Download Chapter 17 Star Stuff

The Life and Death of Stars
What is a Star?
• A star is a sphere of plasma gas that fuses
atomic nuclei in its core and so emits light
• The name star can also be tagged onto a body
that is somewhere on the star evolution
process e.g. proto-star, neutron star.
Stellar Evolution
•
•
•
•
•
Stars begin as a nebula of gas and dust.
Protostar- gravity pulls it together, temperature rises.
Main Sequence- Temperature, color, luminosity, & size
are directly proportional to the mass.
Red Giant or Supergiant- large, luminous, low temp.
Ending is based on initial mass and can be either:
–
A Planetary nebula- outer layers of star blown off causing a:
White Dwarf- small, dim, white star with high temperature. OR/
– A Supernova- star explodes leaving a:
Neutron Star (Pulsar is one type)- only has neutrons, or a
Black Hole- so small and dense that even light cannot escape it.
Star Birth
Stages of Star Birth
• Our goals for learning
• Where and Why do stars form?
• What slows the contraction of a star-forming
cloud?
• How does nuclear fusion begin in a newborn star?
Where and Why do stars form?
Star-Forming Clouds
• Stars form in dark
clouds of dusty gas
in interstellar space
• The gas between the
stars is called the
interstellar
medium
Molecular Clouds
•
•
Most of the matter in star-forming clouds
is in the form of molecules (H2, CO,…)
These molecular clouds have a
temperature of 10-30 K and a density of
about 300 molecules per cubic cm
Fragmentation of a Cloud
Gravity within a
contracting gas
cloud becomes
stronger as the gas
becomes denser
random motions of
different sections of
the cloud cause it to
become lumpy
Each lump in
which gravity can
overcome pressure
can become a star
What slows the contraction of a
star-forming cloud?
Trapping of Thermal Energy
• As contraction packs the molecules and dust particles
of a cloud fragment closer together, it becomes harder
for infrared and radio photons to escape
• Thermal energy then begins to build up inside,
increasing the internal pressure
• Contraction slows down, and the center of the cloud
fragment becomes a protostar
Conservation of
Angular Momentum
•
•
•
Behaves just like solar
system formation,
The rotation speed of the
cloud from which a star
forms increases as the
cloud contracts
Heating, spinning and
flattening occur
Examples of Proplyds in the Orion Nebula
In the animation's visible images,
how do the protostars appear?
A.They are bright points of light.
B.They are bright fuzzy features.
C.They are faint fuzzy features.
D.They do not appear in the image.
How does nuclear fusion begin in
a newborn star?
From Protostar to Main Sequence
• Protostar looks starlike after the surrounding gas is
blown away, but its thermal energy comes from
gravitational contraction, not fusion
• Contraction must continue until the core becomes hot
enough for nuclear fusion
• Contraction stops when the energy released by core
fusion balances energy radiated from the surface—the
star is now a main-sequence star
Birth Stages on a Life Track
•
Life track illustrates star’s surface
temperature and luminosity at different
moments in time
Assembly of a Protostar
•
Luminosity and temperature grow as
matter collects into a protostar
2. Convective Contraction
•
Surface temperature remains near 3,000 K
while convection is main energy transport
mechanism
3. Radiative Contraction
•
Luminosity remains nearly constant during
late stages of contraction, while radiation
is transporting energy through star
4. Self-Sustaining Fusion
•
Core temperature continues to rise until
star arrives on the main sequence
Life Tracks for Different Masses
• Models show that
Sun required about
30 million years to
go from protostar to
main sequence
• Higher-mass stars
form faster
• Lower-mass stars
form more slowly
Open cluster: A few thousand loosely packed stars
What is the initial energy source for
a contracting protostar?
A.pressure
B.gravitational potential energy
C.fusion
D.kinetic energy
What have we learned?
• Where and why do stars form?
