1. What makes a star a star?

Which is ...

... when two atomic nuclei are 'fused' together.

This can only happen when ...

... material is extremely hot and dense.

Why?

Because nuclei have the same electric charge, and so they repel each other, unless pushed close enough together.

Then, suddenly the nuclei are suddenly strongly attracted.

- electric charge pushes them apart
(long-range, electro-magnetic force)

- 'strong' force pulls them together
(a short-range force)

(for a summary of forces, see lecture 2)

If density and temperature (pressure) are high enough, nuclei get 'squeezed' close enough together for the 'strong' force to win out over the electro-magnetic force.

H nuclei (single protons) are easiest to fuse because the have the smallest electric charge of all nuclei (the repellent!)

The fusion releases energy star!

2. Solar structure:      

Core - where fusion occurs

Interior - a radiative transfer zone

Convection zone

Photosphere - the solar disk, very thin      

5800 degrees (K)

Chromosphere - lower atmosphere, where absorption lines are formed

(transition region)

Corona - upper atmosphere - very hot and tenuous      

How is heated?

Energy from magnetic fields?

Trends of temperature and density:      

3. Solar Guts - fusion in the core, in gory detail

Incredible energy production from: fusion +

(i) E = mc²

(ii) total matter and energy are always conserved

The proton-proton chain:      

6 protons (6 H nuclei)

2 protons + 2 neutrinos + 2 positrons + He-4

where,

He-4 (Helium-4) = 2 protons + 2 neutrons

neutrinos: almost massless, weakly interacting

positrons = anti-electrons

- weigh much less than proton or neutron

Add up input and output mass:

input mass > output mass

Energy MUST be released

Mass difference is due to the difference in the greater binding energy of the 2 neutrons + 2 protons in the Helium nucleus.
(Even though proton mass is less than neutron mass, 4 un-bound protons weigh more than 2 neutrons + 2 protons bound together.)

4. Energy transport -- how does the energy get out?      

radiation:

free streaming of photons or other energetic particles

convection:

overturning of cells of matter at different temperatures

Solar radiative zones:

most of the interior of the sun

Core

Interior

and, the Photosphere

Convective zones:

outermost layer of the interior before the photosphere

convective cells get smaller at larger radii

(cooler temperatures, lower densities)

5. Solar surface

Due to, or shaped by magnetic fields:

sunspots      

slightly cooler regions of photosphere
300-1300 degrees (K) cooler

spicules      
prominences      
flares      

Sunspot and Solar cycles:      

11 years - sunspot cycle (minimum and maximum activity)
22 years - solar cycle (orientation of magnetic fields)

Why?

differential rotation twisting of field lines      

Due to convection:

granules (granulation)      

roughly 1000 km in size

super-granules (super-granulation)

roughly 30,000 km in size

• produced by nuclear fusion reactions in core

• so weakly interacting, they should 'free stream'
  directly out from the core to Earth

• where are they? (why don't we detect them?)      

• neutrino oscillations?

  . . . or is our physical understanding of the sun seriously flawed?

The strongest evidence to date favors neutrino oscillations and a standard solar model.

(a) flares

(b) fusion

(c) sunspots

(d) radiation

(e) spicules

(a) mass and energy are convertible and conserved overall

(b) the neutrino can pass through light-years of lead

(c) fusion drives convection in the outer layers of the sun

(d) the mass of the protons are less than the resulting neutrons

(e) proton-proton interactions conserve angular and linear momentum