Transcript 15. Our Star

The Sun: Our Star
29 April 2008
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Announcements
• Telescope open tonight and
Thursday at 8:30pm
• Homework due Friday on
Extrasolar planets (Chap. 13)
• Both Observation Reports
also due Friday.
• TALC: 6-8pm, Evans 264
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Questions about The Sun
1.
2.
3.
4.
5.
6.
7.
8.
9.
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”?
What is the Sun made of?
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
2. How many years will the Sun continue shining?
5 Billion years more
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.
Mass: 2.0 x 1030 kg
=300,000 x Earth
Radius: 7.0 x 105 km
= 110 Earths
Density: 1.4 g/cm3
~ 40% more than Water
Luminosity: 3.8 x 1026 watts
-- - light
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1 sec of Lum. supplies 500,000 yrs
worth of energy for humanity.
Quiz:
Every human being uses 1000 Watts (Ten 100 Watt
light bulbs) on average. How much energy per
second do all humans on Earth use?
a) 108 W
b) 1012 W
c) 1016W
d) 1020W
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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 atmospshere.
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
C 0.3%
Fe 0.2%
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.
• 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.
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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Proton - proton Reaction
makes Deuterium
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
P+P
D + positron + neutrino
D = 2H = Deuterium = proton+neutron
2H:
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“2” is number of
Protons + neutrons
D+P
3He
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
+
photon
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3He
+ 3He
He + 2P
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
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Quiz
What is this object?
a) deuterium
b) tritium
c) helium
d) 2H
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Quiz
•
a)
b)
c)
d)
Which particle has the greatest mass?
helium
electron
proton
deuterium
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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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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, m, is missing !
Converted to Energy:
E=
2
mc
Where c is speed of light, 3x108 m/s.
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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:
- Reactions proceed faster.
- More energy is produced.
• Added energy heats Sun to higher temperature. The Sun expands !
• Expansion causes gases to cool, and gas density to be lower.
• Atoms move more slowly and are farther apart.
• 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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“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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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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Radiation
Zone