Download The Lives of Stars

The Lives
of Stars
Announcements
n 
n 
n 
n 
Quiz # 5 will take place on Tuesday, November 15th.
–  The link `Quizzes on the website contains the
relevant information, including the Textbook units
that are basis for the quiz.
From now on, please try to come for all quizzes/
exams. No much time for make-ups.
Homework # 5 is due on Tuesday Nov 15th.
Homework # 6 starts on Tuesday, Nov 15th. It is due
on Tue, Nov 22nd.
Assigned Reading
n  Units
58, 59, 60, 61.1-2-3, 62, 64, 66
Let s recall:
Hydrostatic Equilibrium of Stars
Thermal
Pressure
Gravitational
Contraction
What happens if we increase the
mass of the star?
n  More
mass = more gravitational
contraction
n  = need for more balancing pressure =
higher temperature at the center (and
on the surface)
n  Higher temperature = more hydrogen
fusion = higher energy production =
more luminous
Thus…
n  More
massive =
n  Higher Temperature
(bluer color) =
n  More luminous
L ~ M3.5
A star 10 times more massive than the Sun is ~3000
times more luminous!
Stellar Lifetimes
A star s lifetime depends on its mass (the tank of fuel)
and its luminosity (the rate at which the fuel is
burned).
Even though a more massive star starts off with more
hydrogen (fuel), it burns it much faster than a less
massive star – so it dies earlier.
tlife ~ M / L ~ M / M3.5 = 1/ M2.5
A star that is 10 times more massive than our Sun has
a lifetime which is about 300 times shorter.
-  Our Sun: 10 billion years
-  A 10 Msun star: 30 million years
More massive stars live shorter lives!
The Role of the Mass in Stars
q  Mass is everything for a star.
q  It determines:
q Its luminosity (L ~ M3.5)
q Its temperature
q Its lifetime ( t ~ 1 / M2.5)
q … and how it dies!
q  Low-mass stars like our Sun are far more
common than high-mass stars.
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram
The Main Sequence
- all main sequence
stars fuse H into He
in their cores
- this is the defining
characteristic of a
main sequence star.
- more massive stars
are more luminous and
hotter: L=4πR2 σT4
The Hertzsprung-Russell Diagram
L=4πR2 σT4
Red Giants
- Red Giant stars
are very large, cool
and quite bright.
Ex. Betelgeuse is
100,000 times more
luminous than the Sun
but is only 3,500K on
the surface. It s radius
is 1,000 times that of the
Sun.
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram
White Dwarfs
- White Dwarfs
are hot but since
they are so small,
they are not very
luminous.
L=4πR2 σT4
The Hertzsprung-Russell Diagram
Mass of
Star
Size of Star
The Hertzsprung-Russell Diagram
Shorter (more mass)
Lifetime
of Star
Longer (less mass)
Measuring Ages with the HR Diagram
We can date
a star cluster
(stars born
together) by
observing its
population of
stars.
The oldest clusters
known have been
measured to be
~13.5 billion years old.
All these stars in the
cluster have burned
themselves out!
Stages in star Birth
1) 
A cloud of gas/dust begins to gravitationally
collapse.
2)  The collapsing gas clouds emit blackbody
radiation and are very bright. They are
protostars.
3)  Eventually, the core of the protostar reaches
10 million Kelvin and nuclear fusion ignites. The
star is now an official Main Sequence star.
What happens to stars once
they reach the Main
Sequence?
Some live fast and die young.
Others plod along for a LONG, LONG time.
How they live, how they die, and what is left over
depends on the MASS.
Mass is everything!
High mass => Mass > 8 solar masses
Low/Interm. mass => 0.08 < Mass < 8 solar masses
Brown Dwarfs => < 0.08 solar masses
Brown dwarfs
Stars that never start fusion!

Stars must have masses at
least 8% of the Sun s



otherwise fusion never starts
(not hot enough)
Jupiter is only 0.1% of Sun s
mass
Between the two are sub stellar brown dwarfs

