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Astronomy: The Solar System and Beyond 5th edition

Astronomy: The Solar System and Beyond 5th edition. Michael Seeds. Chapter 11. All cannot live on the piazza, but everyone may enjoy the sun. Italian proverb. A wit once remarked that astronomers would know a lot more about the sun if it were farther away.

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Astronomy: The Solar System and Beyond 5th edition

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  1. Astronomy:The Solar System and Beyond 5th edition Michael Seeds

  2. Chapter 11 All cannot live on the piazza, but everyone may enjoy the sun. • Italian proverb

  3. A wit once remarked that astronomers would know a lot more about the sun if it were farther away. • This comment contains a grain of truth. • Astronomers know plenty about stars in general, and there are plenty of stars out there for them to observe, in order to test and refine their theories. • However, there is only one star nearby—close enough for astronomers to see swirling currents of gas and arching bridges of magnetic force that their present theories seem inadequate to completely describe.

  4. The more closely the sun is observed, the more complex it seems, and the less astronomers seem to know about it. • It pays to look at this paradox from another direction. • The sun is an average star, and there are billions like it in the sky. • The more astronomers can learn from the convenient example of the sun, the more they will know about other stars, whose distances from Earth render detailed study impossible.

  5. What do astronomers know about the sun? • It is made up of the gases hydrogen and helium, along with a scattering of heavier elements. • It is 109 times Earth’s diameter and 333,000 times its mass.

  6. Nuclear reactions produce a tremendous amount of energy in the interior of the sun. • As this energy escapes to space, it stirs the sun’s layers by generating convection currents. • The sun has a powerful and ever-changing magnetic field that causes dark sunspots and eruptions to appear on its surface.

  7. The Solar Atmosphere • Your grand tour of the sun begins with its atmosphere. • Its atmosphere is made up of three layers. • The photosphere is the thinnest, innermost layer. • The chromosphere lies above the photosphere and is a few times thicker.

  8. The Solar Atmosphere • The corona, the highest layer, is huge and extends very far from the sun.

  9. The Solar Atmosphere • As you study the solar atmosphere, you will find that many phenomena are driven by the outward flow of heat energy from the interior of the sun. • Like a pot of boiling soup on a hot stove, the sun’s surface is in constant activity as heat flows up from below.

  10. The Photosphere • The photosphere is the visible surface of the sun. • It is the source of most of the sun’s light. • However, it is very thin—less than 500 km deep.

  11. The Photosphere • To put its thinness in perspective, imagine a model of the sun the size of a bowling ball. • The photosphere would be no thicker than a piece of tissue paper wrapped around the ball. • Due to the relative thinness of the photosphere, astronomers often refer to it as a surface, or ‘the surface of the sun.’

  12. The Photosphere • Thin as it is, the photosphere emits a great deal of energy. • The temperature at the sun’s surface is about 5,800 K. • At that temperature, each square centimeter must radiate more energy than a 6,000-watt lightbulb. • With all that energy radiating away into space, the sun’s surface would cool rapidly if energy did not flow up from the interior to keep the surface hot.

  13. The Photosphere • The photosphere is dense enough to emit plenty of light but not so dense that the light can’t escape. • Below the photosphere, the gas is denser and hotter and therefore radiates plenty of light. • However, that light cannot escape from the sun because of the outer layers of gas. • So, you cannot detect light from these deeper layers. • Above the photosphere, the gas is less dense and so is unable to radiate or absorb much light.

  14. The Photosphere • Although the photosphere appears to be substantial, it is really a very low-density gas. • Even in the deepest and densest layers visible, the photosphere is 3,400 times less dense than the air you breathe. • To find gases as dense as the air you breathe, you would have to descend about 7 x 104 km below the photosphere—about 10 percent of the way to the sun’s center. • With a fantastically efficient insulation system, you could fly a spaceship right through the photosphere.

  15. The Photosphere • The spectrum of the sun is an absorption spectrum. • That can tell you a great deal about the composition of the photosphere.

  16. The Photosphere • You know from Kirchhoff’s third law that an absorption spectrum is produced when a source of a continuous spectrum is viewed through a gas.

  17. The Photosphere • The deeper layers of the photosphere are dense enough to produce a continuous spectrum. • However, atoms in the photosphere absorb photons of specific wavelengths, producing absorption lines of hydrogen, helium, and other heavier elements.

  18. The Photosphere • In good photographs, the photosphere has a mottled appearance because it is made up of dark-edged regions. • The regions are called granules. • The visual pattern they produce is called granulation.

  19. The Photosphere • A typical granule is about the size of Texas, and exists for only 10 to 20 minutes before fading away. • Faded granules are continuously replaced by new granules.

  20. The Photosphere • Spectra of these granules show that the centers are a few hundred degrees hotter than the edges. • Doppler shifts reveal that the centers are rising and the edges are sinking at speeds of about 0.4 km/s. • From this evidence, astronomers recognize granulation as the upper surface of a convecting layer just below the photosphere.

  21. The Photosphere • Most of the convection occurs below the photosphere, but the tops of the cells extend up into its lower reaches. • Convection occurs when hot fluid rises and cool fluid sinks. • For example, a convection current of hot gas rises above a candle flame.

