Download 12 Stellar Evolution

Chapter 12
Stellar Evolution
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
“We are stardust
Billion year old carbon
We are golden”
Woodstock by Joni Mitchell
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
Question 1
Stars like our
Sun will end
their lives as
Copyright © 2010 Pearson Education, Inc.
a) red giants.
b) pulsars.
c) black holes.
d) white dwarfs.
e) red dwarfs.
Question 1
Stars like our
Sun will end
their lives as
a) red giants.
b) pulsars.
c) black holes.
d) white dwarfs.
e) red dwarfs.
Low-mass stars eventually
swell into red giants, and
their cores later contract
into white dwarfs.
Copyright © 2010 Pearson Education, Inc.
Question 2
Elements heavier than
hydrogen and Helium
were created
Copyright © 2010 Pearson Education, Inc.
a) in the Big Bang.
b) by nucleosynthesis in massive stars.
c) in the cores of stars like the Sun.
d) within planetary nebulae.
e) They have always existed.
Question 2
Elements heavier than
hydrogen and helium
were created
a) in the Big Bang.
b) by nucleosynthesis in massive stars.
c) in the cores of stars like the Sun.
d) within planetary nebula
e) They have always existed.
Massive stars create
enormous core
temperatures as red
supergiants, fusing helium
into carbon, oxygen, and
even heavier elements.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Question 3
The Sun will
evolve away
from the main
sequence when
Copyright © 2010 Pearson Education, Inc.
a) its core begins fusing iron.
b) its supply of hydrogen is used up.
c) the carbon core detonates, and it
explodes as a Type I supernova.
d) helium builds up in the core, while the
hydrogen-burning shell expands.
e) the core loses all of its neutrinos, so all
fusion ceases.
Question 3
The Sun will
evolve away
from the main
sequence when
a) its core begins fusing iron.
b) its supply of hydrogen is used up.
c) the carbon core detonates, and it
explodes as a Type I supernova.
d) helium builds up in the core, while the
hydrogen-burning shell expands.
e) the core loses all of its neutrinos, so all
fusion ceases.
When the Sun’s core becomes
unstable and contracts,
additional H fusion generates
extra pressure, and the star
will swell into a red giant.
Copyright © 2010 Pearson Education, Inc.
Leaving the Main Sequence
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!
End times 1
End times 2
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
Even while on the
main sequence, the
composition of a star’s
core is changing.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
Stages of a star leaving the main sequence.
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
The red giant
stage on the H–
R diagram
Copyright © 2010 Pearson Education, Inc.
Question 4
The helium
flash occurs
Copyright © 2010 Pearson Education, Inc.
a) when T-Tauri bipolar jets shoot out.
b) in the middle of the main sequence stage.
c) in the red giant stage.
d) during the formation of a neutron star.
e) in the planetary nebula stage.
Question 4
The helium
flash occurs
a) when T-Tauri bipolar jets shoot out.
b) in the middle of the main sequence stage.
c) in the red giant stage.
d) during the formation of a neutron star.
e) in the planetary nebula stage.
When the collapsing core of
a red giant reaches high
enough temperatures and
densities, helium can fuse
into carbon quickly – a
helium flash.
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
Stage 10 on the
H–R diagram
Horizontal branch
lasts 10s of
millions of years
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of a Sun-like Star
The star has become
a red giant for the
second time.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Question 5
Stars gradually lose
mass as they
become white
dwarfs during the
Copyright © 2010 Pearson Education, Inc.
a) T-Tauri stage.
b) emission nebula stage.
c) supernova stage.
d) nova stage.
e) planetary nebula stage.
Question 5
Stars gradually lose
mass as they
become white
dwarfs during the
a) T-Tauri stage.
b) emission nebula stage.
c) supernova stage.
d) nova stage.
e) planetary nebula stage.
Low-mass stars forming
white dwarfs slowly lose
their outer atmospheres,
and illuminate these gases
for a relatively short time.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
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.
Copyright © 2010 Pearson Education, Inc.
Question 6
The source of
pressure that
makes a white
dwarf stable is
Copyright © 2010 Pearson Education, Inc.
a) electron degeneracy.
b) neutron degeneracy.
c) thermal pressure from intense core
temperatures.
d) gravitational pressure.
e) helium-carbon fusion.
Question 6
The source of
pressure that
makes a white
dwarf stable is
a) electron degeneracy.
b) neutron degeneracy.
c) thermal pressure from intense core
temperatures.
d) gravitational pressure.
e) helium-carbon fusion.
Electrons in the core cannot
be squeezed infinitely close,
and prevent a low-mass star
from collapsing further.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
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.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
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.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
The small star Sirius B is a white dwarf
companion of the
much larger and
brighter Sirius A.
Copyright © 2010 Pearson Education, Inc.
Question 7
In a white dwarf,
the mass of the
Sun is packed into
the volume of
Copyright © 2010 Pearson Education, Inc.
a) an asteroid.
b) a planet the size of Earth.
c) a planet the size of Jupiter.
d) an object the size of the Moon.
e) an object the size of a sugar cube.
Question 7
In a white dwarf,
the mass of the
Sun is packed into
the volume of
a) an asteroid.
b) a planet the size of Earth.
c) a planet the size of Jupiter.
d) an object the size of the Moon.
e) an object the size of a sugar cube.
The density of a white
dwarf is about a million
times greater than normal
solid matter.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
The Hubble Space Telescope has detected
white dwarf stars in globular clusters
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
As the white dwarf cools, its size does not
change significantly; it simply gets dimmer and
dimmer, and finally ceases to glow.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
A nova is a star that flares up very suddenly and
then returns slowly to its former luminosity.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
A white dwarf that is part of a semi-detached
binary system can undergo repeated novae.
