Download Chapter 12 Stellar Evolution

Chapter 12 Stellar Evolution
Units of Chapter 12
Leaving the Main Sequence
Evolution of a Sun-like Star
The Death of a Low-Mass Star
Evolution of Stars More Massive than the Sun
Supernova Explosions
Observing Stellar Evolution in Star Clusters
The Cycle of Stellar Evolution
Summary of Chapter 12
12.1 Leaving the Main Sequence
During its stay on the main sequence, any
fluctuations in a star’s condition are
quickly restored; the star is in equilibrium.
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!
Even while on the
main sequence, the
composition of a
star’s core is
changing.
12.2 Evolution of a Sun-like Star
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.
Stages of a star leaving the main sequence.
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.
The red giant
stage on the H–R
diagram
Stage 10: Helium fusion
Once the core temperature has risen to
100,000,000 K, the helium in the core starts
to fuse.
The helium flash:
Helium begins to fuse extremely rapidly;
within hours the enormous energy output is
over, and the star once again reaches
equilibrium.
Stage 10 on
the H–R
diagram
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.
The star has
become a red
giant for the
second time.
12.3 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.
There is no more outward fusion pressure being
generated in the core, which continues to contract.
Stage 12: The outer layers of the star expand to form
a planetary nebula.
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.
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.
The small star Sirius B is a white dwarf
companion of the
much larger and
brighter Sirius A.
The Hubble Space Telescope has detected
white dwarf stars in globular clusters
As the white dwarf cools, its size does not
change significantly; it simply gets dimmer and
dimmer, and finally ceases to glow.
A nova is a star that flares up very suddenly and
then returns slowly to its former luminosity.
A white dwarf that is part of a semi-detached
binary system can undergo repeated novas.
Material falls onto
the white dwarf from
its main-sequence
companion.
When enough
material has
accreted, fusion can
reignite very
suddenly, burning
off the new material.
Material keeps being
transferred to the
white dwarf, and the
process repeats.
As the sun ages, the chemical
composition of its core changes
so that it contains a lower
percentage of ______ and a
greater percentage of ______.
A. helium, hydrogen
B. hydrogen, helium
C. uranium, lead
D. oxygen, carbon
Which of the following is not
true of red giants
A. their average density is very
low.
B. molecules are prominent in
their spectra.
C. most are variable stars.
D. most are pre-main sequence
stars.
As a one solar mass star evolves
to the red giant stage:
A. its surface temperature and its
luminosity increase.
B. its surface temperature and its
luminosity decrease.
C. its luminosity decreases and its
surface temperature increases.
D. its luminosity increases and its
surface temperature decreases.
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?
A. by chemical reactions.
B. by other fusion reactions.
C. by gravitational contraction.
D. none of these; the fusion
reactions stop.
Which of the following are old
stars with no current nuclear
reactions?
A. red giants
B. main sequence stars
C. white dwarfs
D. proto stars
12.4 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.
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.
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.
The sequence below, of actual Hubble images,
shows first a very massive star, then a very
unstable red giant star as it emits a burst of
light, illuminating the dust around it.
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.
12.5 Supernova Explosions
A supernova is incredibly luminous, as can be
seen from these curves – more than a million
times as bright as a nova.
A supernova is a one-time event – once it
happens, there is little or nothing left of the
progenitor star.
There are two different types of supernovae,
both equally common:
Type I, which is a carbon-detonation supernova;
Type II, which is the death of a high-mass star.
Carbon-detonation supernova: White dwarf that has
accumulated too much mass from binary companion
If the white dwarf’s mass exceeds 1.4 solar masses,
electron degeneracy can no longer keep the core from
collapsing.
Carbon fusion begins throughout the star almost
simultaneously, resulting in a carbon explosion.
This graphic illustrates the two different types of
supernovae.
Supernovae leave remnants – the expanding
clouds of material from the explosion.
The Crab Nebula is
a remnant from a
supernova
explosion that
occurred in the
year 1054.
12.6 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 cluster as a whole
ages.
After 10 million years, the
most massive stars have
already left the main
sequence, whereas many
of the least massive have
not even reached it yet.
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.
After 10 billion
years, a number of
features are
evident:
The red giant,
subgiant,
asymptotic giant,
and horizontal
branches are all
clearly populated.
White dwarfs, indicating that solar-mass stars
are in their last phases, also appear.
This double cluster, h and  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.
The Hyades cluster, shown here, is also
rather young; its main-sequence turnoff
indicates an age of about 600 million years.
This globular cluster, M80, is about 10-12 billion
years old, much older than the previous
examples.
12.7 The Cycle of Stellar Evolution
Star formation is
cyclical: stars form,
evolve, and die.
In dying, they send
heavy elements into
the interstellar
medium.
These elements then
become parts of new
stars.
And so it goes.
Massive stars have short lifetimes
because they
A. have little available fuel.
B. can't sustain high enough
temperatures.
C. are too large.
D. consume their fuel more
rapidly.
Which of the following is the single
most important indicator of how a
star will evolve?
A. Radius (size).
B. Chemical composition.
C. Mass.
D. Surface temperature.
Which of the following stars is
probably oldest?
A. A one solar mass main
sequence star.
B. A one solar mass white dwarf.
C. A ten solar mass main sequence
star.
D. A ten solar mass red giant.
Which of the following is not a
necessary ingredient in the
construction of a theoretical star
model?
A. A balance between gravity and gas
pressure.
B. A knowledge of the star's position and
motion in space.
C. A knowledge of the star's mass and
chemical composition.
D. A balance between the star's luminosity
and the amount of energy generated.
The more massive a main sequence
star is, then the
A. redder it is.
B. more luminous it is.
C. more time it spends on the main
sequence.
D. greater percentage of heavy
elements it contains.
When a star dies, it becomes a
supernova
A. always.
B. only if it is a few times more
massive than the sun.
C. only if it includes the whole
galaxy.
D. never.
Type I supernovae occur in
A. interstellar clouds.
B. binary star systems.
C. young star clusters.
D. globular clusters.
The crab nebula is
A. a supernova
remnant.
B. a newly forming
star.
C. an h-2 region.
D. a black hole.
A type II supernova explosion
A. involves a massive,
population I star.
B. blows off a large fraction of
the star's mass.
C. peaks about a month after
the explosion begins.
D. all of the above.
E. none of the above.
Stellar remnants with masses
between 1.4 and 3 solar masses
will be
A. white dwarfs.
B. neutron stars.
C. black holes.
D. planetary nebulae.
Summary of Chapter 12
• Once hydrogen is gone in the core, a star
burns hydrogen in the surrounding shell. The
core contracts and heats; the outer atmosphere
expands and cools.
• Helium begins to fuse in the core, as a helium
flash. The star expands into a red giant as the
core continues to collapse. The envelope blows
off, leaving a white dwarf to gradually cool.
• Nova results from material accreting onto a
white dwarf from a companion star.
Summary of Chapter 12, cont.
• Massive stars become hot enough to fuse
carbon, then heavier elements, all the way to
iron. At the end, the core collapses and
rebounds as a Type II supernova.
• Type I supernova is a carbon explosion,
occurring when too much mass falls onto a
white dwarf.
• All heavy elements are formed in stellar cores
or in supernovae.
• Stellar evolution can be understood by
observing star clusters.