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The Sun

The Sun. Announcements. Reading Assignment Review and finish reading Chapter 18 Optional reading March 2006 Scientific American: article by Gene Parker titled “ Shielding Space Travelers ” http://en.wikipedia.org/wiki/Solar_variability

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The Sun

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  1. The Sun Sun 3/1/07

  2. Announcements • Reading Assignment • Review and finish reading Chapter 18 • Optional reading • March 2006 Scientific American: article by Gene Parker titled “Shielding Space Travelers” • http://en.wikipedia.org/wiki/Solar_variability • 3rd Homework due today; 4th homework now posted on course website • Next study-group session is next Wednesday from 10:30AM-12:00Noon – in room 330. Sun 3/1/07

  3. The Sun: Our Star • Distance from Earth • 1 AU • Travel time for Light to Earth • About 8 minutes • Travel time for solar wind to 1 AU • A few days • Mean surface temperature • 5800K • Temperature in the Center • 1.55x107 K • Temperature in the corona • 2 x 106 K Sun 3/1/07

  4. The Sun • Mass • 333,000 Earth Masses • More mass than all of the other objects in the solar system combined (by a long shot) • Diameter • 103 Earth Diameters • Average Density • 1410 kg/m3 • Composition (by mass) • 74% Hydrogen, 25% Helium, 1% other elements Sun seen in Xrays Sun 3/1/07

  5. The Sun’s appearance from the ground • Using a Baader solar filter • Can see the sun’s visible surface, or photosphere • This is how the Sun appears when we simply cut down on its brightness with a filter (i.e. it is seen in “white light”) • These types of filters are very Inexpensive • Many “solar observing glasses” are made out of this material • Used to see sunspots Sun 3/1/07

  6. The Sun’s appearance from the ground • Hα filters • Observes the Sun at a particular wavelength • The a line at 6563 angstroms (Å) which is the strongest of the Sun’s absorption lines associated with the presence of hydrogen in its atmosphere • Much more pricey • Observe the chromosphere • Used to see Prominences, filaments, and sometimes flares • Very impressive Sun seen with an Hα filter Sun 3/1/07

  7. SOLAR CORONA – SEEN DURING A TOTAL ECLIPSE Sun 3/1/07

  8. The Sun’s Appearance from Space Solar Corona As seen in x-rays Solar Corona As with a coronagraph Sun 3/1/07

  9. The Sun’s Energy Source is ThermonuclearFusion in its Core • Proton-proton chain • Four hydrogen nuclei “fuse” to form a single helium nucleus • There is a slight loss of mass in this process which is converted to energy according to Einstein’s famous equation E = mc2 • Thermonuclear fusion occurs only at the very high temperatures at the Sun’s core • Fusion should not be confused with fission ! • Will continue to heat the Sun for another 5 billion years Sun 3/1/07

  10. The Proton-Proton Chain Sun 3/1/07

  11. Detecting Solar Neutrinos • Solar Neutrinos are the only direct probe of the Sun’s interior that we have – hence, they are a valuable tool for understanding the Sun’s interior • Underground detectors are used to avoid interference from cosmic rays • For a long time, it was not clear why there were fewer neutrinos observed than predicted. We now know why this is. Sun 3/1/07

  12. What is the Sun made of? • Mostly hydrogen and helium in the form of plasma • A normal gas (i.e. not a plasma) only exists in the thin region of the photosphere and lower chromosphere • Sunspots (in the photosphere) are cool enough that simple molecules can form • But other than this small region, the Sun is almost entirely made of plasma! Sun 3/1/07

  13. The Structure of the Sun’s interior • Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius • The radiative zone extends from the edge of the core to about 0.71 solar radius • Here energy travels outward through radiative diffusion • The convective zone is the next layer and is a rather opaque gas • Here energy travels outward primarily through convection Sun 3/1/07

  14. The convection zone is just outside the radiative zone • turbulent convective motions cause overturning (bubbling) motions inside the Sun. • Like a pot of boiling water • These are responsible for the granulation pattern seen on the Sun’s surface. Radiative zone Sun 3/1/07

  15. Solar Granulation • These are convection cells that are about 1000 km wide • They are part of the Sun’s atmosphere known as the Photosphere Sun 3/1/07

  16. High-resolution images of granulation and a sunspot Sun 3/1/07

  17. Solar Oscillations • Waves can propagate through the Sun causing a variety of vibrations • In some sense, they are like seismic waves on Earth • Like sound waves • 5-minute oscillations • These are used to infer pressures, densities, chemical compositions, and rotation rates within the Sun • The branch of science that studies solar oscillations is known as Helioseismology Sun 3/1/07

  18. The photosphere is the lowest of three main layers in the Sun’s atmosphere • The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona • Everything below the solar atmosphere is called the solar interior • The visible “surface” of the Sun, the photosphere, is the lowest layer in the solar atmosphere Sun 3/1/07

  19. Limb Darkening • The edges of the Sun’s photosphere appear darker than that seen “straight on” • This is called limb darkening • It is due to the fact that the temperature in the photosphere decreases with altitude Sun 3/1/07

  20. The Origin of Limb Darkening • The light we see at the limb originated higher up in the atmosphere where it is cooler • Thus it will be less bright there Sun 3/1/07

  21. The Chromosphere • Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere “Color Sphere” • Spicules • Regions of rising gas • Filaments • dark, thread-like features • Plage • bright patches surrounding sunspots Sun 3/1/07

  22. Sun 3/1/07

  23. Prominences are the same as filaments, except that they are seen from the side rather than “straight on” Sun 3/1/07

  24. The Corona • The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density • The solar corona blends into the solar wind at great distances from the Sun • Because of the high temperatures, the corona is best viewed in the X-ray part of the spectrum Sun 3/1/07

  25. X-Ray image of the Sun Sun 3/1/07

  26. Sunspots • Existence known since 350 BC (Greece), 28 BC (China) • Lower temperature than surrounding atmosphere • Umbra (darkest part) and penumbra • Associated with Intense magnetic fields • Magnetic Fields are measured by using the Zeeman effect Sun 3/1/07

  27. Zeeman Effect: The splitting of spectral lines by a magnetic field Sun 3/1/07

  28. The 11-year Sunspot Cycle Number of Sunspots versus time – they come and go every 11 years Number of Sunspots versus latitude – forms a “butterfly pattern” Sun 3/1/07

  29. These changes are caused by convection and the Sun’s differential rotation: The Solar Dynamo Sun 3/1/07

  30. Next Class: Solar Variability and its Effect on Earth and its Inhabitants Sun 3/1/07

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