interior of the sun Flashcards
how to figure out lifetime of the sun
internal heat/ luminosity
The Age Crisis: Part I
people thought the sun was powered by chemical or gravitational power.
Geologists estimated that the
Earth was at least a few Million
years old
so atp the sun would be younger than the earth
Kelvin-Helmholtz mechanism: gravitational contraction
Gravity and Pressure in equilibrium
Luminosity radiates away
heat… presure drops
Balance tips in favor of gravity,
sun shrinks
As Sun contracts
gas falls inward
* atoms
* gain speed
* are more
likely to
collide with
other atoms
Temperature
increases
the sun contracting makes
Contraction makes interior heat up,
increasing the internal pressure
Balance restored, but with higher gravity,
pressure & temperature than before.
The Sun could shine by tapping gravitational energy for
30 million years
Today the Sun is in
hydrostatic equilibrium
hydrostatic equilibrium in sun
Gas pressure due to an interior heat source
counteracts the inward pull of gravity
Gas pressure due to an interior heat source
counteracts the inward pull of gravity
Early 20th century: radiometric
dating shows Earth is at least a
billion years old.
again how is the earth older than the sun.REthinking happened again
fusion
lighter nuclei
combine to form heavier
nuclei
fission
heavier nuclei split
to form lighter nuclei
The Sun is powered by
by fusion
* Fuse 1 gram of H into 0.993
grams of He.
* Leftover 0.007 grams is
converted into energy: E = mc2
* Sun converts 600 million tons
of H every second!
Only the Sun’s core is hot enough for fusion to occur
Four protons colliding
at once is unlikely.
* Must be hot:
>10 Million K to get
protons close enough
to fuse together
The energy emitted by the Sun is produced
in a very small region at the very center of the Sun
The chemical composition of the Sun 3 billion years ago was
different from what it is today in that it had
more hydrogen
the proton-proton chain
Fuse 4 protons (1H) into one 4He nucleus plus:
* Energy in the form of Gamma-ray photons
* 2 positrons (positive electrons)
* 2 neutrinos that leave the Sun
Neutrinos
neutral subatomic particles:
* Nearly massless, travel near c
* Interact weakly with matter
* Can pass through lead 1 parsec thick!
Neutrinos created in the Sun’s core should stream out of the Sun
Energy is generated
in the core
Energy is then
transported by
photons in the
Radiation Zone
Energy is next
transported by
mass motions in the
Convection Zone
Solar Oscillations
The Bottom Line
Fuse 4 protons (1H) into one 4He nucleus plus:
* Energy in the form of Gamma-ray photons
* 2 positrons (positive electrons)
* 2 neutrinos that leave the Sun
Fusion Lifetime is ~10 Billion Years
Neutrinos
Neutral subatomic particles:
* Nearly massless, travel near c
* Interact weakly with matter
* Can pass through lead 1 parsec thick!
Neutrinos created in the Sun’s core should stream out of the Sun.
Test: Solar Neutrinos
How do we know that fusion is
occurring in the core of the Sun?
Answer:
* Look for the neutrinos created by
the fusion reactions.
* Neutrinos from nuclear fusion,
with the expected energies,
detected in all experiments
performed to date.
The Solar Interior
1. Energy is generated
in the core.
2. Energy is then
transported by
photons in the
Radiation Zone.
3. Energy is next
transported by
mass motions in the
Convection Zone.
Solar Oscillations
* Doppler measurements
of gas pushed by sound
waves reaching the
surface
* few hundred m/s
* reveal what’s happening
in the Sun’s interior
* helioseismology
Radiative zone
Density drops from ~20
g/cm3 (like gold) at the
base to 0.2 g/cm3 (less
than water) at the top
* Temperature drops to
2x106 K
convection occurs in
outer 30% of the Sun Below 2x106 K, heavier
atoms (C, N, O, etc.)
hold onto more of their
electrons
* Harder for photons to
get through, heating up
gas
* Gas rises to the
photosphere
The Sun is stable as a star because
gravity balances forces from pressure
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The primary source of energy in the Sun i
Fusion of light nuclei (H) to make heavier ones (He)
