Transcript Document

The Sun as a Star
The “Surface” of the Sun and Its Structure
Outer Layers – 3 distinct region
Photosphere
Chromosphere
Corona
Photosphere -- light sphere
The surface in “visible” light
T ~ 6500 - 4000 K
Depth 100’s kms
Granulation – cellular pattern due to convection
Chromosphere – color sphere, seen at solar eclipse
T ~ 6000 - 100,000 K, 2000 km thick
Hot, low density gas, also granular appearance – supergranulation
The Corona or Halo
T ~ 1-2 x 106 K , extends millions kms
Very hot, low density gas
Source of energy to heat chromosphere
and corona ??
Coronal holes -- in X ray images
Flux of charged particles – solar wind - from
the holes, governed by magnetic fields
Heating via magnetic waves and mechanical
flux from convective layers deep in interior
Sunspots and the Solar Activity Cycle
Appear dark – lower temp.
~ 4500 K vs 6000 K
Strong magnetic fields ~
several thousand Gauss –
normal Sun – few Gauss
Opposite polarity between
sides of a large spot group
Magnetic disturbance or storm
The 11 yr sunspot cycle -- magnetic cycle every 22 yrs.
Maunder Minimum and the
“ little ice age”
the Thames 1677 -- frozen
Solar Activity – Prominences associated with large spot groups
Solar Flares -- most violent form of solar activity
A sudden brightening, above large spot group,
between regions of opposite polarity
Outburst of charged particles (cosmic rays),
increase in high energy radiation
Sunspots, prominences, flares all associated with magnetic fields
All increase and reach maximum with 11 yr solar activity cycle
lack of spots, aurora, < 50 spots 1672-1699
Normal in 30 yrs 30,000 – 40,000
Galileo 1614 ~ 100 spots observed
Hevelius 1652-1685, Picard 1653-1685 (LaHire 1718), Flamsteed 1676-1699
LaHire and Durham noted surge in activity 1715
the Maunder minimum and the “little ice age”
1645 -- 1715
global cooling 0.2o C
N. Europe ~ 1 - 2o C
Dalton minimum 1800 – 1830
year without a summer 1816
but in 1815 Tambora volcano – middle of Dalton minimum
Sunspots and short term climate change?
the Solar irradiance – energy flux watts/m2 at upper atmosphere
0.1 – 0.2% variation in flux
Earth’s surface temperature vary by 0.1 – 0.2o C
During little ice age global cooling 0.2o C
Sunspot Cycle 24
Cycle 23 minimum 2008.0 but then a lack of spots
solar irradiance at 2008
minimum – lower 0.1%
global magnetic field at
minimum weaker
solar wind weakening
Is the Sun Missing Its Spots?
NY Times 7/21/2009
The impact on global warming ?
Or – our perception of it
slight increase in solar output ~1900 - 1950
Zero degree reference is
30yr average ~ 1950 - 80
The Solar Interior and the energy source of the Sun and Stars
Gravitational (contraction/collapse)
Nuclear Fission --- radioactive elements
Nuclear Fusion --- ???
p+ + p+ -> ? How?
the Coulomb barrier
Nuclear Fusion and Nucleosynthesis in the Stars
The proton-proton chain or
hydrogen fusion,
requires 107o K
1. p+ + p+  np+ + e+ + neutrino
np+ = deuteron (deuterium)
2. d+ + p+  n2p+ + gamma ray
n2p+ = 3He
3.
Net Result -- 4H  1He
3He+
+ 3He+  4He+ + 2 p+
Alternative -- CNO cycle in more massive stars > 2 Msun
The CNO cycle converts hydrogen to helium
The mass-12 isotope of C captures a proton and emits a gamma-ray producing the mass-13
isotope of N.
N-13 is unstable and beta decays to the mass-13 isotope of C with a half-life of
approximately 10 minutes.
The mass-13 isotope of C captures a proton and emits a gamma-ray to become the mass-14
isotope of N.
The mass-14 isotope of N captures another proton and emits a gamma-ray to become the
mass-15 isotope of O.
The mass-15 isotope of O undergoes a beta decay to become the mass-15 isotope of N.
The mass-15 isotope of N captures a proton and emits an alpha-particle (that is, a nucleus of
helium) to close the cycle and return to C-12.
Beyond Hydrogen Fusion
He Fusion to Carbon, Oxygen
3He4  C12
C12 + He4  O16 requires 108o K
C, O fusion to heavier elements up to Fe (iron) requires 5 x 108o K
C12 + C12  Mg24 , O16 + He4
 Ca20
The Solar Interior
Hydrostatic Equilibrium (Pressure)
gas pressure out = gravity in
Thermal Equilibrium (Temperature)
Energy (heat) in = Energy out
=> Energy production rate = luminosity of the star
Transfer of energy
Radiative (inner) and convection (outer)
Random walk of photons