Transcript Nuclear Powerhouse

The Sun:
A Nuclear Powerhouse
8 March 2005
AST 2010: Chapter 15
1
Happy Sun
8 March 2005
AST 2010: Chapter 15
2
Why Does the Sun Shine?
The Sun gives off energy
The energy must come from somewhere
— there’s no free lunch
Conservation of energy is a fundamental
tenet of physics
Where does the energy come from?
Until the 20th century only 2 possibilities
were known:
Chemical reactions
Gravity
8 March 2005
AST 2010: Chapter 15
3
The Sun’s Energy Output
How bright is the Sun?
The Sun produces 4x1026 watts
The watt is the unit for the rate of
energy use, commonly seen on light
bulbs and appliances.
Our largest power plants
produce around 5 x 109 watts of
power (5,000 megawatts)
Sun’s power = 8 x 1016 of these
power plants (10,000 trillion)
Anyway you look at it, the
Sun gives off a lot of energy
8 March 2005
AST 2010: Chapter 15
4
Chemical Reactions
What are chemical reactions? An example:
Rearrange the atoms in molecules, as in
2H2+ O2  2H2O
This reaction combines hydrogen and oxygen
(gases) to produce water plus energy
Reverse the process: 2H2O  2H2 + O2
By adding energy, we can dissociate water into
hydrogen and oxygen
The energy factor is often left out of
chemical-reaction formulas, for convenience
8 March 2005
AST 2010: Chapter 15
5
Is the Sun Powered by Chemical Reactions?
If the Sun is powered by burning coal or
oil, how long could its fuel last?
Only a few thousand years!
A process that uses fuel more efficiently
is needed — something that gets more
energy out of every kilogram of material
8 March 2005
AST 2010: Chapter 15
6
Gravity Squeeze?
Gravitational contraction: falling layers of the
Sun's material compresses the Sun  heat energy
Drop a book  noise!
Gravitational potential energy turns into sound energy
A contraction of 40 m per day would account for
the Sun’s energy output
Efficiency ~ 1/10,000 %
Gravity could power the Sun for about 100 million
years
but the Sun is thought to be at least 4 billion years
old!
So gravity can't be the Sun's main energy source
but it did help ignite the Sun when it formed
8 March 2005
AST 2010: Chapter 15
7
Mass, Energy, and the Theory of Relativity
To understand the way the Sun produces
its energy, we need to learn a little
about nuclear physics and the special
theory of relativity
Nuclear physics deals with the structure
of the nuclei of atoms
The special theory of relativity deals with
the behavior of things moving at close to
the speed of light
8 March 2005
AST 2010: Chapter 15
8
Converting Mass to Energy
Out of the special theory of relativity
comes the most famous equation in
science: E = mc2
This equation tells us that mass (m) is
just another form of energy (E)!
The c2 is the square of the speed of light
1 gram of matter is equivalent to the
energy obtained by burning 15,000
barrels of oil
8 March 2005
AST 2010: Chapter 15
9
…but there are rules
We can’t simply convert atoms into energy
We rearrange the protons and neutrons in
nuclei to get a lower-mass configuration
The difference between initial mass and final
mass is converted to energy
Chemical energy comes from rearranging atoms to
configurations of lower energy (mass)
Nuclear energy comes from rearranging nuclei to
configurations of lower mass (energy)
In each case, we get out the energy difference
8 March 2005
AST 2010: Chapter 15
10
Elementary Particles
Particle
name
Mass
(MeV/c2)
Charge
(e)
Proton
938.272
+1
Neutron
939.565
0
Electron
0.511
-1
Neutrino
<10-6
0
Photon
0
0
8 March 2005
AST 2010: Chapter 15
5 particles play a
fundamental role
in the Sun
Protons and
neutrons make
atomic nuclei
Electrons orbit
nuclei of atoms
Photons are
emitted by the
Sun
Neutrinos are also
emitted
11
The Atomic Nucleus
Two ways to rearrange nuclei and get energy:
Fission
It produces energy by breaking up massive nuclei like
uranium into less massive nuclei like barium and krypton
A-bombs, nuclear reactors
Fission needs uranium-235 and plutonium-238
Problem: no uranium or plutonium in the Sun
Fusion
It produces energy by fusing
light nuclei like hydrogen to
make more massive nuclei
like helium
H-bomb
The Sun has lots of
Hydrogen!!
8 March 2005
AST 2010: Chapter 15
12
How Does Fusion Work?
