Transcript The Sun: Our Star How does the Sun work ? What is the energy source for the Sun ? The Planets Lecture 15 © 2005

The Sun: Our Star
How does the Sun work ?
What is the energy source for the Sun ?
The Planets
Lecture 15
© 2005 Pearson Education Inc., publishing as Addison-Wesley
The Sun
Homework
• Read Chapter 14: The Sun
• MasteringAstronomy: Assignment Chapter 14
Due Friday, Oct 24, 6pm
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Partial Solar Eclipse
Meet on grass area
across from Pimentel Hall
Thursday 2:30-4:00pm
Questions about The Sun
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is the Sun made of?
What produces the enormous light energy ?
How many years will the Sun continue shining?
Is the Sun’s light output constant, or variable?
What doesn’t the Sun contract, due to its gravity?
How did the Sun form ?
What are sunspots? And those loops on the surface?
What is the “sunspot cycle”?
Does the Sun have layers inside, like the Earth?
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Answers about The Sun
1.
What produces the enormous light energy ?
Nuclear reactions: 4H
He : Fusion
2. How many years will the Sun continue shining?
5 Billion years
3.
Is the Sun’s light output constant, or variable?
Constant! Within 0.1%
4.
What doesn’t the Sun contract due to its gravity?
Gas Pressure pushes outward.
5.
How did the Sun form ?
A massive gas cloud collapsed by its own gravity.
6.
What are sunspots?
Dark regions with strong magnetic fields. .
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Observable Properties of the Sun
Distance: 1.5 x 108 km
= 1 A.U.
Luminosity: 3.8 x 1026 watts
Light spreads out spherically
Mass = 300,000 x Earth
Radius = 110 x Earth
Density = 1.4 g/cm3
~ 40% more than Water
1 sec of Luminosity supplies
500,000 yrs of energy for humanity.
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Quiz:
Each person uses roughly 1000 Watts (i.e., 10
100-Watt light bulbs). How much energy per
second do humans use in the entire world?
a) 7x108 W
b) 7x1012 W Approach: Find approximate answer
9)
•
7
billion
people
(7
x
10
c) 7x1016 W • 1000 watts per person
d) 7x1020 W • Total power: 7 x 10129 x 1000
= 7x10 Watts
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Quiz:
How much energy, in Joules, do all humans
use in one year?
a) 2x108 W
b) 2x1012 W
c) 2x1016 W Approach:12Find approximate answer
• 7 x 10 Watts
d) 2x1020 W
X • One year =
•
365 days x 24 hour/day x 3600 sec/hour
= 3 x 107 seconds
Total Energy = 7 x 1012 Watts x 3 x 107 sec
= 21 x 1019 Joules
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Quiz:
How much energy, in Joules, do humans use
in one year?
a) 2x108 J
b) 2x1012 J
Approach: Find approximate answer
c) 2x1016 J
• 7 x 1012 Watts
d) 2x1020 J
• One year =
•
365 days x 24 hour/day x 3600 sec/hour
= 3 x 107 seconds
Total Energy = 7 x 1012 Watts x 3 x 107 sec
= 21 x 1019 Joules
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Properties of the Sun
Density: 1.4 g/cm3
~ 40% more than Water
Implies: Sun is Gaseous
compressed by gravity
No hard surface
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Composition of the Sun
Dark spectral lines are caused by
absorption of light by atoms
in the Sun’s atmosphere.
Magnesium
Calcium
Sodium
Iron
You can measure the amount of different atoms
from darkness of the absorption lines.
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Composition of the Sun
(by Mass)
C, N, O, Ne, Fe, Others: 2%
He
28%
Hydrogen 70%
Representative of the Universe as a whole:
Hydrogen and Helium Dominate.
(But not for Earth.)
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H
He
O 0.3%
C 0.2%
Fe
Gravity Balanced by Pressure
• Gas pressure supports the star
against the inward force of gravity.
• At Sun’s center, pressure is huge.
