Wednesday February 3rd 1999
Announcements:

Lecture notes:

Class 11

Nuclear fusion powers the sun.
H -> He
Only occurs in the core (very center) of the sun

Core has T of about 14-15 million degrees
very hot - > makes short wavelength radiation

In fact the core of the sun is so hot it makes x and gamma rays as the main type of radiation - these are the shortest wavelength radiation.

Now x/gamma rays are harmful to people - so if x/gamma rays are made in huge amounts in the core of the sun then why don't they kill us?

To understand we will follow the path of a single gamma ray from creation in the core to escape from the solar surface.  We shall see that it's not a direct route but takes long and
twisting route to escape as the gamma ray leaves the core it passes into the radiation
zone.
 

• This is a layer of gas above the core made of H and He.  There is no nuclear burning here and a it has a temperature of about 4x10⁶K
• Great deal of matter here that gamma rays must pass through.
• Photons interact with matter via absorption and re-emission.
• When absorbed and re-emitted it has a new energy corresponding to region where emitted - lower T so lower E.
• So energy is stolen away from a gamma ray photon via this process of absorption and re-emission.

Happens many many times - takes 10 000 years to make it to surface of radiation zone.  Could fly out in just 3 seconds if not blocked.

Then photon enters the convective zone.  Somewhat like pot of boiling water.  It has huge circulating "convection" patterns. Each bubble is about the size of the USA.  The temperature is
about 1x10⁶K
 

• Then it goes to the photosphere - "photon sphere" or "edge of the sun"
• This is the visible surface of the sun.  It has low density and no matter to block photons so they can fly freely away.
• The temperature is about 5700K
• Thin layer (300 km thick).  By the time it gets to the surface our gamma ray has been turned into visible light.

Now above the photosphere is another layer that we talked about before in the eclipse section
Chromosphere - cool low density atmosphere of the sun.  4500 K
Doesn't block photons much.  You can't normally see it.  Can see during some solar eclipses has no color.

Weather on the Sun
The surface is not bland and uniform but has sever kinds of features.

Granulation.
Solar surface is granulated - particles of light and dark.
Each granule is about the size of the USA and lives for 5-10 minutes
Arises from bubbling up from convection zone like boiling soup bubbles.

Sunspots - dark spots on solar surface
about the size of the Earth
Cooler than photosphere but not really black -- live for 1-5 weeks
Caused by magnetic field sticking up out of the sun - blocks convection below.

Prominence
loops of gas ejected from solar surface
"ride" the magnetic field and can be huge ( > 10 times the Earth)

Flares - giant explosions caused when magnetic fields get ripped apart

• last for about 1 hour
• 100 million H bombs worth of energy
• occur near sunspots.

Sun does emit some x-rays but these are not made in the core.
These x-rays arise due to energetic processes occuring above the photosphere (e.g. magnetic fields being torn apart).

This is far less than if x/gamma rays escaped from core
 
 

Rotation on Sun

• sunspots and other features let us measure how the sun rotates - can watch them move across sun as it rotates.
• Faster at equator - 27 days
• Slower at poles - 31 days

This is possible because Sun is not a solid body.
This is called differential rotation.

Sunspot Cycle
Count the number of spots on the sun and watch over time.
Number of spots reaches a maximum roughly every 11 years and then falls off
Most recent maximum was in 1990
Also new spots appear closer to the equator as cycle progresses.

Part of 22 year solar cycle.

• Sunspot cycle is not a perfect clock
• 11 year period actually varies between 7-15 years.
• also it stopped from 1645-1715.  For 70 years there was little solar activity - few or no
• sunspots.
• Period of particularly cold winters on earth called "little ice
• age"

Why solar-cycle?
Not well understood in detail
Magnetic fields twisted up by differential rotation and then realign

How the Sun affects people?
Main way - allows for life on Earth
powers photosynthesis
Sunburns and skin cancer from UV radiation
Climate connections - for example the little ice age.
Details are tricky but generally fewer sunspots - > cooler weather.

Solar flares can lead to power outages
high energy photons hit Earth and make currents that damage circuits
March 1989 flares

• Quebec province power system collapsed.
• Local power stations  here in PA had short outages

Also a solar wind that flows out from corona

Corona = hot (2 million degree) low density surface layer of sun
Why hot?  magnetic heating
Can see during some eclipses when both photosphere and chromosphere are blocked

Solar wind - hot gas from corona that flows out into space.
Mostly protons and electrons with some heavier particles.
Blows past earth at 300-800 km/s with irregular gusts up to 1000 km/s

Aurora - Northern Lights and Southern Lights.

Finally in 5 billion years when the sun starts to die it will expand dramatically and its luminosity output will sharply increase.  It will incinerate life on Earth and it may expand so much that it swallows the Earth.  This is the final way that the sun will affect life on Earth.  Pretty final!.