Leacture 11
Why does the Sun shine?
Nuclear fusion requires extremely high temperatures and densities
Recall that our Sun was born about 41 billion years ago from a
collapsing cloud of interstellar gas. The contraction of the cloud released
gravitational potential energy, raising the interior temperature and
pressure. With- out another source of energy to replace the energy
output from its surface, the Sun continued to contract until its central
temperature finally rose high enough to sustain nuclear fusion. Then the
Sun was finally able to replace the energy lost from the surface and
stopped contracting.
2 kinds of balance to make sun stable:
o gravitational equilibrium (or hydrostatic equilibrium), is
between the outward push of internal gas pressure and the inward
pull of gravity. A stack of acrobats provides a simple example of
gravitational equilibrium
The Sun’s internal pressure precisely balances gravity at
every point within it, thereby keeping the Sun stable in
size
the pressure must increase with depth. Deep in the Sun’s
core, the pressure makes the gas hot and dense enough to
sustain nuclear fusion. The energy released by fusion, in
turn, heats the gas and maintains the pressure that keeps
the Sun in balance against the inward pull of gravity.
o energy balance between the rate at which fusion releases energy
in the Sun’s core and the rate at which the Sun’s surface radiates
this energy into space.
without it the balance between pressure and gravity would
not remain steady. If fusion in the core did not replace the
energy radiated from the surface, thereby keeping the total
thermal energy content constant, then gravitational
contraction would cause the Sun to shrink and force its
core temperature to rise.
Summary : 41 billion years ago gravitational contraction made the Sun
hot enough to sustain nuclear fusion in its core. Ever since, energy
liberated by fusion has maintained gravitational equilibrium and energy
balance within the Sun, keeping it shining steadily and supplying the
light and heat that sustain life on Earth.
Sun’s structure (out to in)
1. solar wind—the stream of charged particles continually blown outward in all directions from the Sun. Recall that the solar wind helps shape the
magnetospheres of planets and blows back the material that forms the
plasma tails of comets
2. outermost layer of this atmosphere, called the corona
a. highest temp
b. emits most X-ray
c. low density
3. Nearer the surface, the temperature suddenly drops to about 10,000 K in
the chromosphere, the middle layer of the solar atmosphere that
radiates most of the Sun’s ultraviolet radiation.
a. Reddish color due to strong H emission
b. Less dense and less bright than photosphere
4. Lowest layer of the atmosphere, the photosphere, which is the visible
surface of the Sun. Although the photosphere looks like a well-defined
surface from Earth, it consists of gas far less dense than Earth’s
atmosphere. (6000K)
a. Sunspots
i. Migrate in latitude over the 11-year cycle
b. Intense magnetic fields
c. Granulation – convection (hotter gar rise, cooler gas sinks)
5. convection zone, where energy generated in the solar core travels
upward, transported by the rising of hot gas and falling of cool gas
called convection
6. calmer plasma of the radiation zo
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