Nuclear fusion:
a process by which two light nuclei combine to form
a single larger nucleus
However, nuclei are positively charged
Like charges repel
Two nuclei naturally repel each other and thus
cannot fuse spontaneously
For fusion, electrical repulsion must be “overcome”
When two nuclei are very close, the strong
nuclear force takes over and holds them
together
How do two nuclei get close enough?
8 March 2005
AST 2010: Chapter 15
13
Fusion needs fast moving nuclei
Fast moving nuclei
can overcome the
repulsion
Low speed
They get a running
start
Lots of fast moving
nuclei means high
temperature
High speed
The core of the Sun
has a temperature of
15 million degrees
kelvin
8 March 2005
AST 2010: Chapter 15
14
Fusion Powers the Sun
Temperatures in the cores of stars are
estimated to be above the 8 million K
needed to fuse hydrogen nuclei together
Calculations have shown that the
observed power output of the Sun is
consistent with the power produced by
the fusion of hydrogen nuclei
The observed neutrinos from the Sun
produced are expected as one of the
byproducts of fusion reactions
Hypothesize: all stars produce energy by
nuclear fusion
8 March 2005
AST 2010: Chapter 15
15
Proton-Proton Chain
H  H 2H  e  
2
8 March 2005
• Fuse two hydrogen
(H=1 proton) to make
deuterium (2H=1
proton+1 neutron),
neutrino, and positron
• Fuse one deuterium and
one hydrogen to make
helium-3 (3He=1
proton+2 neutrons) and
a gamma ray (energetic
photon)
• Fuse two helium-3 to
make helium-4 (4He)
and two hydrogen
H  H 3He  
AST 2010: Chapter 15
3
He  3He  4He  H  H
16
Why a Complicated Chain?
Fusion would be simpler if four protons would
collide simultaneously to make one helium
nucleus
That is simpler, but less likely
rare for four objects to collide simultaneously with
high enough energy
chance of this happening are very, very small
rate too slow to power the Sun
The proton-proton chain: each step involves
collision of two particles
chance of two particles colliding and fusing is much
higher
so nature slowly builds up the helium nucleus
8 March 2005
AST 2010: Chapter 15
17
Fusion and Solar Structure
Fusion occurs only in Sun's core
This is the only place that is hot enough
Heat from fusion determines the Sun's
structure
8 March 2005
AST 2010: Chapter 15
18
Heat from Core Determines Sun's Size
Force equilibrium
Hydrostatic equilibrium:
balance between
thermal pressure from the
hot core pushing outwards
gravity squeezes the star
collapse to the very center
Nuclear-fusion rate is very
sensitive to temperature
A slight increase/decrease in temperature
causes fusion rate to increase/decrease by a
large amount
8 March 2005
AST 2010: Chapter 15
19
Gravity and Pressure
Force equilibrium
Newton's second law: F = ma
Static equilibrium: no acceleration if
forces on object balance
Gravity tries to pull 1/4 pounder to
center of the Earth
Pressure from table opposes gravity
Hydrostatic equilibrium in the Sun
“Cloud of gas" (like 1/4 pounder)
Gravity pulls cloud to the center
Pressure from gas below opposes
gravity
Heat from fusion in the hot core
increases pressure
Energy output controls size of sun!
