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  1. OUR UNIVERSE Week 8

  2. The Sun & the Stars. Visible UV

  3. Sun’s Structure

  4. Sun’s Structure The ultra-hot core extends outward from the star's center to about 20% of its radius. The temperature at the centre of the core is around 15 million kelvin, and it gradually decreases further from the center. The core is the location within the Sun in which hydrogen fusion occurs.

  5. Above the core is the radiation zone. It extends from the top of the core outward to about 70% of the Sun's radius. Temperatures varies from 10 to 5 million kelvin. Energy is carried through the radiation zone via electromagnetic radiation (photons).

  6. Above the radiation zone, and extending all the way to the "surface" of the Sun, is the convection zone. This part of the Sun is relatively "cool", with temperatures ranging downward from a peak of around 2 million kelvin. Energy flows upward through this area in a different manner than in the underlying radiation zone.

  7. Gigantic blobs of matter, heated by the radiation zone below, rise to the Sun's surface, carrying heat with them. As these blobs of plasma emit their energy into space at the Sun's surface, they cool somewhat; enough so that their densities increase and they sink back down. This convective motion is akin to that seen in a lava lamp.

  8. Solar Granulation is due to convection cells

  9. The Sun’s internal Structure

  10. Convection cells

  11. At the topmost boundary of the convection zone lies the photosphere. The photosphere is often referred to as the "surface" of the Sun. The photosphere marks an abrupt transition in the optical properties of the material that makes up the Sun.

  12. Below the photosphere, the photons bounce around so much that they don't travel direct paths to viewers on Earth. Hence we cannot see deeper into the Sun than the photosphere. So the photosphere is the "visible surface" of the Sun. The temperature of the photosphere is about 5800 K.

  13. Above the photosphere the Sun's vast “atmosphere” extends outward into interplanetary space. In the case of the Sun, the density of material in the solar atmosphere is much less than is the case below the photosphere within the Sun's "interior". Also, the physical properties that control motions of material and the temperatures encountered are far different in the Sun's atmosphere than in the layers of the Sun beneath the photosphere.

  14. There are two major regions within the Sun's atmosphere: the lower and much smaller chromosphere, and the upper and much larger corona.

  15. The relatively thin chromosphere is just a few thousand kilometers deep, less than Earth's diameter. Although temperatures within the Sun gradually decrease as one moves outward, (from 15 million kelvin in the core to 5,800 kelvin at the photosphere) they begin to climb once again as we rise through the Sun's atmosphere.

  16. The temperature of the chromosphere increases from 4,300 kelvin (slightly above the photosphere) to around 50,000 kelvin (near the corona). Powerful magnetic fields in the Sun's atmosphere accelerate the plasma as they transfer energy to it, heating the material in ways that scientists still don't fully understand.

  17. Until relatively recent times, when special filters and space-based telescopes became available, the Sun's atmosphere was only visible during total solar eclipses. During an eclipse, the chromosphere could be seen as a colorful reddish zone around the edge of the occluded solar disk, thus earning the region its name (Greek "chromos" = "color").

  18. Chromosphere

  19. The Sun's much larger upper atmosphere, the corona, extends unevenly for millions of kilometers into space. The temperature of the solar atmosphere climbs sharply in a narrow transition region between the chromosphere and the corona. The temperatures in the corona range from around 800,0000 K to 3 million K

  20. corona,

  21. Matter is continuously flung outward by the Sun. An electrically charged "soup" of protons, electrons, and lesser numbers of heavier atomic nuclei flows outward into space. This extremely tenuous plasma is called the solar wind. In a sense, the solar wind is a vast extension of the Sun's atmosphere.

  22. The solar wind flows past Earth and beyond. All of the planets are within the gigantic "bubble" of the solar wind. Eventually, on the far edge of our solar system, the solar wind merges with the outpourings of other stars, and the extended solar atmosphere ends.

  23. The gigantic region within this solar wind "bubble" is called the heliosphere. The boundary of the heliosphere, where the extended atmosphere of the Sun finally gives way to interstellar space, is called the heliopause. The location of the heliopause is something like 70AU from the Sun.

  24. Solar Prominence in UV

  25. Sunspots

  26. It appears that sunspots are the visible counterparts of magnetic flux tubes in the sun's convective zone that get "wound up" by differential rotation.

  27. Convection cells

  28. Differential rotation causes field lines to be wrapped around the Sun Sunspots migrate to equator where they cancel out and eventually reverse the overall field

  29. If the stress on the tubes reaches a certain limit, they curl up like a rubber band and puncture the sun's surface. Convection is inhibited at the puncture points; the energy flux from the sun's interior decreases; and with it surface temperature.

  30. Sunspots are depressions on the sun's surface. Sunspots come in pairs with opposite magnetic polarity. From cycle to cycle, the polarities of leading and trailing with respect to the solar rotation] sunspots change from north/south to south/north and back. Sunspots usually appear in groups.

  31. Sunspot Structure

  32. The sunspot itself can be divided into two parts: • The central umbra, • which is the darkest part, where the magnetic field is approximately vertical. • The surrounding penumbra, • which is lighter, where the magnetic field lines are more inclined.

  33. Sunspot activity cycles about every eleven years. • The point of highest sunspot activity during this cycle is known as Solar Maximum, and the point of lowest activity is Solar Minimum. • Early in the cycle, sunspots appear in the higher latitudes and then move towards the equator as the cycle approaches maximum.

  34. Sun spots have a 11 year cycle (magnetic field reversal)

  35. Differential rotation causes field lines to be wrapped around the Sun Sunspots migrate to equator where they cancel out and eventually reverse the overall field

  36. The Solar Corona Visible

  37. The Solar Corona A corona is a type of plasma "atmosphere" of the Sun. It extends millions of kilometers into space Most easily seen during a total solar eclipse, but also observable in a coronagraph. Visible

  38. The Solar Corona A corona is a type of plasma "atmosphere" of the Sun. It extends millions of kilometers into space Most easily seen during a total solar eclipse, but also observable in a coronagraph. Visible

  39. Light from the corona comes from three primary sources (called K, F and E), which are called by different names although all of them share the same volume of space.

  40. The K-corona (K for kontinuierlich, "continuous" in German). Created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectrum the appearance of a continuum with no absorption lines.

  41. The F-corona (F for Fraunhofer). Created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the Zodiacal light.

  42. The E-corona (E for emission). Result from spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.

  43. The Solar Neighbourhood Transparency

  44. circles at 5, 10, 15 ly Transparency

  45. Measuring Stars Temperature T Distance d Luminosity L Radius R Mass M Element Abundances

  46. Measuring Stars • Distances (d) • needed for finding L • Stellar parallax (Hipparchos satellite yielded a revolutionary improvement) • Proper motion studies • Moving clusters - stars seem to get closer as the cluster recedes. • Comparison with standard stars of known distance

  47. Measuring the Stars. First a Reminder: