April 12, 2013, jeb

Outline

• Basic facts
• Standard Solar Model
• Energy Source: Thermonuclear Fusion
• The Solar Interior
• The Sun's Atmosphere
• Solar Vibrations
• Features of the Solar Atmosphere for a "Quiet Sun"
• Solar Neutrinos
• The Active Sun

Basic facts:

• Central temperature: 15,000,000 K
  
• Surface temperature: 6,000 K
  
• Distance from Earth: 150,000,000 kilometers (1 Astronomical Unit)
  
• Size: 1,400,000 kilometer diameter
  
• Mass: 2 x 10 ³⁰ kilograms (300,000 x the Earth's mass)
  
• Average density: 1400 kg/m³(1.4 x water)
  
• Solar energy reaching Earth: 1400 Watts/m² (solar constant)
  
• Total Luminosity of the Sun
  • equivalent to the energy flow through a sphere surrounding the Sun
    • This is an example of the inverse square law
  • The surface area of this sphere is 2.8 x 10 ²³ m²
  • So, the total luminosity is:
    • 1400 Watts/m² x 2.8 x 10 ²³ m²
    • = 4 x 10 ²⁶ Watts

Standard Solar Model

• Core
  • radius = 0 to 200,000 km
  • Temperature(inner radius) = 15,000,000 K
  • Energy generated by nuclear fusion
• Radiation Zone
  • radius = 200,000 to 496,000 km
  • Temperature(inner radius) = 7,000,000 K
  • Energy transported by electromagnetic radiation
• Convection Zone
  • radius = 496,000 to 696,000 km
  • Temperature(inner radius) = 2,000,000 K
  • Energy transported by convection
• Photosphere
  • radius = 696,000 to 696,500 km
  • Temperature(inner radius) = 5800 K
  • Electromagnetic radiation escapes - visible surface of the Sun
• Chromosphere
  • radius = 696,500 to 698,000 km
  • Temperature(inner radius) = 4500 K
  • Cool lower atmosphere
• Transition Zone
  • radius = 698,000 to 706,000 km
  • Temperature(inner radius) = 8000 K
  • Temperature rising rapidly
• Corona
  • radius = 706,000 km out
  • Temperature(inner radius) = 3,000,000 K
  • Hot, low-density upper atmosphere
• Solar Wind
  • radius = 10,000,000 km out
  • Temperature(inner radius) = >1,000,000 K
  • Material escaping and flowing through the solar system

Energy Source: Thermonuclear Fusion

The Energy generation of the Sun is tremendous


For about 5 billion years, the Sun has been emitting
4 x 10 ²⁶ Watts


• this is about 3 x 10 ¹³ Joules per kilogram
  • hydrocarbons -> 40-50 MegaJoules/kilogram
  • dynamite -> about 5 MegaJoules/kilogram
• What can possibly be the source of so much energy?
  
  
Thermonuclear Fusion


H + H + H + H - ---> He (effectively)
with slight loss of mass

• The Helium nucleus weighs slightly less than four times the Hydrogen nucleus
• This process actually proceeds through a chain of interactions known as the proton-proton chain
  • the first reaction is p + p -> p + n + positron + neutrino
The small amount of lost mass turns into energy
Following Einstein's famous equation:

E=mc²

(hydrogen burning)
(1 million times the energy of TNT per unit mass)

Hydrostatic Equilibrium

1. Tremendous weight of the mass of the Sun presses inward under the force of gravity.
2. Enormous pressure (generated by Thermonuclear fusion) inside the Sun pushes back.


HYDROSTATIC EQUILIBRIUM occurs when these two forces are balanced throughout the Sun.

