Transcript astro7a_sun_shortv3

The Sun:
The Internal Physics
25 November 2008
Astronomy 7A
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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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worth of energy for humanity.
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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Quiz
What is this object?
a) deuterium
b) tritium
c) helium
d) 2H
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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
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
photosphere
Convective
Zone
Core
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Radiation
Zone
Convective Transport of Energy
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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 Visible at
Surface of the Sun
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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
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• Photosphere:
opaque “surface”
human eye sees.
• Granulation
(convection)
• Sunspots
Journey Into the Sun
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•
•
•
•
Photosphere
Convection Zone
Radiation Zone
Core: proton-proton
nuclear reactions:
Helium
Photospheric Features
Sunspots: dark spots on the surface where
the temperature is cooler.
Granulation: the
tops of convection
cells seen “bubbling”
on the Solar surface
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National Solar Observatory/AURA/NSF
Sunspots and Convection at
Surface of the Sun
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Layers of the Sun
Solar
Wind
photosphere
Convective
Zone
Core
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Radiation
Zone
Solar Chromosphere
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Chromosphere
Temp = 10,000 K
Hydrogen Emission
n = 3 to 2.
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Chromosphere
• T = 10,000 K;
• Depth: Thin and patchy over surface
• A thin hot layer above the photosphere
where most of the Sun’s UV light is emitted.
• UV image of the Sun
Light emitted from
Helium at 20,000 K
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SOHO
Prominences from the Chromosphere
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Hydrogen Alpha:
Electrons drop from
3rd - 2nd level.
Wait 10 sec
For movie.
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Prominences – Gas trapped in the magnetic fields is heated
and elevated above the photosphere and chromosphere.
X-ray images from NASA’s TRACE mission.
Movie. Click to launch.
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Solar Flares: Magnetic Explosions
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Solar Flares: Magnetic Explosions
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The Corona
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Corona
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Corona
• T = 2 Million K
• Thickness  Radius of Sun (700,000 km)
• The hot, ionized gas which surrounds the Sun.
– it emits mostly X-rays
• It can be seen in visible light during an eclipse.
X-ray image (YOHKOH telescope)
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Visible image
Solar Wind
• The stream of electrons, protons, Helium nuclei and
other ions which flow out from the Sun.
• It extends out beyond Pluto.
X-ray image of corona
UV image of solar wind
Visible image of solar wind
comet SOHO-6 (fell into Sun)
Sagittarius
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Solar Wind
electrons, protons, He nuclei expelled by flares
Interact with Earth’s magnetic field to cause…
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The Aurorae
the Northern & Southern Lights
• A strong Solar
wind can affect
human technology
by:
• interfering with
communications
• knocking out
power grids
• damage electronics
in space vehicles
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Solar Magnetic Activity
• The photosphere of the Sun is covered with sunspots.
• Sunspots are not constant; they appear & disappear.
• They do so 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
Umbra, Penumbra
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What causes a sunspot?
Magnetic field
slows down
convection;
Less heat is
transported to
surface;
so that part of
photosphere is
cooler
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11-year Sunspot Cycle
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Magnetic Activity changes with Time :
11-year Cycle (Last Maximum in Year 2000)
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Sunspot Cycle
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Rotation Period of Sun: 30 days
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The Sun:
How long will it Shine ?
Until it burns up its available Hydrogen
(in the core where T > 2 million degrees)
At Current Rate of Energy production:
5 billion more years
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The Death of the Sun
in 5 Billion Years
• Core becomes pure helium! No Hydrogen burning possible.
• The Helium core begins to collapse.
– H shell (around Helium) heats up and H fusion begins there.
– Outer layers of the Sun expand.
– The Sun enters giant phase of its life.
Original Sun
Expanding:
“Giant Star”
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The Dying Sun: 5 billions years from now
Giant Star Phase
• The He core collapses until it heats to 108 K
– He fusion begins ( 3 He
•
C)
Carbon forms!
The star, called a Giant, is once again stable.
–
–
Gravity balanced by pressure, from He fusion reactions
Giant stars create, and release, most of the Carbon in the
universe: Key ingredient for organic molecules and life.
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