– Stars form in dark, dusty clouds of molecular
gas with temperatures of 10-30 K
– Stars form in clouds that are massive enough
for gravity to overcome thermal pressure (and
any other forms of resistance)
– Such a cloud contracts and breaks up into
pieces called proplyds that go on to form
stars
What have we learned?
• What slows the contraction of a star-forming
cloud?
– The contraction of a cloud fragment slows when
thermal pressure builds up because infrared and
radio photons can no longer escape
• How does nuclear fusion begin in a newborn
star?
—
Nuclear fusion begins when contraction causes the
star’s core to grow hot enough for fusion
Forces Controlling Stars
• Our goals for learning
• How does nuclear fusion occur in stars?
• What is thermal pressure?
• What is degeneracy pressure?
• How do these control the mass of a star?
How does nuclear fusion occur in stars?
High temperatures
cause particles to
smash together.
Enables nuclear
fusion to happen in
the core
Sun releases energy by fusing four hydrogen nuclei into one helium
nucleus
Total mass is 0.7% lower
Stable Star
• Star’s core fuses hydrogen into helium,
releasing energy.
• The energy causes gas
expansion pushing outward
against the gravity pulling
inward. This is called
thermal pressure.
• When the pressures balance,
the star is in equilibrium and
burns steadily.
Stellar Thermostat
• The balancing act between thermal expansion pushing out
and gravity contracting inward, forces the temperature to
stay constant.
Stellar Thermostat
If temperature falls because
fusion rate drops :
Thermal pressure drops,
gravity gains
=>core contracts and
heats back up
If temperature rises because
fusion rate increases:
Thermal pressure increases
over gravity.
=>core expands, which
causes core to cool down
When star run out of fuel other atomic forces becomes
important
Pauli Exclusion Principal:
Laws of quantum mechanics prohibit two electrons
from occupying same state in same place
Electron Degeneracy Pressure
•Electrons can never
occupy the same state at
the same time.
•As a result, they cannot
be completely crushed
without being forced into
each other, so they exert a
force outward acting
against gravity.
•This is called degeneracy
pressure.
Thermal Pressure:
Depends on heat content
The main form of pressure
in most stars
Degeneracy Pressure:
Particles can’t be in same
state in same place
Doesn’t depend on heat
content
How do these Forces Control the Mass of a Star?
– Low mass means low gravitational force so electrons
are not crushed and degeneracy pressure is effective.
– Fusion will not begin if contraction stops before the
core temperature rises above 107 K.
– At very high mass, high gravitational force causes
extreme amounts of fusion and very high thermal
and radiation pressure.
– If radiation pressure is high, then star will be large.
Too high and the star will explode
Limiting the Mass of Stars
> 100 MSun
Stable stars
< 0.08 MSun
(MSun is the mass
of the Sun)
• Radiation pressure so great,
star blows apart
• Gravity and heat overcome
degeneracy pressure in the
core, core fuses - stars shine!
• Degeneracy pressure supports
the core of the star, no fusion!
Brown Dwarfs
• Degeneracy pressure
halts the contraction
of objects with
<0.08MSun before
core temperature
become hot enough
for fusion
• Starlike objects not
massive enough to
start fusion are
brown dwarfs
What have we learned?
• How does nuclear fusion occur in stars?
– The core’s extreme temperature and density are
just right for nuclear fusion of hydrogen to
helium through the proton-proton chain
– Gravitational equilibrium acts as a thermostat to
regulate the core temperature because fusion
rate is very sensitive to temperature
What have we learned?
• What is thermal pressure?
– hot gases moving more, cause expansion,
which exerts an outward pressure inside a star
• What is degeneracy pressure?
– electrons cannot be at the same state at the
same time and exert a repulsion force to prevent
it from happening.
A star's life is a struggle between
________ wanting to crush it, and
________ wanting to expand it.
A.gravity, nuclear energy
B.gravity, convection
C.gas pressure, radiation
D.nuclear energy, radiation
Lives in the Balance
• Our goals for learning
• How does a star’s mass affect its life
and death?
