Chapter 12 Star Stuff

1
Chapter 12Star Stuff
2
12.1 Star Birth

• Our Goals for Learning

How do stars form? How massive are newborn
stars?
3
We are star stuff because the elements
necessary for life were made in stars
4
How do stars form?
5
Stars are born in molecular clouds consisting
mostly of hydrogen molecules
6
Stars form in places where gravity can overcome
thermal pressure in a cloud
7
Cloud heats up as gravity causes it to
contract Conservation of energy Contraction can
continue if thermal energy is radiated away
8
Star-forming clouds emit infrared light because
of the heat generated as stars form
9
Orion Nebula is one of the closest star-forming
clouds
Infrared light from Orion
10
Solar-system formation is a good example of star
birth
11
As gravity forces a cloud to become smaller, it
begins to spin faster and faster
12
As gravity forces a cloud to become smaller, it
begins to spin faster and faster Conservation
of angular momentum
13
As gravity forces a cloud to become smaller, it
begins to spin faster and faster Conservation
of angular momentum Gas settles into a spinning
disk because spin hampers collapse perpendicular
to spin axis
14
Angular momentum leads to Rotation of
protostar Disk formation and sometimes
Jets from protostar Fragmentation into
binary
15
(No Transcript)
16
(No Transcript)
17
Protostar to Main Sequence

• Protostar contracts and heats until core
  temperature is sufficient for hydrogen fusion.
• Contraction ends when energy released by hydrogen
  fusion balances energy radiated from surface.
• Takes 50 million years for star like Sun (less
  time for more massive stars)

18

• Summary of Star Birth
• Gravity causes gas cloud to shrink and fragment
• Core of shrinking cloud heats up
• When core gets hot enough, fusion begins and
  stops the shrinking
• New star achieves long-lasting state of balance

19
How massive are newborn stars?
20
A cluster of many stars can form out of a single
cloud.
21
Very massive stars are rare
Luminosity
Low-mass stars are common
Temperature
22
Stars more massive than 100 MSun would blow apart
Luminosity
Stars less massive than 0.08 MSun cant sustain
fusion
Temperature
23
Pressure Gravity

• If M 0.08 MSun, then gravitational contraction
  heats core until fusion begins
• If M gravitational contraction before fusion can begin

24
Degeneracy Pressure Laws of quantum mechanics
prohibit two electrons from occupying same state
in same place
25
Thermal Pressure Depends on heat content The
main form of pressure in most stars
Degeneracy Pressure Particles cant be in same
state in same place Doesnt depend on heat
content
26
Brown Dwarf

• An object less massive than 0.08 MSun
• Radiates infrared light
• Has thermal energy from gravitational contraction
• Cools off after degeneracy pressure stops
  contraction

27
What have we learned?

• How do stars form?
• Stars are born in cold, relatively dense
  molecular clouds.
• As a cloud fragment collapses under gravity, it
  becomes a protostar surrounded by a spinning disk
  of gas.
• The protostar may also fire jets of matter
  outward along its poles. Protostars rotate
  rapidly, and some may spin so fast that they
  split to form close binary star systems.

28
What have we learned?

• How massive are newborn stars?
• Newborn stars come in a range of masses, but
  cannot be less massive than 0.08MSun.
• Below this mass, degeneracy pressure prevents
  gravity from making the core hot enough for
  efficient hydrogen fusion, and the object becomes
  a failed star known as a brown dwarf.

29
12.2 Life as a Low-Mass Star

• Our Goals for Learning

What are the life stages of a low-mass star?
How does a low-mass star die?
30
What are the life stages of a low mass star?
31
High-Mass Stars
8 MSun
Intermediate-Mass Stars
Low-Mass Stars
Brown Dwarfs
32
A star remains on the main sequence as long as it
can fuse hydrogen into helium in its core
33
Life stages of a low-mass star like the Sun
34
Thought Question

• What happens when a star can no longer fuse
  hydrogen to helium in its core?
• A. Core cools off
• B. Core shrinks and heats up
• C. Core expands and heats up
• D. Helium fusion immediately begins

35
Thought Question

• What happens when a star can no longer fuse
  hydrogen to helium in its core?
• A. Core cools off
• B. Core shrinks and heats up
• C. Core expands and heats up
• D. Helium fusion immediately begins

36
Helium fusion requires higher temperatures than
hydrogen fusion because larger charge leads to
greater repulsion Fusion of two helium nuclei
doesnt work, so helium fusion must combine three
He nuclei to make carbon
37
Thought Question

