VIII. Understanding the Sun Our Star

1. Interior of the sun

A) Energy source

Possibilities that have been discussed are:

a) Sun of pure coal and oxygen

Life of approx. 10000 years -> much too short

b) Shrinkage under gravity

-> increase pressure -> increase T -> radiate energy
Life of approx. a few million years -> too short

The Energy Source of the Sun:

Factoid: The Sun is already 4.5 billion years old
Source How? How Long?


Coal & Oxygen

Chemical reaction (burning)

10,000 years No

Sun Shrinks

Pressure Increased -->
Heat is produced

10 Million years No
Hydrogen Nuclei

Nuclear Fusion:

4 H --> He + 2 Positrons + g Photons + Neutrinos

He weighs less than 4 H

E = mc2

10 Billion years Yes


c) Nuclear Fusion (Bethe: Nobel Prize)

4H --> He + gamma photons + neutrinos
stay in sun escape
(collide with matter) (interact very weakly with matter)

He has less mass than 4 H atoms, and the 'mass difference' is converted into energy

via E = m c2 (Einstein)

The energy appears as gamma photons and neutrinos.

Fusion requires high temperature to overcome electric repulsion between
positively charged nuclei ("fusion ignition temperature").
Fusion requires high density for lots of collisions. Thus
Fusion occurs only in a small central 'core'. (T approx. 15 million K)

On Earth so far only used in Hydrogen Bombs (ignited by a nuclear bomb)

nuclear fusion reactor difficult -> hot stuff flies apart too fast
try: magnetic containment

Why does the sun not explode like a nuclear bomb?

more fusion -> T increases -> pressure increases
-> sun expands -> T decreases -> less fusion

B) Hydrostatic equilibrium

Pressure increases inwards. since:
Density increases inwards (more dense sinks) and
Temperature increases inwards (to carry energy outwards)
gas under pressure gets heated (as pump for bicycle)

C) Information from the interior

a) Neutrinos: Detection = proof of fusion
They have no mass[?] but energy -> One "view" of sun's interior
can pass through matter not electromagnetic radiation
"Solar Neutrino Problem": only 1/3 of expected flux detected

Possible explanations (?):

- detector problems? checked with other detectors
- nuclear data incorrect? checked with accelerators
- model of sun incorrect? only one side reaction checked
-> get neutrinos from pp reaction
- Neutrinos change on way -> neutrinos would have small mass
and/or interact with sun's magnetism

All stars make neutrinos -> universe full of neutrinos

Can neutrinos with mass solve the missing mass problem?

GALLEX experiment: main reaction neutrinos react with Gallium

-> close to expected amount found

-> neutrino mass very very small or 0
-> problem not fully solved yet

b) Helioseismology (like earthquakes)

Doppler effect --> Surface moves up and down (like the ocean)
Period about 5 minutes "5 minute oscillations"
Sound waves trapped inside sun

Our 2nd "view" of sun's interior

Results: Do not solve neutrino problem
Do not agree with models of magnetic field generation
So far a riddle -> extended measurements with SOHO

D) Energy transport to surface

a) Energy flow by photons:

b) Energy flow by convection (outer shell)

- Hot rises
- Cool descends

2. Solar atmosphere

A) Photosphere:

a) Definition: where energy (visible light) comes from

b) Structure:

Granulation - All over sun (except in sunspots)
- small scale convection on surface (like boiling teapot)

Velocities can be measured with Doppler effect:

c) Activity

Zeeman splitting of spectral lines Þ magnetism measured

Sunspots: strong magnetism.
Sunspots with complex magnetism --> flares

Fewer sunspots --> fewer faculae --> sun dimmer
--> colder climate (no sunspots in 17th century) --> 'little ice age'

Alternate view: dust from volcanoes made the little ice age and other cool periods.

B) Chromosphere:

Chromosphere: thin layer below corona
Prominences: chromospheric material held up by magnetism (in dilute corona)

C) Corona:

Coronal heating - a riddle

Current best model:

b) Corona is structured by magnetism: lines and loops

Due to: electrons spiralling around magnetic lines

Magnetic lines are like taut strings

i. Waves carry energy (Alfvén waves - Nobel prize) --> heat

ii. Magnetic reconnection turns magnetic energy into motion (kinetic energy)

D) Solar Wind

a) The corona overcomes solar gravity

Solar Wind: regions of the corona flow out to Earth and beyond
Strong solar wind originates over poles of the sun:
Goal of Ulysses (over solar poles in 1995/96)

Competition gravity and magnetism hold atmosphere down
temperature escape --> solar wind

Solar wind flows with supersonic speed

When it runs into obstacles (Earth's, other planets' magnetic field), it forms a

Bow Shock = "Supersonic Boom of the Earth in space"

b) Solar wind carries sun's magnetism to Earth and beyond
Sun's magnetism + Earth's magnetism --> aurora etc.

c) The corona and solar wind change --> affects Earth

i. Solar cycle (number of sunspots)

ii. Coronal transients (coronal mass ejections) --> ejection of plasma and magnetism

d) Shield against cosmic rays

Altogether there is a 3 layer shield around Earth:

i. Solar wind (sun's magnetic field)
ii. Earth's magnetic field
iii. Earth's atmosphere

Go to Chapter IX.