Stellar Mass and Life Path
• High mass stars (> 8 MSun)
end with an explosion
• Lower mass stars < 4 MSun)
end by material loss
Life as a Low-Mass Star
• Our goals for learning
• What are the final life stages of a low-mass
star?
• How does a low-mass star die?
Stellar Old-Age
• A star remains on the main sequence as long as it can
fuse hydrogen into helium in its core.
• It starts to leave the main sequence once the core is
mostly made of helium
• Observations of star clusters show that a star becomes
larger, redder, and brighter after its time on the main
sequence is over.
The Changing Star
• He is inert, so is generating no
thermal pressure.
=> Core contracts
• H begins fusing in a shell
around the core.
• Shell fusion is even more fast
burning than before.
=> brightness increases
• Thermal pressure increases.
=> Star expands into red giant
Feedback Loop
• degeneracy pressure not
thermal pressure now operates
in the core.
• The increasing fusion rate in
the shell does not stop core
contraction
=> Causes even more shell
fusion, brightness and star
expansion.
• This is a runaway process until
the core reaches a critical mass
of helium.
Hot enough for Helium fusion :
three He nuclei combine to make carbon
Helium fusion does not begin right away because it requires
higher temperatures than hydrogen fusion.
Helium Flash
• Core temperature rises rapidly
when helium fusion begins
• Helium fusion rate skyrockets
until thermal pressure takes
over and expands core again.
• The core has switched back on
and the star contracts and
burns evenly.
Helium burning stars neither shrink nor grow
because core thermostat is temporarily fixed.
Life Track after Main Sequence
• Changes in brightness
or temperature show up
on the H-R Diagram
• Dying stars form their
own branch away from
the main sequence line
Life Track after Helium Flash
• Models show that a
red giant should
shrink and become
less luminous after
helium fusion begins
in the core
When a low-mass star can no longer fuse
hydrogen into helium in its core_______
A. hydrogen fusion will begin in a shell
around the core.
B. helium will begin to fuse into carbon in
the core.
C. all fusion reactions stop and the star
becomes a white dwarf.
D. the outer layers of the star blow off in a
slow but massive stellar wind.
As the helium core collapses;
the luminosity _________,
and the temperature ________ .
A.increases, increases
B.increases, decreases
C.decreases, stays the same
D.stays the same, increases
How does a low-mass star die?
Double Shell Burning
• The core fuses until a
critical mass of inert
carbon is in the core and
helium fusion stops.
• He fuses into carbon in a
shell around the carbon
core, and H fuses to He in
a shell around the helium
layer.
• fusion rate spikes in a
series of thermal pulses.
Planetary Nebulae
• A final pulse ejects
the H and He into
space as a planetary
nebula
• The core left behind
becomes a white
dwarf
Planetary Nebulae
Side view
Angled view
End view
The apparent shape of a planetary nebula
depends on the angle at which we see it.
End of Fusion
• Fusion progresses no further in a low-mass star
because the core temperature never grows hot
enough for fusion of heavier elements (some He
fuses to C to make oxygen)
• Degeneracy pressure supports the white dwarf
against gravity
Low-Mass Star Summary
1. Protostar
2. Main Sequence:H fuses to He in core
3. Red Giant: H fuses to He in shell
around He core
4. Helium Core Burning:
He fuses to C in core while H fuses
to He in shell
5. Double Shell Burning:
H and He both fuse in shells
Not to scale!
6. Planetary Nebula leaves white dwarf
behind
Life Track of a Sun-Like Star
• Helium burning stars
form a horizontal branch
• Double shell burning
stars are called
Assymtotic Branch
Giants (ABGs)
What have we learned?
• How does a star’s mass affect its life?
– Mass affects life length and how explosive the stars
death becomes.
• What are the life stages of a low-mass star?
