (Cont.)
Evolution in Close Binary Systems: Evolution of Nova (Cont.)
Evolution in Close Binary Systems: The White Dwarf Limit
 The maximum mass for a white
dwarf is 1.4 M, known as the
Chandrasekhar limit.
 If material dumped on the white
dwarf pushes it over this limit, the
white dwarf will explode.
Evolution in Close Binary Systems: The White Dwarf Limit (Cont.)
Evolution in Close Binary Systems: The White Dwarf Limit (Cont.)
Evolution in Close Binary Systems: The White Dwarf Limit (Cont.)
Evolution in Close Binary Systems: Type Ia Supernova
 This explosion is called a Type
Ia supernova.
 The explosion is briefly as luminous
as 10 billion Suns.
 Nothing of the white dwarf is left
behind; the other star continues to
evolve on its own.
Evolution in Close Binary Systems: Type Ia Supernova (Cont.)
Evolution in Close Binary Systems: Type Ia Supernova (Cont.)
Evolution in Close Binary Systems: Type Ia Supernova (Cont.)
Class Question
Which of the following is the correct order for
the stages of evolution of a low-mass star, like
the Sun?
A. Main sequence, white dwarf, planetary
nebula, red giant.
B. Main sequence, red giant, white dwarf,
planetary nebula.
C. Main sequence, red giant, planetary nebula,
white dwarf.
D. Main sequence, planetary nebula, red giant,
white dwarf.
Chapter Summary
 Low-mass stars burn through their nuclear fuel more
slowly and have longer lifetimes than high-mass stars.
 A low-mass star will evolve as hydrogen fusion stops in
its core.
 The evolutionary result of an isolated low-mass star:
planetary nebula + white dwarf
 H-R diagrams help us understand stellar evolution.
Nebraska Applet
HR-Diagram Explorer
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Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture slides for
CHAPTER 12: Evolution of
Low-Mass Stars
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company