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”

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

Gravitational (contraction/collapse)

Nuclear Fission --- radioactive elements

Nuclear Fusion --- ???p+ + p+ -> ? How?

the Coulomb barrier

The Solar Interior and the energy source of the Sun and Stars

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. 3He+ + 3He+ 4He+ + 2 p+

Net Result -- 4H 1He

Alternative -- CNO cycle in more massive stars > 2 Msun

The CNO cycle converts hydrogen to heliumThe 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, Oxygen3He4 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