The Solar Interior

Thermal Equilibrium

Energy can be transferred by three processes: conduction, convection, and radiation

Only convection and radiation are important in the Sun




Transport of heat to the Solar surface

Energy is transported from the core to the Solar surface, through the Radiation Zone and the Convection Zone



Transport can take tens of thousands of years



Radiation Zone

Radiation passes freely through the Radiation Zone to the base of the Convection Zone



Convection Zone

Circulation of Solar mass moves heat outward

The Sun's Atmosphere



1. Photosphere
   Layer from which most visible light comes

   radius about 700,000 km

   temperature about 6000 K

   absorption lines
   reveal the composition of the Sun

   • tens of thousands of spectral lines have been observed and catalogued
   • 67 elements have been identified
   • Ten most common elements:
     • Hydrogen (91.2 % of atoms)
     • Helium (8.7% of atoms)
     • Oxygen (0.078%)
     • Carbon (0.043%)
     • Nitrogen (0.0088%)
     • Silicon (0.0045%)
     • Magnesium (0.0038%)
     • Neon (0.0035%)
     • Iron (0.0030%)
     • Sulfur (0.0015%)

   
   

   Chromosphere

   Above the Photosphere

   about 2,000 km thick

   dominated by emission lines

   
   

   Corona

   Outermost region

   millions of kilometers thick

   temperature greater than 1,000,000 K

   very "thin" ionized gas

Solar Vibrations

Sun "rings" like a bell
Measuring vibrations on the surface is a link to the interior

Features of the Solar Atmosphere for a "Quiet Sun"



• Photosphere

  "granules" - giant bubbles at top of convection zone

  
  
• Chromosphere

  "spicules" - spears of gas spurting out 10,000 km

  
  
• Corona

  heated tremendously (1,000,000 K at inner radius) by magnetic fields

  beautifully visible during solar eclipse

  
  
• Solar Wind

  outflow of material, captured in the magnetic field

Solar Neutrinos


Neutrinos are produced in nuclear reactions
• Nuclear reactors on Earth
• Thermonuclear Fusion in the Sun
• Supernova explosions
• Particle Accelerators in Laboratories

Solar neutrinos are coming directly from the interior of the Sun
• traveling at the speed of light (300,000 km/sec)
  neutrinos will pass through the radius of the Sun (700,000 km)
  in 2.3 seconds
• window on present day energy rate

Observations of Solar Neutrinos passing through the Earth
• deep swimming-pool-sized detector in Japan which serves as a "neutrino telescope"

Properties of Neutrinos
• carry no charge
• carry very little mass
• Interact hardly at all
  (neutrinos from the Sun usually pass through the entire Earth freely!)

Fewer neutrinos are detected than are expected from our model of the Sun
• We now understand this as a change in the properties of neutrinos
• Neutrino oscillations
• Our Model of the Sun appears OK

The Active Sun

Sunspots

Irregularly shaped dark spots on the Photosphere

Cooler than surrounding surface (4000 K)

• umbra - dark core
• penumbra - less dark border




reveal the differential rotation of Photosphere

• equator rotates more rapidly than pole
• 25 days vs. 35 days




very strong magnetic fields

thousands of times the Earth's field

distorted by differential rotation




number of sunspots peaks every 11 years

magnetic polarity reverses each 11 year cycle

sunspot activity varies over recorded history

Prediction of future




Up-to-date monitor of Sunspot activity




Prominences
cool clouds of hydrogen gas


Flares
burst of electromagnetic energy and particles


Coronal mass ejection
about 1/week at solar minimum, and 2-3/day at solar maximum
often (not always) associated with flares and prominences


Coronal Holes
hole in the corona
source of solar wind

Summary

Standard Solar Model
• Core
• Radiation Zone
• Convection Zone
• Photosphere
• Chromosphere
• Transition Zone
• Corona
• Solar Wind
Thermonuclear Fusion
Hyrdostatic Equilibrium
Thermal Equilibrium
Solar Neutrinos
Active Sun

• Sunspots
• Prominences
• Flares
• Coronal Mass Ejection

The Sun: a pictorial Introduction by P. Charbonneau and O.R. White
The Genesis Mission