Adequate?
Chemical reaction (burning)
Sun
Shrinks
Pressure Increased -->
Heat is produced
Nuclear Fusion:
4 H --> He + 2 Positrons + g Photons + Neutrinos
He weighs less than 4 H
E = mc2
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? |
Adequate? |
|---|---|---|---|
| Coal & Oxygen |
Chemical reaction (burning)
|
10,000 years | No |
| Gravity |
Pressure Increased --> |
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 |
| 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) 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.
| Chromosphere: | thin layer below corona |
| Prominences: | chromospheric material held up by magnetism (in dilute 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)
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
-> first shield against cosmic rays for the Earth
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 |