• What happens as a stars inert helium core starts
  to shrink?
• A. Hydrogen fuses in shell around core
• B. Helium fusion slowly begins
• C. Helium fusion rate rapidly rises
• D. Core pressure sharply drops

38
Thought Question

• What happens as a stars inert helium core starts
  to shrink?
• A. Hydrogen fuses in shell around core
• B. Helium fusion slowly begins
• C. Helium fusion rate rapidly rises
• D. Core pressure sharply drops

39
Broken thermostat rising fusion rate in shell
does not expand core, so luminosity continues to
rise.
40
Thought Question

• What happens in a low-mass star when core
  temperature rises enough for helium fusion to
  begin?
• A. Helium fusion slowly starts up
• B. Hydrogen fusion stops
• C. Helium fusion rises very sharply
• Hint Degeneracy pressure is the main form of
  pressure in the inert helium core

41
Thought Question

• What happens in a low-mass star when core
  temperature rises enough for helium fusion to
  begin?
• A. Helium fusion slowly starts up
• B. Hydrogen fusion stops
• C. Helium fusion rises very sharply
• Hint Degeneracy pressure is the main form of
  pressure in the inert helium core

42
Helium Flash

• Thermostat is broken in low-mass red giant
  because degeneracy pressure supports core
• Core temperature rises rapidly when helium fusion
  begins
• Helium fusion rate skyrockets until thermal
  pressure takes over and expands core again

43
Helium burning stars neither shrink nor grow
because thermostat is temporarily fixed.
44
Thought Question

• What happens when the stars core runs out of
  helium?
• A. The star explodes
• B. Carbon fusion begins
• C. The core cools off
• D. Helium fuses in a shell around the core

45
Thought Question

• What happens when the stars core runs out of
  helium?
• A. The star explodes
• B. Carbon fusion begins
• C. The core cools off
• D. Helium fuses in a shell around the core

46
Life stages of a low-mass star like the Sun
47
Star clusters help us test models of stellar
evolution because they contain stars of same age
but at different life stages
48
(No Transcript)
49
How does a low mass star die?
50
A star like our sun dies by puffing off its outer
layers, creating a planetary nebula. Only a
white dwarf is left behind
51
A star like our sun dies by puffing off its outer
layers, creating a planetary nebula. Only a
white dwarf is left behind
52
A star like our sun dies by puffing off its outer
layers, creating a planetary nebula. Only a
white dwarf is left behind
53
A star like our sun dies by puffing off its outer
layers, creating a planetary nebula. Only a
white dwarf is left behind
54
Thought Question

• What happens to Earths orbit as Sun loses mass
  late in its life?
• A. Earths orbit gets bigger
• B. Earths orbit gets smaller
• C. Earths orbit stays the same

55
Thought Question

• What happens to Earths orbit as Sun loses mass
  late in its life?
• A. Earths orbit gets bigger
• B. Earths orbit gets smaller
• C. Earths orbit stays the same

56
(No Transcript)
57
(No Transcript)
58
What have we learned?

• What are the life stages of a low-mass star?
• A low-mass star spends most of its life
  generating energy by fusing hydrogen in its core.
  Then it becomes a red giant, with a hydrogen
  shell burning around an inert helium core. Next
  comes helium core burning, followed by
  doubleshell burning of hydrogen and helium shells
  around an inert carbon core.
• c In the late

59
What have we learned?

• How does a low-mass star die?
• A low-mass star like the Sun never gets hot
  enough to fuse carbon in its core. It expels its
  outer layers into space as a planetary nebula,
  leaving behind a white dwarf.

60
12.3 Life as a High-Mass Star

• Our Goals for Learning

What are the life stages of a high mass star?
How do high-mass stars make the elements
necessary for life? How does a high-mass star
die?
61
What are the life stages of a high mass star?
62
High-Mass Stars
8 MSun
Intermediate-Mass Stars
Low-Mass Stars
Brown Dwarfs
63
High-Mass Stars Life

• Early stages are similar to those of low-mass
  star
• Main Sequence H fuses to He in core
• Red Supergiant H fuses to He in shell around
  inert He core
• Helium Core Burning He fuses to C in core (no
  flash)