(Weight of material above is huge.)
• Huge Pressure
Huge temperature and densities
at Sun’s center.
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Center of the Sun
Computer models (balancing gravity with pressure) show:
Temperature = 15 Million K
Collisions between atoms so violent:
• electrons removed from atoms: Atoms are Ionized
• leaving bare nucleus of each atom.
Nuclei of atoms collide & react
Nuclear Reactions
Fusion of Hydrogen to Helium
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Fusion occurs
only in the Sun’s core
• Nuclear fusion
• a reaction where Hydrogen combines
(fuses) to form Helium nuclei.
+
+
• Electric force: nuclei repel each other.
• Nuclei have positively charged protons
• For fusion to occur, nuclei must be moving
fast enough to overcome electric repulsion
• This requires high temperatures
At low speeds, electric
repulsion prevents protons
from coming close.
+
+
• When nuclei touch, the nuclear force binds
them together
At high speeds, protons overcome
electric repulsion. Come close.
Nuclear reaction!
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Neutrons
Neutrons are not stable! They do not exist alone for long!
+ e + ¯ e
-
n
p+
p+
n + e+ + e (inverse -decay)
(-decay)
e is a neutrino ---- a weakly interacting
particle which has almost no mass and
travels at nearly the speed of light.
e- = electron
e+ = positron (anti-electron)
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Note: Charge conserved.
Energy Generation: Nuclear Fusion in the Sun:
Proton-Proton Chain
IN: 6 H, (2 e-)
OUT: He, 2 H, 2 e, 4 
4 H nuclei are converted into 1 He
nucleus and energy is released.
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Energy Generation: Nuclear Fusion in the Sun:
Proton-Proton Chain
IN: 6 H, (2 e-)
OUT: He, 2 H, 2 e, 4 
4 H nuclei are converted into 1 He
nucleus
and energy is released.
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Quiz
•
a)
b)
c)
d)
Which particle has the greatest mass?
helium
electron
proton
deuterium
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Proton - proton Reaction
makes Deuterium
P+P
D + positron + neutrino
D = 2H = Deuterium = proton+neutron
2H:
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“2” is number of
Protons + neutrons
D+P
3He
+
photon
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3He
+ 3He
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He + 2P
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Quiz
What is this object?
a) deuterium
b) tritium
c) helium
d) 2H
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Quiz
What is this object?
a) deuterium
b) tritium
c) helium
d) 2H
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Mass Accounting
Mass Input: 4 p + 2 eMass Output: 1 He (2p + 2n)
Look up Masses of particles:
Mass Input > Mass Output
Mass Input = 1.007 Mass Output
Mass is missing ! Mass is not conserved!
Converted to Energy:
2
E = mc
8 m/s.
Where
c
is
speed
of
light,
3x10
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Nuclear Reactors on Earth work by Fission
A heavy nucleus, such a Uranium 235 is split apart
by a fast-moving neutron. The resulting nuclei have
lower energy. The extra energy comes out as fast-moving
particles, notably neutrons that provide the energy
of the reactor, heating the material to high temperatures.
Fission:
http://www.youtube.com/watch?v=G1jtWR_tcX4&feature=elated
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The Solar Thermostat
Suppose the Sun Heats Up at little accidentally
Is there a negative feedback to bring temperature back ?
• Higher Temp causes faster collisions:
- Nuclear reactions proceed faster.
- More energy is produced.
• Added energy heats Sun to higher temperature. The Sun expands !
• Expansion causes gas density to be lower.
• So, atoms are farther apart.
• Nuclear reaction rate declines.
• Sun cools - - - Back to normal Temp.
Sun’s energy output (luminosity) remains stable:
Thermostat
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The Solar Luminosity
has Risen 30% in Past 4 Billions years
• During the past 4.6 billion years:
• 4 Hydrogen atoms fused into Helium
• Core now has fewer atoms. Lower pressure: the Sun’s core contracts,
causing it to heat up
• The fusion rate increases (until higher pressure balances gravity)
• A new equilibrium is reached at a higher energy output
• Thus, the Sun’s luminosity increases.
