Download The Life And Times Of A Star

Lecture 21
Stellar Evolution
Announcements
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Homework 11 due now
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Homework 12 – Due Monday April 30
– Unit
– Unit
– Unit
– Unit
62:
64:
65:
67:
RQ1,
RQ1,
RQ3,
RQ5,
P3, TY1
3, TY1
TY1, 2
P2, TY1
Pressure vs. Gravity
The PressureTemperature Thermostat
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Hotter temperatures:
–
–
–
–
–
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Atomic nuclei move faster.
They “touch” more frequently.
More fusion reactions each second.
More energy released.
HIGHER PRESSURES PUSH OUTWARD.
Cooler temperatures:
– Atomic nuclei move more slowly.
– Less fusion.
– PRESSURE GOES DOWN.
The Main Sequence
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More Mass means:
– More gravity, so the star weighs more.
– The star needs to create more internal pressure
to support its weight.
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What Creates More Internal Pressure?
– More nuclear fusion!
– More fusion = more heat and light!
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So more massive stars are brighter and
hotter.
Recall the H-R Diagram
Relates
temperature vs.
luminosity
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-3
-1
1
Stars are found
only in certain
regions of the
diagram.
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Main Sequence
Most (90%) of
stars are found in
this band that
runs diagonally
from the upper
left to the lower
right. This band
is called the
main sequence.
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1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Main Sequence
These stars are
brighter than main
sequence stars of
the same
temperature, so
they must be larger.
They are called
giants (or red
giants, because
many are cool, red
stars).
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1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Main Sequence
These stars are
brighter (and
therefore larger)
than even giants of
the same
temperature.
They’re called
supergiants.
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1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Main Sequence
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1
Finally, these stars
are hot (white and
blue-white), but
very dim, so they
must be very small.
They are called
white dwarfs.
3
5
7
9
40,000 20,000 10,000 5,000 2,500
But What Are These
Types of Stars?
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The Main Sequence are normal stars.
They burn hydrogen to helium in their cores.
They have a relationship between mass, temperature, and
luminosity:
– More massive = bigger (higher R)
– More massive = hotter (higher T)
– More massive = brighter (higher L)
But What Are These
Types of Stars?
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The (Red) Giants are very old stars nearing the end of their
life.
They have run out of hydrogen fuel in their cores.
They are low and intermediate mass (typically less than 8
times the sun’s mass).
They are large (R = 10 to 100) and bright (L = 100 to 10,000)
One Day The Sun Will
Become A Red Giant!
Supergiants
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The supergiants are also stars near the end of their lives.
They have run out of hydrogen fuel in their cores.
They are high mass (more than 8 solar masses).
They are very large (R = 100 to 1,000) and extremely bright
(L = 10,000 to 1,000,000).
The White Dwarfs
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White dwarfs are what low and intermediate
mass stars become when they die.
They are tiny, hot, collapsed stars that have totally
run out of nuclear fuel.
What Makes The Stars
Shine?
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Stars shine because of NUCLEAR
FUSION.
Atomic nuclei all have positive electric
charge.
Two nuclei repel each other with a
very powerful force.
The bigger the nuclei, the more
powerful the repulsive force.
What Makes The Stars
Shine?
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Normally this force keeps nuclei from
getting close together.
At very high temperatures, the nuclei
move so quickly that they can “touch”
(sort of) before the repulsive force can
push them apart.
What Makes The Stars
Shine?
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The nuclei then “fuse” into a single nucleus.
IF the two nuclei were lighter than iron:
– The new nucleus is just a bit lighter than both
old nuclei together.
– The “missing” mass is converted into energy:
heat and light.
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IF the two nuclei were as heavy/heavier
than iron:
– The new nucleus is just a bit heavier than both
old nuclei together.
– Heat energy is converted into some extra mass.
Nuclear Fusion
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So fusing elements lighter than iron
gives off energy.
Fusing elements heavier than iron
takes in energy.
Some Like It Hot…
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Fusion produces a LOT of energy, but things need
to be VERY hot and under very high pressure to get
it started.
Heavier nuclei than hydrogen require even HIGHER
temperatures and pressures to start fusion!
The Fuel Of The Stars
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Nearly all stars fuse
hydrogen into
helium.
BUT at high enough
temperatures, a star
can fuse heavier
elements as fuel:
– Helium into Carbon,
Nitrogen, and Oxygen.
– Oxygen into Magnesium
and Neon.
– And others….
