Download lecture16

Structure of the Sun
The Core is where all the action is.

The core is the only place in the Sun where
the temperature (10 million K) and density
are high enough to support nuclear fusion
(hydrogen bomb needs atomic bomb for ignition!)


Every second, about 600 million tons of
Hydrogen are fused into 596 million tons of
helium.
The remaining mass (4 million tons) is
converted to energy in line with Einstein’s
formula
E = mc2
Nuclear Fusion in the Sun

Hydrogen is constantly
being transformed into
helium in the Sun’s core.
The proton-proton chain Reaction
(main branch)
 - neutrinos; + - positrons;  - gamma rays.
1
4H
-->
4He
+ energy ( E =
2
mc )
mass(4He) = 0.993{mass(H)+mass(H)+mass(H)+mass(H)}
mass loss is 0.007{mass(4H)} = 5 x 10-29 kg
E=mc2=(5 x 10-29)(3 x 108)2 = 4 x 10-12 joules
How many fusions per second?
Solor Luminosity = 4 x 1026 joules/sec
4 x 1026 joules/sec
----------------------- = 1038 fusions/sec = 200 million tons/sec!!!
4 x 10-12 joules
Actually, about 500 million tons/sec are needed!
Balance in the stars
Thermal
Pressure
Gravitational
Contraction
Pressure and Temperature of a Gas
How does a
star hold itself?
This balance between
weight and pressure is
called hydrostatic
equilibrium.
The Sun's core, for
example, has a
temperature of about 16
million K.
My House’s Thermostat
Air temperature inside drops below
the set point of the thermostat
Electrical signal triggers
ignition of furnace
Furnace shuts off
Furnace burns oil/gas
and generates heat
Air warms thermostat until
electrical signal is shut off
Radiators transfer energy
from water to air
Heat is absorbed by water
running near furnace
causing water temperature
to rise
Hot water is circulated
throughout house
The Solar Thermostat
Outward thermal pressure of core
is larger than inward gravitational
pressure
Core expands
Nuclear fusion rate
rises dramatically
Contracting core heats up
Core contracts
Expanding core cools
Nuclear fusion rate
drops dramatically
Outward thermal pressure
of core drops (and becomes
smaller than inward grav. pressure)
Balance

If the star contracts at all, the temperature
goes up, the rate of fusion increases, the
pressure increases, and the star re-expands.

If the star expands too much, the
temperature drops, the rate of fusion drops,
the pressure drops, and the star contracts.
How is the energy transported
from the center to the surface?

Radiative transfer


Photons are continually absorbed and re-emitted
(reprocessing of radiation), and when they are reemitted from more external layers, which are colder,
the loose energy (but total energy is conserved,
because more photons are emitted)
Convection

Large scale motions of gas. Hot gas moves upward
and carries energy there. Cold gas move downward
to restore balance
Radiative Transport



Photons are absorbed and re-emitted in random
walk motion.
Each photon mean-free path is very, very short
It takes each γ photon produced by nuclear
fusion million years to reach the surface, and
when it does, it has been reprocessed into
~1,800 cooler visible-light photons (energy is
consesrved!!)
Convection Activity
Structure of the Sun
An aside about nuclear fusion
as a viable energy source
A few key points about the P-P Chain:

It is a “clean” form of energy production.

Put 4 protons (Hydrogen) in and get one Helium and some
energy back out.
Given fuel (in the form of Hydrogen) it is a selfsustaining reaction.
 It requires *very* high temperatures (107 K)

The energy emitted by the Sun is
produced




in a small region at the very center of the Sun.
uniformly throughout the entire Sun.
throughout the entire Sun but more in the
center than at the surface.
from radioactive elements created in the Big
Bang.
The bulk of the violent surface activity on the
Sun is due to “seasonal” variations in the
Sun’s _________.
1) energy output
2) radius
3) electric fields
4) magnetic fields
What is burning in stars?




Gasoline
Nuclear fission
Nuclear fusion
Natural gas
Sun Activity: or Weather
on the Sun
•
Two key points to remember here:
1. nearly all “solar weather” is a result of changes
in the magnetic fields that penetrate the
Photosphere.
2. Magnetic Fields are the result of convection
in the Sun (=large scale motions of
electrically charged particles generate manetic
fields)
Due to change in magnetic field
Magnetic field lines
Measured by
Zeeman effect
Why sunspot is colder
Differential Rotation of the Sun
causes magnetic cycles
Rotation period:
about a month
The middle rotates
faster than the north
or south.
Time-lapse (27 days) movie of x-ray emission from the Sun's
corona. Source: Yohkoh Public Outreach Project.
Babcock Model
The Solar Cycle
Solar activity peaks
roughly each 11 years.
The Solar Constant







The S.C tells us the amount of energy the Sun
produces:
It is 1360 Joules per second per square meter…
…or about the luminosity (=ENERGY PER
SECOND) of 60 W bulb every square millimeter.
A stable change of ~1% in the S.C. would change
Earth’s temperature by 1-2 degrees
Random variation ~0.1% over days or weeks
Long-term, cyclical variation 0.018% per year (period
longer than the 22 years of the sunspots)
Small random fluctuations do not affect Earth’s climate