• Computer Models indicate the Sun’s luminosity has increased
30% since it formed 4.6 billion years ago.
• From 2.9 x 1026 watts to today’s 3.8 x 1026 watts
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Core
• T = 15 million K; Depth = inner 1/4 of Sun
• Where the Sun’s energy is generated.
Interior Zones
• Energy is transported from center outward.
• The interior is divided into two zones:
• Radiation Zone (energy carried by light)
• Convection Zone (energy carried by rising hot gas)
• Boundary between them is at:
• T = 2 x 106 K; Distance from center: 0.70 RSun
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Layers of the Sun
Solar
Wind
photosphere
Convective
Zone
Core
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Radiative
Zone
Photon Transport of Energy
“Radiation Transport”
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Energy Transport by Photons (Light)
• Radiation Zone
• Energy travels as photons of light, which continually collide with particles
• Photons scatter, changing direction (random walk), and change wavelengths
• This is called radiative diffusion
Path of photon,
scattered by electrons
and atoms.
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• This is a slow process!
• It takes about 1 million
years for energy to
travel from the core to
the surface.
Layers of the Sun
Solar
Wind
Convective
Zone
Core
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Radiative
Zone
Convective Transport of Energy
Wait 10 sec
For flame
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Convective Energy Transport
• Convection: Hot air rises; carries heat with it.
• The bottom of the convection zone is heated … hot gas rises to the top
• cooler gas sinks to the bottom…similar to boiling a pot of water!
• Energy is brought to the surface via bulk motions of matter
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Convection in the Sun: Hot gases rise, cool gases fall
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Layers of the Sun
Solar
Wind
photosphere
Convective
Zone
Core
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Radiation
Zone
Photosphere
• T = 5,800 K; depth = 400 km
• This is the yellow “surface” that we see.
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The Photosphere:
Visible Surface of the Sun
• Photosphere:
opaque “surface”
human eye sees.
• Granulation
(convection)
• Sunspots
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The Sun: 1 Hour Ago
Sun in
Visible
Light
Nov 19, 2013
4pm (PST)
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http://sdo.gsfc.nasa.gov/data/
http://www.spaceweather.com/
Solar Magnetic Activity
• The photosphere of the Sun is covered with sunspots.
• Sunspots are not constant; they appear & disappear.
• They appear and disappear in a cycle, lasting 11 years.
• Sun’s magnetic field switches polarity (N-S) every 11 yrs
• So the entire cycle repeats every 22 yrs
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Sunspots: Cool, Magnetic Regions
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Sunspots: Umbra and Penumbra
Umbra
Penumbra
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Partial Solar Eclipse
Thursday 2:30-4:00pm
Meet on grass area
across from Pimentel Hall
Partial Solar Eclipse
The Sun: Our Star
End of Lecture 20
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Extra Slides
Neutrinos from the Sun
• Neutrinos are created in the proton-proton reaction.
• We have detected them, proving that the theory of nuclear
fusion reactions is correct!
• But we only detect about 30% of the neutrinos predicted by
theoretical models.
• Reason: Three types of neutrinos:
• electron (e), muon (), and tau ()
• our neutrino detectors can register only electron neutrinos
• Neutrinos can change type after being created, allowing us to
detect only 1/3 of them
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“Observing” the Solar Interior
• The Sun’s interior is opaque…
• we can not see directly into it with light
• We can construct mathematical computer models of it.
• the models are a grid of temperature, pressure, & density vs. depth
• these values are calculated using known laws of physics
• they are tested against the Sun’s observable quantities
• We can directly measure sound waves
moving through the interior
• we observe “sunquakes” in the photosphere
by using Doppler shifts
• motion of sound waves can be checked
against interior conditions predicted by
models
• There is another way to see directly into
the core…neutrinos!
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