Copyright © 2010 Pearson Education, Inc.
The Death of a Low-Mass Star
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of Stars More Massive
than the Sun
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of Stars More Massive
than the Sun
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of Stars More Massive
than the Sun
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.
Eta Carinae ~ 100 solar masses
Copyright © 2010 Pearson Education, Inc.
Evolution of Stars More Massive
than the Sun
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.
Copyright © 2010 Pearson Education, Inc.
Evolution of Stars More Massive
than the Sun
Copyright © 2010 Pearson Education, Inc.
Supernova Explosions
A supernova is incredibly luminous, as can be
seen from these curves – more than a million
times as bright as a nova.
Copyright © 2010 Pearson Education, Inc.
Supernova Explosions
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.
Copyright © 2010 Pearson Education, Inc.
Supernova Explosions
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.
Copyright © 2010 Pearson Education, Inc.
Supernova Explosions
This graphic illustrates the two different types of
supernovae.
Copyright © 2010 Pearson Education, Inc.
Question 8
A Type II
supernova
occurs when
Copyright © 2010 Pearson Education, Inc.
a) hydrogen fusion shuts off.
b) uranium decays into lead.
c) iron in the core starts to fuse.
d) helium is exhausted in the outer layers.
e) a white dwarf gains mass.
Question 8
A Type II
supernova
occurs when
a) hydrogen fusion shuts off.
b) uranium decays into lead.
c) iron in the core starts to fuse.
d) helium is exhausted in the outer layers.
e) a white dwarf gains mass.
Fusion of iron does not produce energy or provide pressure; the
star’s core collapses immediately, triggering a supernova explosion.
Copyright © 2010 Pearson Education, Inc.
Supernova Explosions
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.
Copyright © 2010 Pearson Education, Inc.
Which is the Supernova?
A
or
B
Copyright © 2010 Pearson Education, Inc.
Question 9
Astronomers
determine the age
of star clusters by
observing
Copyright © 2010 Pearson Education, Inc.
a) the number of main sequence stars.
b) the ratio of giants to supergiants.
c) the luminosity of stars at the turnoff
point.
d) the number of white dwarfs.
e) supernova explosions.
Question 9
Astronomers
determine the age
of star clusters by
observing
a) the number of main sequence stars.
b) the ratio of giants to supergiants.
c) the luminosity of stars at the turnoff
point.
d) the number of white dwarfs.
e) supernova explosions.
The H–R diagram of a cluster can
indicate its approximate age.
Turnoff point from the main
sequence
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Observing Stellar Evolution in Star
Clusters
After 100 million years,
a distinct mainsequence turnoff
begins to develop. This
shows the highestmass stars that are still
on the main sequence.
After 1 billion years, the
main-sequence turnoff
is much clearer.
Copyright © 2010 Pearson Education, Inc.
Question 10
In a young star
cluster, when more
massive stars are
evolving into red
giants, the least
massive stars are
Copyright © 2010 Pearson Education, Inc.
a) ending their main-sequence stage.
b) also evolving into red giants.
c) forming planetary nebulae.
d) barely starting to fuse hydrogen.
e) starting the nova stage.
Question 10
In a young star
cluster, when more
massive stars are
evolving into red
giants, the least
massive stars are
a) ending their main-sequence stage.
b) also evolving into red giants.
c) forming planetary nebulae.
d) barely starting to fuse hydrogen.
e) starting the nova stage.
More massive stars form much
faster, and have much shorter
main-sequence lifetimes.
Low-mass stars form more
slowly.
Copyright © 2010 Pearson Education, Inc.
Observing Stellar Evolution in Star
Clusters
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.
Copyright © 2010 Pearson Education, Inc.
Observing Stellar Evolution in Star
Clusters
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.
Copyright © 2010 Pearson Education, Inc.
Observing Stellar Evolution in Star
Clusters
The Hyades cluster, shown here, is also rather
young; its main-sequence turnoff indicates an
age of about 600 million years.
Copyright © 2010 Pearson Education, Inc.
Observing Stellar Evolution in Star
Clusters
This globular cluster, M80, is about 10-12 billion
years old, much older than the previous examples.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Question 11
A star will spend
most of its
“shining” lifetime
Copyright © 2010 Pearson Education, Inc.
a) as a protostar.
b) as a red giant.
c) as a main-sequence star.
d) as a white dwarf.
e) evolving from type O to type M.
Question 11
A star will spend
most of its
“shining” lifetime
a) as a protostar.
b) as a red giant.
c) as a main-sequence star.
d) as a white dwarf.
e) evolving from type O to type M.
In the main-sequence stage,
hydrogen fuses to helium.
Pressure from light and
heat pushing out balances
gravitational pressure
pushing inward.
Copyright © 2010 Pearson Education, Inc.
Question 12
As stars
evolve during
their mainsequence
lifetime
Copyright © 2010 Pearson Education, Inc.
a) they gradually become cooler and
dimmer (spectral type O to type M).
b) they gradually become hotter and
brighter (spectral type M to type O).
c) they don’t change their spectral type.
Question 12
As stars
evolve during
their mainsequence
lifetime
a) they gradually become cooler and
dimmer (spectral type O to type M).
b) they gradually become hotter and
brighter (spectral type M to type O).
c) they don’t change their spectral type.
A star’s main-sequence characteristics of surface
temperature and brightness are based on its mass.
Stars of different initial mass become different spectral
types on the main sequence.
Copyright © 2010 Pearson Education, Inc.