Transcript Announcements

Announcements
• Star Assignment 3,
due Wednesday March 17
– READ chapter 16,
– Do Angel quiz,
– Do Astronomy Place tutorial “Measuring Cosmic Distances: Lesson 2,
Stellar Parallax”
• Global Warming Project,
due Wednesday March 17
– Sample 4 web sites taking different positions on whether Global Warming
is occurring & whether people are responsible.
– For each site, evaluate the science - Is it good or poor?
– Criteria (list from class discussion)
– Submit via email
Gravitational
equilibrium:
The outward push
of pressure
balances the
inward pull of
gravity
Pressure
Pressure is force exerted by colliding particles
• Higher density --> particles closer together
--> more collisions --> higher pressure
• Higher temperature --> particles move
faster --> more & harder collisions -->
higher pressure
Equilibrium
Pressure balances Gravity
•Pressure = weight of overlying material
•Pressure increases toward center
to balance larger gravity toward center
Energy Source:
Nuclear Fusion
1
4
4 H --> 1 He + 2  + Energy

Fusion occurs ONLY in the Sun’s
CORE ?
• Nuclear fusion
• Lighter nuclei are fused into heavier
nuclei
• all nuclei are positively charged
• Electromagnetic force causes nuclei
to repel each other.
• for fusion to occur, nuclei must be
moving fast enough to overcome E-M
repulsion
• this requires high temperatures
• When nuclei touch, the nuclear
force binds them together
Electric Barrier
Answers to Explain the Thermostat
• Good - gives details
Gravitational equilibrium acts as the Sun’s
internal thermostat. If there is a small rise
in the core temperature, the fusion rate will
rise dramatically. The increased energy
creates increased thermal pressure that can
overcome gravity, causing the core to
expand and cool which restores the fusion
rate to normal. If there is a small decrease
in the core temperature, there will be a
dramatic decrease in the fusion rate. The
decrease in energy will allow gravity to
compress the core. This will heat up the
core and in turn restore the fusion rate to
normal.
• Bad - does not answer
question:
Nuclear fusion transforms hydrogen
into helium and creates extra energy.
This energy is so great it heats up to
temperatures no spacecraft could
survive if it ever attempted to journey
to its core.
Energy Loss
• Heat is produced in the core
• Heat is lost (radiated away to space) from
the surface
• How is heat transported from the hot core to
the (relatively) cool surface?
Inner 2/3 of Sun Heat is transported by
RADIATION
Radiative zone: Energy gradually diffuses outward (in about a million
years) by randomly bouncing photons. Each hot core gamma ray photon
becomes many visible cool surface photons.
Random Walk Activity
• Each person in center of classroom take a
balloon. Blow it up and tie it. Tap it up.
• Everyone tap balloons UP when one comes
to you
• Time how long it takes half of balloons to
reach a wall
• Now compare with time it takes if balloon
are tapped directly towards a wall.
Rate of energy loss
Determined by bulk of star outside of
core
If star is good insulator -> rate of energy
loss smaller
If star is poor insulator -> rate of energy
loss greater
Why does energy take so long to reach
the surface?
• Good - gives details
Gamma-ray photon collide with
electrons often, sending them
bounced in random directions.
With each bounce the photon drifts
farther from its initial location. As
a result it has to go a much longer
distance than a straight line and so
takes a very long time to reach the
surface
• Bad - no details
Energy generated in the core takes
about a million years to reach the
surface, because the transportation
process (via photons and
convection) is so complex.
Outer 1/3 of Sun Energy is transported
by CONVECTION
Granule ~ fountain
Hot gas rises, spreads out, falls back
Convection zone: Hot gas rises, radiates away its heat at the surface,
becomes cool gas and sinks, pulled down by gravity.
Convection
pattern
at solar
surfact
Hot gas rises
(floats up)
-> Brighter
Cool gas sinks
(pulled down
by gravity)
-> Darker
~ 1000 km
Convection
pattern
at the
solar
surface
Sunspots:
Magnetic
fields inhibit
convection
-> cooler
-> Darker
(Fig. 15.15)
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Equilibrium:
The Structure of a Star is determined
by two balance conditions
1.Energy Production
(by nuclear fusion in core)
= Energy Loss
(by radiation from surface)
2.Pressure (pushing out)
= Gravity (pulling in)
Feedback - the Solar Thermostat
(Fig. 15.8)
The Solar Thermostat
Temperature increase
 increased fusion
energy production greater than energy loss
core heats up
 pressure increases
 pressure greater than gravity
 core expands
core cools (convert KE to PE)
 energy generation decreases
Tests: Compare predictions of models
with observations
1. Observations of resonant sound wave
oscillations (helio-seismology)
2. Neutrino observations
Sound Waves in the Sun
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
If wave comes
back on itself,
it is reinforced
= resonant
Sound
waves
penetrate
different
distances
probe the
solar
interior.
Global Oscillation Network Group
Observed as Doppler Shift
at the solar surface
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Test Solar Models:
Compare
frequencies
of observed
resonant
oscillation
modes with
frequencies
calculated
from solar
models
Agreement
Excellent
Model vs. Helioseismic Observations
Fig. 15.10
Density water = 1 g/cm2
Sun in Neutrinos
Solar Neutrino Flux:
Theory vs. Observation
Theory is biased by Experiment
Conclusion:
either
1. Something wrong with models of the Sun
2. Something wrong with theory of neutrinos
Resolution (recent)
•
•
•
•
•
•
3 types of neutrinos - e, mu, tau
Sun produces only e type
SNO observing only e-type, sees 0.35
predicted number
Super-Kamiokanda, sensitive to e + some
mu & tau, sees 0.46 predicted number
Conclusion: e-type <-> mu & tau - types
Hence, see only 1/3 predicted if observe
only e-type