PES 105        Fall 2001 Previous Lecture General Astronomy I Next Lecture

Lecture Notes:

Sun

text: Chapter 11

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Introduction and General Properties

Basic Information and Links : http://seds.lpl.arizona.edu/billa/tnp/sol.html

Link to good information, pictures and movies of the Sun: http://planetscapes.com/solar/eng/sun.htm

Pictures : http://antwrp.gsfc.nasa.gov/apod/index/Sun.html

Solar Environment Center (Pictures of the Sun today!): http://www.sec.NOAA.gov/

Sunspots, Solar cycles and Sun videos: http://www.sunspotcycle.com/

Sun is a typical star

stars are balls of hot gas

Basic numbers:

• mass = 330,000 x Earth
• volume = 1 million x Earth
• surface gravity = 28 x Earth
• luminosity = 4 x 10²⁶ watts
• surface temperature = 5780 K
• core temperature = 15 million K
• composition:
  • 73% Hydrogen
  • 25 % Helium
  • 2 % other
• differential rotation
  • 25 days at equator
  • 30 days near poles

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Structure of the Sun

1. Core:

source of energy: nuclear fusion

• atomic nuclei combine
• converts mass to energy (E = mc²)
• requires very high temperatures and pressures

2. Radiative Zone

energy from core transferred out by absorption and re-radiation of light

temperature drops gradually

3. Convective Zone

energy transferred out by convection

boiling motion in the gas

4. Photosphere

Sun's "surface"

very thin layer (300 - 400 km thick (like Earth's atmosphere)

very low density (about 1/1000 of an atmosphere)

gas is cooler here => absorbs light => source of absorption line spectra from Sun

surface is divided into cells = "granulation"

Granules are 500 - 1500 km across (Texas is about 1500 km across) lifetime of a cell is about 10 minutes

5. Chromosphere

cloud of hot rarefied hydrogen

heat => emission line spectra from here

observe larger granulation and jets of gas (spicules) penetrating layer

6. Corona

irregularly shaped cloud of rarefied ions

very hot (not clear why it is so hot)

shape changes

7. Solar wind

flow of charges particles away from the Sun

• comes out of coronal holes
• low density
• fast (about 400 km/sec at Earth)
• extends out beyond Neptune
• governed by magnetic fields

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Magnetic Fields and Sunspots

Sun has strong magnetic field

Field flips every 11 years

returns to same direction every 22 years

Sunspots

• dark regions on Sun
• 1 - 3 x size of Earth
• cooler than surrounding areas (4000 K vs. 6000 K)
• last up to several months
• usually appear in pairs
  • pairs have strong magnetic field with one spot a North pole and one a South pole
• number of sunspots varies in 11 year cycle
  • next maximum in 2000-2001

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Evolution of Sun

• Hydrogen Burning
  • lasts 10 billion years
  • half way through this stage
• Red Giant
  • when hydrogen in core is used up
  • sun expands to almost Earth's orbit
  • lasts 500 million years
• Helium Burning
  • lasts 10 - 100 million years
• Red Giant II
  • when helium in core is used up
  • sun expands beyond Earth's orbit
  • lasts 1 million years
• Mass loss
  • lasts100,000 years
  • outer layers of Sun are slowly blown off
• White Dwarf
  • only core remains
  • no more nuclear reactions - so slowly cools
  • lasts 5 - 20 billion years
• Black Dwarf
  • eternity
  • cold, dark ball of gas

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