The Mass-Luminosity
Relation
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There is a definite
relationship
between a main
sequence star’s
mass and it’s
luminosity:
M=0.2
L=0.0036
M=1
L=1
L = M3.5
M=3.2
L=58.6
The Mass-Luminosity
Relation
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The Mass-Luminosity relation can be used to
figure out how long before a star begins to
run out of hydrogen fuel.
– A star’s mass determines how much hydrogen
fuel it has to burn (more matter = more fuel)
– A star’s luminosity determines how fast the
hydrogen fuel is burned (more luminous =
burning fuel faster)
Stars = Cars?
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A low-mass star is like
an economy car:
– Small fuel tank
– Poor performance (low
energy output)
– Excellent “gas mileage.”
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Low mass stars burn
their fuel very, very
slowly.
So they last a very
long time.
Stars = Cars?
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A high-mass star is like
a sports car:
– Larger fuel tank
– High performance (high
energy output)
– Poor “gas mileage.”
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High mass stars burn
their fuel very, very
quickly.
So they run out of fuel
very quickly.
Live Fast, Die Young
Lifetime of a mainsequence star:
t = 1/M2.5 × 10 billion
(years)
…gives the length of
time before the star
runs out of
hydrogen fuel.
Something’s Wrong…
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But let’s do a quick
test…
The sun’s mass is
2×1030 kg. About 75%
of that mass is
hydrogen.
The sun converts about
6×1011 kg of hydrogen
into helium every
second.
Something’s Wrong…
So the sun has 0.75 × (2 × 1030) =
1.5×1030 kg of hydrogen available.
Every second it uses 6×1011 kg of that
hydrogen.
So the sun should be able to fuse
hydrogen into helium for:
1.5×1030 / 6×1011 = 2.5×1018 s
That’s equal to 80 billion years.
What Gives?
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So why will the sun
run out of fuel in
only 10 billion
years?
Has to do with the
structure of the
sun!
The Sun’s Fuel Tank
Remember the sun’s inside is
divided into layers…
Convection Zone
Radiative Zone
Core
The Sun’s Fuel Tank
Only the CORE
has temperatures
and pressures high
enough for
hydrogen to
helium fusion! So
once the core runs
out of hydrogen,
the sun has
effectively run out
of fuel!
Different On The Inside
Here’s the inside of the sun
again. Other main sequence
stars look different on the
inside.
Convection Zone – the
gas here is “churning”
or “boiling”
Radiative Zone – the
gas doesn’t “flow”
here – light and heat
radiate through this
layer.
Core – where fusion is
happening
Different On The Inside
High mass stars have radiative
outsides and convective
insides.
Radiative Zone – the
gas doesn’t “flow”
here – light and heat
radiate through this
layer.
Convection Zone – the
gas here is “churning”
or “boiling”
Core – where fusion is
happening
Different On The Inside
Very low mass stars are
convective all the way through.
Convection Zone – the
gas here is “churning”
or “boiling”
Core – where fusion is
happening
The Life And Times Of A
Star
Here’s a diagram of
a main sequence
star near the
beginning of its life.
Hydrogen is being
fused into helium in
the star’s core.
The Life And Times Of A
Star
Helium is heavier
than hydrogen, so it
sinks down to the
very center of the
star and collects.
The Life And Times Of A
Star
The star’s core isn’t
hot enough to fuse
helium.
So the tiny little
knot of helium starts
to collapse under its
own weight (no
fusion to provide
internal pressure)
The Life And Times Of A
Star
The little knot of
helium heats up a
little as it collapses.
The Life And Times Of A
Star
And this extra heat
flows out into the
core, heating up the
core (just a little
bit).
The Life And Times Of A
Star
Because the core is just a
little bit hotter, it fuses
hydrogen to helium just
a little bit faster. Two
things happen.
1. Helium “ash” is
produced a little faster
than before.
2. The extra heat from
the faster fusion flows
out into the rest of the
star.
The Life And Times Of A
Star
The extra internal
pressure from the extra
heat makes the star
swell up (just a tiny little
bit) and cool off (just a
tiny little bit).
Why cool off?
Remember when you
expand a gas (swell up),
you cause it to cool off
a bit.
The Life And Times Of A
Star
This process is very
slow.
Over time, the helium
core gets bigger.
The star also gets
bigger and cooler, BUT:
The core is hotter, so
there is more fusion, so
the star is also getting
brighter.
The Life And Times Of A
Star
For a very, very long
time this process is very,
very slow.
Over the last 5 BILLION
years, the sun has
doubled in brightness
and increased in size by
perhaps 10%.
The Life And Times Of A
Star
Over the next 5
BILLION years the sun
will again double in
brightness, and continue
to slowly grow in size
(and get cooler).
Main Sequence Evolution
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So as they age, main sequence stars:
– Very slowly get brighter.
