PES 106        Spring 2003 Previous Lecture General Astronomy II Next Lecture

Lecture Notes:

Sun

text: Chapter 11

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/

great pictures of Sun: http://www.solarphysics.kva.se/NatureNov2002/press_images_eng.html

description of the Sun and how it works: http://science.howstuffworks.com/sun.htm

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Introduction and General Properties

Sun is a typical star

stars are balls of hot gas

held together by a balance between gravity pulling in and the pressure of hot gas pushing out = "hydrostatic equilibrium"

pressure of gas increases for higher density gasses and for hotter gasses

Sun: Basic numbers:

• mass = 2x10³⁰ kg (330,000 x Earth)
• radius = 700,000 km (109 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:
  • 71% Hydrogen
  • 27 % Helium
  • 2 % other
• differential rotation
  • gas balls do not have to rotate together, equator can spin faster than poles !
  • 25 days at equator
  • 30 days near poles

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Structure of the Sun

 temperatures come from computer models of the Sun

Figure 11.2 shows how density and temperature vary inside the Sun. Link to Figure 11.2

1. Core:

high density (about150 g/cm³) gas - (about 100x more dense than water on Earth)

source of energy: nuclear fusion

• atomic nuclei combine
• converts mass to energy (E = mc²)
• requires very high temperatures and pressures
• in Sun: 4 hydrogen nuclei combine to form 1 Helium nucleus + 2 neutrinos + 1 gamma ray
  • neutrinos are tiny particles with very little mass that do not interact much with anything
    • this means they should escape quickly from the Sun
    • We do not see as many as we expect from models
      • they are hard to detect ?
      • model of Sun is wrong ?
      • we do not understand neutrinos ?
      • all of the above ?
  • gamma ray is a high energy form of electromagnetic radiation
    • it will be absorbed quickly by gas atoms
    • the gas atoms will then re-radiate (in any direction)
    • each time the electromagnetic energy is absorbed and reradiated, a little energy is lost
    • after about 100,000 years, the electromagnetic radiation created in the core will reach the surface of the sun and will be radiated away as light (much lower energy than gamma rays).
    • 

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 (hot gas rises, cool gas falls)

boiling motion in the gas

gas rises and falls at a rate of about 1 km/second

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

granulation is related to the convection in the layer below

bright areas are hot gas rising

darker areas are cool gas falling (difference between hot and cool is only about 300K)

5. Chromosphere

cloud of hot rarefied hydrogen (about 2500 km thick)

hot => emission line spectra from here

emission lines are usually washed out by bright sun (observe during eclipse)

heat may be related to magnetic fields interacting with the gas

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

spicules can be about 1 Earth diameter high and last a few minutes each (typically 300,000 at any time) - Link to further description and photo of spicules: http://csep10.phys.utk.edu/astr162/lect/sun/chromosphere.html

super granulation has cells with diameter about 3x Earth's diameter which last about a day

6. Corona

irregularly shaped cloud of rarefied ions

made up of ions (atoms which have lost electrons) => very hot (heated by magnetic waves speeding up atoms)

shape changes

Figure 11.7 in the text is a picture of the Corona - Link to Figure 11.7

Figure 11.16 shows solar prominences - Link to Figure 11.16

More pictures and description available: http://csep10.phys.utk.edu/astr162/lect/sun/corona.html

7. Solar wind

flow of charges particles away from the Sun

• comes out of coronal holes
• low density (a few hundred atoms per cubic centimeter)
• fast (about 500 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

[DEMO: magnet and filings]

 

Field flips every 11 years

returns to same direction every 22 years

possibly caused by the magnetic field getting wound up in the sun by differential rotation ?

Sunspots

• dark regions on Sun
• 1 - 3 x size of Earth
• cooler than surrounding areas (4500 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
  • last maximum in 2000-2001
• location on Sun also varies
  • when lots of spots - they appear near 30 degrees above and below equator
  • when few spots - they appear close to equator
• regions with sunspots are very active
  • solar flares are common there
    • huge outbursts of charged particles
• origin of sunspots appears to be related to magnetism preventing hot gas from rising
  • See Figure 11.15 in the text for a sketch of this process. - Link to Figure 11.15
• close-up image of sunspots from Swedish telescope - scale marks are 1000 km apart, granulation is also apparent
  • http://www.solarphysics.kva.se/NatureNov2002/images/AR10030_4877_color.gif

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