Sun

Sun

Eclipse of the Sun
Layers/Regions
Energy Transport
Magnetic Field Lines
Hydrostatic Equilibrium
The Sunspot Cycles:
   Dynamo Theory
   Coronal Holes
        see also the Stars...
        see also the Solar System...

"Our sun is the greatest source of energy available to us and the simplest to obtain.
We have the means and the technology to do this. 
Currently we use other sources of energy most of which are finite, polluting and most importantly could be put to the better use of humanity.
From a "value added" economic perspective fossil fuel resources would be much better utilised in making much more valuable plastics, chemicals, lubricants... in the long term.
The long term interests of humanity offset by the short term interests of the few.  Repeated lessons in history not learned.
Yet in the western democracies easily resolved by the simple casting of an informed vote."

Eclipse of the Sun

An eclipse of the sun occurs when the moon crosses, in front of and temporarily blocks, the light of the sun as viewed by an observer on earth



Image of total eclipse in north Queensland Australia - courtesy Joe. Michna - note solar flares at bottom

Layers/Regions:

Core: 

• Central energy producing region
• Thermonuclear fusion occurs here.
• Energy transport by radiation. 
• Temperature: ~16 x 106 K
• Density: ~160 g/cm3 

 Intermediate Interior:

• large region of interior photons scattered on the way to the solar surface.
• Temperature: ~8 x 106 K 
• Density: ~20 g/cm3

Convective Zone: Transition region between interior
  and surface. Energy transport by convection.
  Temperature: ~5 x 105 K
  Density: ~0.01 g/cm3
 
Photosphere: Visible surface of the Sun.
  Convective "bubbles" from below visible
  Granulation. Location of Sunspots.
  Temperature: ~5.8 x 103 K
  Density: ~4 x 10-7 g/cm3



Chromosphere:
Beginning of solar "atmosphere".

• Location of prominences and solar flares.
• Temperature: ~5 x 104 K
• Density: ~8 x 10-8 g/cm3
• Transition Region: Region between corona and chromosphere.
• Temperature changes from ~50,000 K in chromosphere to ~2 x 106 K in corona over
• a few tens of kilometers!!!!
• Corona: Hot, thin, large solar "atmosphere". Coronal holes are source of the solar
• wind.
• Temperature: ~2 x 106 K
• Density: ~1 x 10-14 g/cm3

Granulation:   Provide evidence of convection below solar surface


  
 Diagram of convection.   Sunspots:

Regions of intense magnetic fields.
Temperature ~ 4200 K cooler than photosphere, which makes them appear as dark spots.
Can be used to measure solar rotation (25 days at equator, 31 days at poles).
Umbra - dark inside, Penumbra - lighter outside

   


  Spicules:
  Chromosphere contains many dark, brush-like spikes that protrude upward.  These are called spicules.They are "jets" of gas surging away from the sun at 20 km/s.




  Prominences/Solar Flares
:


  Huge, arching columns of gas often appearing above sunspots. Prominences are regions along a magnetic field line where conditions are right for light to be emitted.



Occasionally, kinks and stresses occur on magnetic field lines discharging amounts  of energy (known as Solar Flares). The amount of energy released is equivalent to a 2 billion megaton bomb. Flares release large numbers of particles into the corona.


 

Energy Transport:

  There are three ways in which energy can be transported from one place to another:

• 1.Radiative Transport


• Energy is transported by photons.
• Energy loss by scattering and absorption by atoms.
• Very efficient mechanism in stars (also efficient on cloudless evenings very chilly nights).


• 2.Convective Transport


• Energy transported by bulk motion of mass.
• Important mechanism in stars.
• Very poorly understood process.


• 3.Conductive Transport


• Energy transport by atoms colliding with one another, gaining and releasing energy.

Efficient mechanism in solids, but not in stars.

• Energy:
• Examples of Energies:
• Energy to lift a sheet of paper 1 cm: ~ 1 erg
• Chemical energy in a barrel of oil: ~ 1017 ergs
• Nuclear energy in a gram of water: ~ 7 x 1019 ergs
• Total energy emitted by Sun in its lifetime: ~ 1050 ergs
• Total energy emitted by single supernova: ~ 1051 ergs

There are 3 types of energy of concern to us here:
 
1.Gravitational Energy:
  Energy resulting from the motion of matter when interacting with other matter.
2.Chemical Energy:
  Energy resulting from, a change in the electron structure of atoms when bonding with other atoms. "Burning".
    3.Nuclear Energy:
  Energy resulting from changes in the nuclear structure of atoms.
 