– Very slowly get bigger.
– Very slowly get cooler (a bit redder).
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Eventually (after about 10 billion years
for our sun), the helium core will begin
to take up most of the sun’s core.
The Life And Times Of A
Star
Eventually (after about 10
billion years for our sun), the
helium core will begin to take
up most of the sun’s core.
The sun’s core will be much
hotter than when it was first
born.
Hydrogen is now burning in a
thick shell around a “dead”
helium core. And it’s
burning very, very fast!
The Life And Times Of A
Star
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The hydrogen fusion is happening very, very
fast now, so a LOT of waste helium is
getting produced and dumped onto the
dead helium core.
The dead helium core is now very big and
massive, but still isn’t producing energy
using nuclear fusion, so it continues to
collapse under its own weight.
The Life And Times Of A
Star
As the helium core
collapses it produces
enormous amounts of
heat.
The fusion becomes even
faster in the hydrogen
shell.
Also areas once too cool
for fusion are now hot
enough to begin to
convert hydrogen into
helium.
The Life And Times Of A
Star
The tremendous heat
produced floods into the
star’s outer layers.
The outer layers continue
to swell up and cool off.
This is like what was
happening when the star
was main sequence, but
it’s happening MUCH,
MUCH faster.
The star is swelling into a
RED GIANT!
The End Of The Main
Sequence
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The star’s core is now full of helium ash collapsing
under it’s own weight.
The star has run out of hydrogen fuel in its core.
The hydrogen fusion is now happening in a thin
shell surrounding the “dead” core.
All the extra energy has swelled the stars outer
layers a great deal, and caused the star to cool off.
It has left the main sequence and become a red
giant.
The Red Giant Phase
The Power Source:
Hydrogen-burning shell
slowing burning out
from the core.
Waste helium is
dumped onto core.
The Red Giant Phase
The Helium Core:
Getting heavier (more
massive) with time.
Slowly shrinking.
Heating up as it shrinks.
Extra heat is speeding up
fusion in the surrounding
hydrogen shell.
The Red Giant Phase
Outer Layers
(Envelope):
Slowly expanding away
from the core.
Star is getting larger with
time.
Getting cooler as they
expand.
Star is getting redder.
The Red Giant Phase
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So as the red giant
ages:
– It gets bigger.
– It gets brighter.
– It gets cooler and
redder.
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Similar to a main
sequence star, but
it’s happening
much, much faster!
About How Big Will Our
Sun Get?
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Here’s the sun today …
About How Big Will Our
Sun Get?
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A few million years after leaving the
main sequence, the sun will be much
bigger and brighter…
L = 50
T = 3,700 K
R = 17
About How Big Will Our
Sun Get?
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For the next few million years, the sun
continues to swell and become
brighter…
L = 2,100
T = 2,900 K
R = 183
How Big Will The Sun
Get?
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This is large enough to swallow Mercury and
Venus, but the Earth is spared.
Red Giants Lose Mass
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The sun’s mass hasn’t gone up. So
sun’s surface gravity is now over
10,000 times weaker (because the
surface is over 100 times farther from
the center).
The escape velocity from the sun’s
surface is reduced by 10 times.
The fastest moving gas atoms at the
sun’s surface can now easily escape
into space.
Red Giants Lose Mass
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THE RESULT: The solar wind becomes much
more intense.
The sun begins to quickly lose several percent
of its mass every few million years.
– The sun is “recycling” hydrogen back into
space.
– As it becomes lighter, it’s gravity weakens.
– Mass loss speeds up with time.
– The remaining planets drift outward a bit.
A Second Birth…
In the core:
The temperatures
eventually reach 100
million K.
Helium begins to fuse into
Carbon, Nitrogen, and
Oxygen.
Helium Ignition Has
Begun!
The Helium Flash
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When the sun was first born hydrogen
to helium fusion began slowly.
Helium fusion begins very quickly:
– Called the Helium Flash!
– Core Temperature rises to over 350
million K!
– Energy output spikes dramatically (goes
up by millions of times).
Reset The Thermostat!
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The Pressure-Temperature Thermostat
kicks back in:
– The expansion cools the star’s core off.
– Puts out the hydrogen burning shell.
– Star’s luminosity and size go down, and
the star shrinks in size a little.
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The star becomes a stable heliumburning giant.
The Sun’s Second Life
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The sun is now a horizontal branch
star (we’ll see why next time).
L = 40
T = 4,500 K
R = 10.5
For Next Time
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Look over Units 64, 65 and 66
We will discuss the final fate of stars
including the Sun (and the Earth).
– aka, the kaboom lecture