8 March 2005
AST 2010: Chapter 15
pressure
from
table
weight
from
gravity
pressure
from hot
gas
cloud
weight
from
gravity
20
Temperature and
Pressure
Temperature
corresponds to the
random motion of
atoms in a gas
Pressure is the amount
of force per unit area
on piston from gas
Generally pressure
increases with
increasing temperature
8 March 2005
AST 2010: Chapter 15
21
Balancing Fusion, Gravity, and Pressure
If the fusion rate increases, then
thermal pressure increases causing the star to expand
star expands to a new point where gravity would balance the
thermal pressure
the expansion would reduce compression of the core
the temperature in the core would drop
the nuclear fusion rate would subsequently slow down
the thermal pressure would then drop
the star would shrink
the temperature would rise again and the nuclear fusion rate
would increase
stability would be re-established between the nuclear reaction
rates and the gravity compression
8 March 2005
AST 2010: Chapter 15
22
Hydrostatic Equilibrium
The balance between pressure, heat from fusion, and
gravity determines the Sun's size
Big stars have cooler cores
Small stars have hotter cores and, thus, are more
compressed
8 March 2005
AST 2010: Chapter 15
23
Other Particles
Helium is not the only product in the
fusion of hydrogen
Two other particles are produced
Positrons
Neutrinos
8 March 2005
AST 2010: Chapter 15
24
Gamma-Ray Propagation in the Sun
The positrons quickly annihilate the electrons
Photons produced in core of the Sun take
about a million years to move to the surface
This migration is slow because
they scatter off the dense gas
particles
The photons move about only
a centimeter between collisions
In each collision, they transfer
some of their energy to the gas particles
As they reach the photosphere, gamma rays
have become visible photons
Because the photons have lost some energy in their
journey through the Sun
8 March 2005
AST 2010: Chapter 15
25
Neutrinos
These particles have no charge and are
nearly massless
They rarely interact with ordinary matter
Neutrinos travel extremely fast
at almost the speed of light if their mass is
tiny
Neutrinos pass from the core of the Sun
to its surface in only two seconds
They take less than 8.5 minutes to
travel the distance from the Sun to the
Earth
8 March 2005
AST 2010: Chapter 15
26
Neutrino Counting
In principle
We can use neutrino count at Earth as
indicator of the Sun’s energy output
The problem:
Neutrinos have a very low probability of
interacting with matter
They could pass through a light year of lead
and not be stopped by any of the lead
atoms!
8 March 2005
AST 2010: Chapter 15
27
Neutrino Abundance
The Sun produces a lot of neutrinos
In one second several million billion
neutrinos pass through your body
Do you feel them?
Not to worry!
The neutrinos do not damage anything
The great majority of neutrinos pass right
through the entire Earth as if it weren’t there
8 March 2005
AST 2010: Chapter 15
28
Detecting Neutrinos
Increase the odds of detecting neutrinos
by using a LARGE amount of a material
that reacts with neutrinos in a
measurable way
A chlorine isotope changes to a radioactive
isotope of argon when hit by a neutrino
A gallium isotope changes to a radioactive
isotope of germanium
Neutrinos can interact with protons and
neutrons and produce an electron
The electron can be detected
8 March 2005
AST 2010: Chapter 15
29
Neutrino Detectors
Neutrino detectors use
hundreds of thousands of
liters of these materials in a
container buried under
many tens of meters of rock
to shield the detectors from
other energetic particles
from space called cosmic
rays
Even the largest detectors
can detect only a few
neutrinos per day
8 March 2005
AST 2010: Chapter 15
30
Solar Neutrino Production (1)
Number of neutrinos produced in the Sun is directly
proportional to the number of nuclear reactions taking
place in the Sun's core
Same principle with neutrinos produced via the CarbonNitrogen-Oxygen chain
The more reactions there are, the more neutrinos are
produced and the more that should be detected here
on the Earth
Physicists find that the number of neutrinos coming
from the Sun is smaller than expected
Early experiments detected only 1/3 of the expected number
of neutrinos
These experiments used hundreds of thousands of liters of
cleaning fluid (composed of chlorine compounds) or very pure
water
8 March 2005
AST 2010: Chapter 15
31
Solar Neutrino Production (2)
Later experiments using many tons of
gallium were able to detect the more
abundant low-energy neutrinos
However, those experiments also found
the same problem
Too few neutrinos (the gallium experiments
found about 2/3 the expected number)
The puzzling lack of neutrinos from the
Sun is called the solar neutrino problem
8 March 2005
AST 2010: Chapter 15
32
The Solar Neutrino Problem
Physicists evaluated a number of possible
reasons for the problem
Nuclear fusion is not the Sun's power source?
Not supported by observations, not likely to be the correct
reason
The experiments were not calibrated correctly?
Unlikely that all carefully-tuned experiments were tuned in
the same wrong way. Experiments independently verified
by many other scientists; astronomers think that the
results are correct.
The nuclear reaction rate in the Sun is lower than
what our calculations say?
Possible, but many people have checked and re-checked
the physics of the reaction rates
Strong constraints in how much one can lower the
temperature in the core of the Sun to slow down the
reactions
8 March 2005
AST 2010: Chapter 15
33
Solar Neutrino Solution
Three types of neutrinos exist
The Sun produces only one type, called
electron neutrinos
The experiments detect only the electron type
On their way from the Sun, neutrinos can
transform from one type to another
This can explain why we only detect 1/3 of the mix
at Earth
This also implies that neutrinos have mass,
which is very small, but not zero
8 March 2005
AST 2010: Chapter 15
34