The Sun

1
The Sun

• Nearest star can be studied in detail
• Completely Gaseous
• Gravity Balanced by Gas Pressure
• We see only the surface layers (so-called
  atmosphere)
• Out-flowing gas from Sun extended past Earths
  orbit

2
Photosphere

• 500 km deep 5,800K
• Below photosphere denser gas light cannot
  escape
• Above less dense gas doesnt radiate as much
  light
• The photosphere is opaque most H is neutral, but
  enough free electrons to make
• H e- ? H-
• Good at absorbing photons also this reaction
  produces a continuous spectrum

3

• Deeper layers of photosphere are denser
  produces continuous spectrum
• Higher layers are less dense, and as light passes
  through ? absorption lines (Kirchoffs 3rd law).
• Granules structures in photosphere, centers
  rise, edges fall show heat transfer due to
  Convection

4
(No Transcript)
5
Chromosphere

• Lies above photosphere
• 1000 times fainter
• Seen during total eclipses appears pink due to
  Balmer lines
• Absorption lines from lower parts (Balmer, Ca II
  H and K lines)
• Emission lines from outer parts
• Low density, ionized gas

6
(No Transcript)
7
(No Transcript)
8
Corona

• Faint, detected during solar eclipses, or with
  coronagraphs
• Extends to 10 of the distance to the Earth
• Shines by sunlight reflected of 1) dust grains,
  2) free electrons (it is highly ionized)
• Temperature 5 x 105 to 2 x 106 K , heated by
  magnetic fields
• Solar Wind gas flows along magnetic field lines
  towards the Earth

9
(No Transcript)
10
(No Transcript)
11
Back to the Photosphere

• Sunspots
• Typically twice the diameter of the Earth
• Occur in pairs, groups ( 100 in active region)
• Why dark? Cooler than surrounding region
• Magnetic fields stronger near sunspots, charged
  particles cannot move freely, so convection is
  slowed
• These magnetic fields extend into the
  Chromosphere and Corona
• 11-year cycle -- start at /-350, then begin to
  appear at lower latitudes (Maunder butterfly
  diagram)
• Cycle is result of changing magnetic fields

12

• Sun experiences differential rotation equatorial
  region rotates faster than at higher latitudes
• Babcock Model magnetic fields are frozen into
  the gas differential rotation wraps, twists the
  magnetic fields around the Sun
• Once the tangling becomes extreme, adjacent
  regions change fields to match their neighbors
  and the process begins again
• Note Sunspot pairs are like bar magnets, with
  one a north pole, one a south pole the rising
  and sinking of gas twists the field line into
  tubes, where the sunspot pairs appear.

13

• Prominences hot gas trapped in magnetic fields,
  extending into the lower corona
• Solar Flares violent outbursts, linked to the
  magnetic fields (reconnection of arches of
  magnetic force) emit X-ray, UV, visible light
  and produce shock waves
• Flares send high energy charged particles towards
  the Earth -- Aurorae

14
Coronal Activity

• Sunspot minimum flattened corona
• Sunspot maximum extended, circular corona
• Corona can be observed with UV, X-ray telescopes
  (from space) coronagraphs
• When magnetic fields do not loop back to trap hot
  gas get coronal holes (from which the Solar
  Wind escapes)

15
Source of Suns Energy

• Nuclear Fusion
• 4 known forces in nature
• Gravity
• Electromagnetic
• Weak (radioactive decay)
• Strong (nuclear energy)

16

• Fission splitting of U235 into lighter elements
• Fusion light nuclei combine to form heavier
  nuclei
• 4 hydrogen ? 1 helium energy
• 4 H nuclei 6.693 x 10-27 kg
• 1 He nucleus 6.645 x 10-27 kg
• So 0.048 x 10-27 kg is missing!
• Mass is converted into energy ? E Mc 2

17

• Within the Sun 1038 reactions per second
• In order for the reaction to occur, the nuclei
  must overcome repulsive electromagnetic force
  (the Coulomb barrier)
• Need hot gas 107 K
• Also need high density the core of the Sun is
  denser than solid lead (but still a gas!)
• Lots of collisions occur but only a small
  fraction result in Fusion.

18
(No Transcript)
19
Proton Proton Chain

• Dominant at temperatures up to 107 K
• Source of 90 of the Suns energy
• Energy released in form of ?-rays these are
  absorbed by surrounding gas in a fraction of a
  second
• Also positron/electron annihilation produces
  ?-rays
• Also get energy of motion of the newly created
  nuclei
• Takes time for this energy to escape from the
  interior of the Sun

20
(No Transcript)
21

• Neutrinos (?) 2 of the energy created during
  the fusion process
• Rarely interact with matter 1012 of them pass
  through you every second!
• Davis experiment 100,000 gallons of C2Cl4, in a
  mine. One Chlorine atom/day converted into Argon.
• But, only 1/3 of the predicted amount of ?s.

22

• Three types electron, muon, and tau neutrinos
  (Davis experiment only sensitive to
  electron-type)
• Oscillations of neutrinos 2/3 could have become
  undetectable types on passage from Sun
• Super Kamiokande neutrino detector latest
  results show that neutrinos have rest mass and
  oscillate.