The boundary between stars
and planets is observationally
hard to define
first brown
dwarf seen:
Gliese 229B
How do stars that can start fusion (M> 8% Msun) age?
They spend
most of their
life cycle on the
Main Sequence.
Main Sequence
stars fuse
hydrogen into
helium in their
cores.
Main Sequence
stars are in
hydrostatic
equilibrium.
What happen when the star runs out of
hydrogen in the center?
Hydrogen
burning core
shell
Hydrogen
fuel
Thermal
pressure from
layers above core
compresses the
hydrogen layer
just above the
Helium `ashes’
and causes the
hydrogen to
initiate fusion in
a shell
Helium
“ash”
Core of a star
Up the red giant branch
Eventually, hydrogen will burn only in the outer parts
of the mostly-helium core. The star will swell to
enormous size and luminosity, and its temperature will
drop, becoming a red giant.
Sun in ~5 Gyr
Sun today
Not to scale!
Stars become Red Giants
Cool and Large
Why are Red Giants much rarer
than Main Sequence stars?
Main Sequence Star
time
Protostar
Red Giant
Survey Question
(L=4πR2 σT4)
When Hydrogen shell fusion ignites, there is a dramatic
increase in the energy production of the core of the star.
Follow the energy: What happens to the outer layers of the
star?
1) the extra energy causes the outer layers to heat up
2) the extra energy creates thermal pressure which
pushes out on the outer layers
3) the extra energy is lost in the long and tortuous
journey out of the star
4) more than one of the above happens
Survey Question
(L=4πR2 σT4)
When Hydrogen shell fusion ignites, there is a dramatic
increase in the energy production of the core of the star.
Follow the energy: What happens to the outer layers of the
star?
1) the extra energy causes the outer layers to heat up
2) the extra energy creates thermal pressure which
pushes out on the outer layers
3) the extra energy is lost in the long and tortuous
journey out of the star
4) more than one of the above happens
Survey Question
The dramatic changes in the appearance of a
star during its red giant phase are primarily
due to
1) the changing chemical composition of the outer layers
affecting the fusion temperature and rate in the core.
2) the changing temperature and surface area of the outer
layers affecting the fusion temperature and rate in the
core.
3) the changing fusion temperature and rate in the core
affecting the chemical composition of the outer layers
4) The changing fusion temperature and rate in the core
affecting the temperature and surface area of the outer
layers
Stars like the Sun (M< 8 Msun)
n 
n 
n 
n 
We left our Sun-like star
as a Hydrogen-shell
burning red giant
Now the core is hot
(100,000,000 K) and
`heavy enough to start
Helium-core burning
The star contracts to a
yellow giant.
Helium burns and
produces an element
“crucial” to our existence:
– 
CARBON
The Core of a Star changes composition…
Core Composition of the Sun
70%
50%
100%
50%
30%
H
He
5 billion
years ago
H
He
today
Carbon
5 billion
years from
now
Survey Question
Fusion in a Main Sequence star changes
the chemical composition of the core.
What happens to the material outside the
core?
1) Helium becomes more abundant outside the core.
2) The chemical composition outside the core changes very
little.
3) The same changes occur outside the core as within the
core.
4) Hydrogen becomes more abundant outside the core.
After helium fusion gets going in a shell…
The Sun will expand and cool again, becoming a red
(super) giant. Earth, cooked to a cinder during the red
giant phase, will be engulfed and vaporized within the
Sun. At the end of this stage, the Sun’s core will consist
mostly of carbon, with a little oxygen.
The end of the line
In its last phase the grossly
distended Sun will begin to
pulse, becoming unstable.
Eventually, the outer parts of
the Sun, about half its mass,
will break away and the Sun
will die.
The expanding cloud of gas
will resemble a planet in
appearance (glowing,
roundish) and be called a
planetary nebula. The hot
core will be a white dwarf.
Helix Nebula--125 pc
Summary:
Life Stages of
a low-mass star
White dwarfs have
typically 1/100 the
radius of the Sun
(about the size of the
Earth), and a mass <
1.4 Msun (they shed a
lot of mass during
their lifetime!).
Life of a Low Mass (< 8 Msun) Star on the HR Diagram:
White dwarfs
start very hot
(they are the
nuclei of stars,
incredibly dense
balls of Helium
and Carbon!), and
passively cool
down to become
black dwarfs.
What happens in massive stars
(>8 Msun)
Fusion continues up to the heaviest possible element (iron)
The lead-up to disaster in massive stars
n 
n 
n 
Iron cores do not
immediately collapse (the
electrons cannot be
`squeezed’).
If the density continues to
rise, eventually the
electrons are forced to
combine with the protons
– resulting in neutrons.
What comes next is …
core collapse.
Supernova 1987a before/after
Massive Star Explosions:
Supernovae
n 
The gravitational collapse of the core releases an
enormous amount of energy (gravitational potential energy
is converted to radiative energy as the material falls in; the
layers outside the iron core ignite and cause a run-off fusion).
n 
All the shells ignite, and the stars literally explodes
- A neutron star or black hole (core cadaver) is left
n 
100 times the total amount of energy produced by the Sun
in its lifetime is released in a matter of seconds.
–  For a few days, the star is, sometimes, ~as luminous as a whole
galaxy!!!
Supernovae are pretty easy to
`see
Some supernovae have been detected from the deepest past of
our Universe! (this is how `dark energy has first been revealed!!!)
Supernova Remnant: Crab
Nebula
The supernova
explosion that
created the
Crab was seen
on about July
4, 1054 AD.
Summary:
Life stages of a high
mass star.
A massive (e.g., 25
M_sun) star burns:
• H in 7 million years
• O in 6 months
• Si in 1 day
• Then… Booom
• Core collapse in
~0.001-0.01 sec
Life of a High Mass (> 8 Msun) Star on the HR Diagram:
Supernova
explosion
Survey Question
The Sun will never supernova because
1) It will become a white dwarf before it has the chance.
2) Its surface temperature is not high enough.
3) It is not large enough.
4) It is not bright enough.
5) It is not massive enough.
Stellar Evolution in a Nutshell
M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mass controls the
evolution of a star!
Mcore > 3MSun
End Products of Stars
M > 8 Msun à Supernova + neutron star
or a black hole
n  0.08 Msun < M < 8 Msun à White dwarf
n  M < 0.08 Msun à Brown dwarf (fusion never
starts)
n 
Key points of where elements are produced:
Elements Generated in the Big Bang
Elements Produced in Low Mass Stars
Elements Produced in High Mass Stars
Elements Produced in Supernovae
We are supernova remnants