Gravitational Energy As A Source Of Solar Energy:
 
 

Total gravitational energy ÷ Luminosity = How long sun could be powered.
  (1 × 1048 ergs) ÷ (4 × 1033 ergs) = Could power the sun for about 100 million years.

  Chemical Burning As A Source Of Solar Energy:
  Total chemical energy ÷ Luminosity = How long sun could be powered.
  (1 × 1046 ergs) ÷ (4 × 1033 ergs) = Could power the sun for about 10,000 years.

  Until the late 1800s, gravitational energy was thought to be sufficient, since it was not thought that the Universe was that old. However, for two reasons it was later thought that something more than gravitational energy powered the sun: Darwin (1809-1882): Theories of evolution required much longer than 108 years for evolution to proceed.  Geological dating of rocks indicated that the Earth was several x 109 years old.
 
So What Powers The Sun?
  Einstein, getting energy from hydrogen fusion, came up with the following equation that explains it all:

 E = mc2
 E = Energy,    m = mass,   c = speed of light

Fundamental Structure Of The Atom:



    The Nucleons:
P+   Proton  (charge: +,  mass: 1.67 × 10-24 g)
    n    Neutron (charge: Ø, mass: 1.67 × 10-24 g)
    The Electron:
    e-   Electron (charge: -, mass: 9 × 10-28 g)
 

The Proton-Proton Chain:   Nuclear Fusion:
 
  High Temperatures Are Needed For Fusion. Why? Recall that like charge repel.



High temperatures (energy) are required in order to overcome this repulsion.Actual details require descriptions provided by Quantum Mechanics.   Temperature required for the p-p chain:    T ~ 107 K
 
Temperature is found in the Sun's core, where density is ~ 160 g/cm3 . Even at this density, reaction probability is very low. Single proton takes ~ 7 x 109 years before reacting. Large number of protons make up for low probability.  

Hydrostatic Equilibrium

Why doesn't the Sun collapse under its own weight, or blow itself apart because of nuclear fusion?

• Core nuclear processes produce outward pressure.
• Gravitational forces produce inward pressure.
• When the two pressures are equal (but opposite directions) the star (Sun) is stable against collapse.

  Energy Output Of The Sun:
 
 The total amount of energy, in a given time, over a given area above the Earth's atmosphere,  that reaches us from the Sun is called: Solar Constant.
Numerically it is ~ 2 calories/cm2/minute.
Models suggest that a 1% change will result in a 1º-2º change in Earth's temperature.
During the last ice age (~20,000 years ago), Earth was ~5º cooler than today.
A 10% (~10º) change would result in an ice covered Earth.
 

Magnetic Field Lines:

1.Magnetic field: term that describes regions of space modified by
moving electrical charges.
2.Magnetic fields around planets/Sun resemble those around bar
magnets.
3.Lines indicating direction of the magnetic field are called magnetic
field lines.
4.Planetary and solar magnetic fields are thought to arise in a fluid,
conducting interior.
5.Magnetic fields arise from circulating electrical currents generated by
the object's rotation.
 

The Sunspot Cycles:

  Is The Solar Constant, Constant?  The Sunspot Cycle:

  The number of sunspots observed on the Sun varies in an 11 year cycle called the Sunspot Cycle.



  The 11 year cycle is actually part of a 22 year magnetic sunspot cycle.

1. Differential rotation winds up the magnetic field.
2. Field becomes tangled and breaks through the surface sunspot pairs.
3. After 11 years, field decays.
4. Differential rotation winds up the magnetic field.
5. Newly organized field is reversed.

One theory that attempts to explain the cycle is the

Dynamo Theory

Differential rotation and convection twist interior magnetic fields and
bring them to the surface.
Magnetic fields appear as sunspot pairs.
Magnetic fields decay and switch polarity.
Charged particles move along magnetic field lines - they cannot cross them.

Coronal Holes:

Coronal holes are regions where magnetic field lines are open, that is, they do not return to Sun's surface. Charged particles can, therefore, escape the Sun in the form of Solar Wind.




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