– H fusion in core (main sequence)
– H fusion in shell around contracting core (red giant)
– He fusion in core (horizontal branch)
– Double-shell burning (red giant)
• How does a low-mass star die?
– Ejection of H and He in a planetary nebula leaves
behind an inert white dwarf
Life as a High-Mass Star
• Our goals for learning
• What are the final life stages of a high-mass star?
• How does a high-mass star die?
What are the life stages of a highmass star?
High-Mass Old Age
• Late life stages of high-mass stars are similar to
those of low-mass stars:
– Hydrogen core fusion (main sequence)
– Hydrogen shell burning (giant)
– Helium core fusion (supergiant)
– Feed-back loop continues with carbon core
fusion
Helium Capture
•
stars with >4MSun have core temperatures that
allow helium to fuse to make heavier elements
Advanced Nuclear Burning
•
Core temperatures in stars with >8MSun
allow fusion of elements as heavy as iron
Multiple Shell Burning
• Advanced nuclear
burning proceeds in a
series of nested shells
• This is a Red Supergiant
Life Tracks of
High Mass Stars
How does a high-mass star die?
• Iron is a dead end
for fusion because
nuclear reactions
involving iron do
not release energy
• (Fe has lowest mass
per nuclear particle)
• In an >8MSun star, iron
builds up in core until
degeneracy pressure is
overwhelmed.
• The core suddenly
collapses, then core
bounce causes a
supernova explosion
Type II Supernova Explosion
• Core degeneracy pressure
goes away because
electrons combine with
protons, making neutrons
and neutrinos
• Neutrons collapse to the
center, forming a neutron
star
Supernova Remnant
• Energy released by
collapse of core
drives outer layers
into space
• Heavier elements
are created in the
explosion.
Energy and neutrons released in supernova explosion enable elements
heavier than iron to form, including Au and U
Supernova Remnant
• Energy released by
collapse of core
drives outer layers
into space
• The Crab Nebula is
the remnant of the
supernova seen in
A.D. 1054
Neutron stars are barely as large as a small town
Life Stages of High-Mass Star
1. Protostar
2. Main Sequence:H fuses to He in core
3. Red Supergiant: H fuses to He in
shell around He core
4. Helium Core Burning:
He fuses to C in core while H fuses
to He in shell
5. Multiple Shell Burning:
Many elements fuse in shells
6. Supernova leaves neutron star behind
Not to scale!
What have we learned?
• What are the life stages of a high-mass star?
– They are similar to the life stages of a low-mass
star, but go further
– Higher masses produce higher core temperatures
that enable fusion of heavier elements
• How does a high-mass star die?
– Iron core collapses, leading to a supernova
How does a star’s mass
determine its life story?
Role of Mass
• A star’s mass determines its entire life story
because it determines its core temperature
• High-mass stars with >8MSun have short
lives, eventually becoming hot enough to
make iron, and end in supernova explosions
• Low-mass stars with <2MSun have long
lives, never become hot enough to fuse
carbon nuclei, and end as white dwarfs
• Intermediate mass stars can make elements
heavier than carbon but end as white dwarfs
The Mass of Stellar Corpses
• <1.44 Solar Masses, Degeneracy pressure
maintains the corpse => White Dwarf
• 1.44 - 3 Solar Masses, Degeneracy pressure
overwhelmed, Plasma becomes neutrons
=>Neutron Star
• >3 Solar Masses, Neutron pressure
overwhelmed, star collapses to a point
=>Black Hole!
What have we learned?
• How does a star’s mass determine its life story?
– A star’s mass determines its entire life story because it
determines its core temperature and how fast a star
uses its fuel.
– Low-mass stars have long lives, never become hot
enough to fuse carbon nuclei, and end as white dwarfs
– High-mass stars have short lives, eventually becoming
hot enough to make iron, and end in supernova
explosions.
– Intermediate mass stars can make elements heavier
than carbon but mostly end as white dwarfs.