64
CNO cycle is just another way to fuse H into He,
using carbon, nitrogen, and oxygen as
catalysts CNO cycle is main mechanism for H
fusion in high mass stars because core
temperature is higher
65
High-mass stars become supergiants after core H
runs out Luminosity doesnt change much but
radius gets far larger
66
How do high mass stars make the elements
necessary for life?
67
(No Transcript)
68
Big Bang made 75 H, 25 He stars make
everything else
69
Helium fusion can make carbon in low-mass stars
70
CNO cycle can change C into N and O
71
Helium-capture reactions add two protons at a time
72
Helium capture builds C into O, Ne, Mg,
73
Advanced nuclear fusion reactions require
extremely high temperatures Only high-mass stars
can attain high enough core temperatures before
degeneracy pressure stops contraction
74
Advanced reactions make heavier elements
75
Advanced nuclear burning occurs in multiple shells
76
Iron is dead end for fusion because nuclear
reactions involving iron do not release
energy (Fe has lowest mass per nuclear particle)
77
Evidence for helium capture Higher abundances
of elements with even numbers of protons
78
How does a high mass star die?
79
Iron builds up in core until degeneracy pressure
can no longer resist gravity Core then suddenly
collapses, creating supernova explosion
80
Core degeneracy pressure goes away because
electrons combine with protons, making neutrons
and neutrinos Neutrons collapse to the center,
forming a neutron star
81
Energy and neutrons released in supernova
explosion enables elements heavier than iron to
form
82
Elements made during supernova explosion
83
Crab Nebula Remnant of supernova observed in
1054 A.D.
84
before
after
Supernova 1987A is the nearest supernova observed
in the last 400 years
85
The next nearby supernova?
86
What have we learned?

• What are the life stages of a high-mass star?
• A high-mass star lives a much shorter life than a
  low-mass star, fusing hydrogen into helium via
  the CNO cycle. After exhausting its core
  hydrogen, a high-mass star begins hydrogen shell
  burning and then goes through a series of stages
  burning successively heavier elements. The
  furious rate of this fusion makes the star swell
  in size to become a supergiant.

87
What have we learned?

• How do high-mass stars make the elements
  necessary for life?
• In its final stages of life, a high-mass stars
  core becomes hot enough to fuse carbon and other
  heavy elements. The variety of different fusion
  reactions produces a wide range of elements
  including all the elements necessary for
  lifethat are then released into space when the
  star dies.

88
What have we learned?

• How does a high-mass star die?
• A high-mass star dies in the explosion of a
  supernova, scattering newly produced elements
  into space and leaving a neutron star or black
  hole behind.
• The supernova occurs after fusion begins to pile
  up iron in the high-mass stars core. Because
  iron fusion cannot release energy, the core
  cannot hold off the crush of gravity for long. In
  the instant that gravity overcomes degeneracy
  pressure, the core collapses and the star
  explodes.

89
12.4 Summary of Stellar Lives

• Our Goals for Learning

How does a stars mass determine its life
story? How are the lives of stars with close
companions different?
90
How does a stars mass determine its life story?
91

• Low-Mass Star Summary
• Main Sequence H fuses to He in core
• Red Giant H fuses to He in shell around He core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Double Shell Burning
• H and He both fuse in shells
• 5. Planetary Nebula leaves white dwarf behind

Not to scale!
92

• Reasons for Life Stages
• Core shrinks and heats until its hot enough for
  fusion
• Nuclei with larger charge require higher
  temperature for fusion
• Core thermostat is broken while core is not hot
  enough for fusion (shell burning)
• Core fusion cant happen if degeneracy pressure
  keeps core from shrinking

Not to scale!
93

• Life Stages of High-Mass Star
• Main Sequence H fuses to He in core
• Red Supergiant H fuses to He in shell around He
  core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Multiple Shell Burning
• Many elements fuse in shells
• 5. Supernova leaves neutron star behind

Not to scale!
94
Life of a 1 MSun star
Life of a 20 MSun star
95
How are the lives of stars with close companions
different?
96
Thought Question

• The binary star Algol consists of a 3.7 MSun main
  sequence star and a 0.8 MSun subgiant star.
• Whats strange about this pairing?
• How did it come about?

97
Stars in Algol are close enough that matter can
flow from subgiant onto main-sequence star
98
Star that is now a subgiant was originally more
massive As it reached the end of its life and
started to grow, it began to transfer mass to its
companion Now the companion star is more massive
99
What have we learned?

• How does a stars mass determine its life
  story?
• A stars mass determines how it lives its life.
• Low-mass stars never get hot enough to fuse
  carbon or heavier elements in their cores, and
  end their lives by expelling their outer layers
  and leaving a white dwarf behind.
• High-mass stars live short but brilliant lives,
  ultimately dying in supernova explosions.
• Sun

100
What have we learned?

• How are the lives of stars with close
  companions different?
• When one star in a close binary system begins to
  swell in size at the end of its hydrogen-burning
  life, it can begin to transfer mass to its
  companion. This mass exchange can then change the
  remaining life histories of both stars.
• Sun