More About the Sun

The Sun

The Sun: Basics

  • Radius: 696,000 km (109 times Earth’s radius)
  • Mass: 2×10³º kg (332,946 times Earth’s mass)
  • Temperature: 15,000,000 K – center; 5,780 K – photosphere; 2-5 million K – corona
  • Luminosity: 3.8×1026 watts
  • Rotation Period at Equator: 25.4 days
  • Photospheric Composition (by number of atoms): 92.1% hydrogen, 7.8% helium, 0.1% other elements
  • Photospheric Composition (by mass %): 73.5% hydrogen, 24.9% helium, 1.6% other elements

Energy Transport Mechanisms

  • Conduction: most important in solids (e.g. pot over open flame or stove)
  • Radiation: transport of energy by motion of photons; efficiency depends on how opaqueness of the matter (e.g. heater in a cold room)
  • Convection: bulk transport of packets of matter in a liquid or gas (e.g. boiling water — hot water rises, cold water sinks)

Layers of the Sun

  • Radiative Zone: the sun’s center is opaque, so energy takes hundreds of thousands of years to escape
  • Convective Zone: hot gases rise, cold gases sink

Solar Atmosphere (photosphere, corona, chromosphere)

  • Photosphere: 5,800 K; 500 km thick; granulation in solar atmosphere
  • Chromosphere: 10,000 K; 1,000 km thick; red color, Balmer series emission line of hydrogen
  • Corona: 2 million K; large region of high-density plasma

Solar Magnetic Activity

  • Sunspots: magnetic field prevents convective bubbles; lower temperature than rest of Sun’s surface; has magnetic storms; maximum number of sunspots at 11 years, which is half of a 22-year cycle; every 11 years, sunspots’ magnetic fields change
  • Solar Flares: powerful, energetic eruptions that releases magnetic energy, and up to 20 million K
  • Corona Mass Ejections: huge flows of hot gas at 1,500 km/sec

Detecting Solar Neutrinos

  • Neutrinos: matter that interact very weakly with normal matter; the interior of the Sun is transparent to neutrinos (discovered in 1956 by Clyde Cowan and Frederick Reines)
  • First neutrino “telescope” at Homestead gold mine, South Dakota: used 400,000 liters of dry-cleaning fluid (perchloroethylene -C2Cl4) because a neutrino can interact with a chlorine nucleus to form an argon nucleus
  • Only 1 out of 10²² passing neutrinos reacted, once every two days
  • 1/3 of expected neutrinos detected based on understanding of the proton-proton chain
  • Other experiments: Sudbury Neutrino Observatory (1,000 tons of heavy water) and Super-Kamiokande experiment in Japan (50,000 tons of water)
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Solar Flares in 2012

Solar Flare

2012 marks the height of another 11-year solar cycle as the number of solar flares peak. Throughout 2012, solar flares will disrupt satellite transmissions and radio signals. A solar flare is a high-energy ejection from the sun’s corona, emitting up to 6 × 1025 Joules and reaching up to the edge of the solar system!