  22. The Photosphere • You can observe convection in a liquid by adding a bit of cool nondairy creamer to an unstirred cup of hot coffee. • As the coffee in the cup convects, the creamer colors some of the currents, so that you are able to see them. • Convection cells form in hot coffee as the surface of the drink cools: the cooler coffee sinks as warmer coffee rises from the bottom to replace it.

  23. The Photosphere • This process is invisible until you pour in the creamer. • When you do, you may see small regions on the surface of the coffee that mark the tops of convection currents. • Viewed from above, these regions look much like solar granules.

  24. The Photosphere • In the sun, the tops of rising currents of hot gas are brighter than their surroundings. • As the gas cools slightly, it is pushed aside by rising gas from below. • The cooler gas, sinking at the edge of the granule, is slightly dimmer. • Consequently, granules have bright centers and dimmer edges.

  25. The Photosphere • Less obvious structures called supergranules appear to be caused by larger, deeper convection currents in the sun’s layers. • These features can be over twice the size of Earth. • The presence of granulation, and the convection that it implies, is clear evidence that energy is flowing upward through the photosphere.

  26. The Chromosphere • Above the photosphere lies the chromosphere. • Solar astronomers define the lower edge of the chromosphere as lying just above the visible surface of the sun, its upper regions blending gradually with the corona. • You can think of the chromosphere as being an irregular layer with an average depth of less than Earth’s diameter.

  27. The Chromosphere • As the chromosphere is roughly 1,000 times fainter than the photosphere, you can see it with your unaided eyes only during a total solar eclipse—when the moon covers the brilliant photosphere. • Then, the chromosphere flashes into view as a thin line of pink just above the photosphere.

  28. The Chromosphere • The pink color is produced by the combined light of the red, blue, and violet Balmer emission lines of hydrogen. • The word chromosphere comes from the Greek word chroma, meaning ‘color.’

  29. The Chromosphere • As the chromosphere is roughly 1,000 times fainter than the photosphere, you can see it with your unaided eyes only during a total solar eclipse—when the moon covers the brilliant photosphere. • Then, the chromosphere flashes into view as a thin line of pink just above the photosphere.

  30. The Chromosphere • Astronomers know a great deal about the chromosphere from its spectrum. • As the chromosphere produces an emission spectrum, Kirchhoff ’s second law tells you the chromosphere must be an excited, low-density gas. • The density is about 108 times less dense than the air you breathe.

  31. The Chromosphere • Spectra reveal that atoms in the lower layers of the chromosphere are ionized and atoms in the higher layers are even more highly ionized. • That is, they have lost more electrons. • From the ionization state of the gas, astronomers can find the temperature in different parts of the chromosphere.

  32. The Chromosphere • Just above the photosphere, the temperature falls to a minimum of about 4,500 K and then rises rapidly to the extremely high temperatures of the corona.

  33. The Chromosphere • This fact is a little surprising. • You would expect the outer layers of the sun to get progressively cooler. • You will learn later what makes the chromosphere and corona hotter than the photosphere.

  34. The Chromosphere • Solar astronomers can take advantage of some elegant physics to study the chromosphere. • The gases of the chromosphere are transparent to nearly all visible light. • However, atoms in the gas are very good at absorbing photons of specific wavelengths. • This produces certain dark lines in the absorption spectrum of the photosphere.

  35. The Chromosphere • A photon having one of those wavelengths that is emitted in a deeper layer of the sun is very unlikely to escape through the chromosphere without being absorbed. • If a photon of one of these particular wavelengths reaches Earth, you can be sure that it came from higher in the atmosphere.

  36. The Chromosphere • A filtergram is a photograph made using light in one of those dark absorption lines. • Filtergrams reveal detail in the upper layers of the chromosphere. • A filtergram made at the wavelength of the Hα Balmer line is displayed. • It reveals complex structure in the chromosphere.

  37. The Chromosphere • Spicules are flamelike jets of gas rising upward into the corona that last 5 to 15 minutes. • They appear to be cooler gas from the lower chromosphere extending upward into hotter regions.

  38. The Chromosphere • Seen at the limb (edge) of the sun’s disk, spicules blend together to look like flames covering a burning prairie. • Filtergrams of spicules located at the center of the solar disk show that they spring up around the edges of supergranules.

  39. The Chromosphere • Spectroscopic analysis of the chromosphere reveals that it is a low-density gas in constant motion, where the temperature increases rapidly with height. • Just above the chromosphere lies even hotter gas.

  40. The Solar Corona • The outermost part of the sun’s atmosphere is called the corona, after the Greek word for ‘crown.’ • The corona is so dim that it is not visible in the daytime sky because of glare from the brilliant photosphere.

  41. The Solar Corona • During a total solar eclipse, the moon covers the photosphere and the corona shines with a pearly glow not quite as bright as the full moon. • Special telescopes on earth, and in space, can block the light from the photosphere and image the corona out beyond 20 solar radii, almost 10 percent of the way to Earth.

  42. The Solar Corona • The images show streamers in the corona that follow the lines of force of the sun’s magnetic field.

  43. The Solar Corona • The spectrum of the corona can tell you a great deal about the coronal gases and simultaneously illustrate how astronomers can analyze a spectrum. • Some of the light from the outer corona produces a spectrum with absorption lines the same as the photosphere’s spectrum. • This light is just sunlight reflected from dust particles in the corona.

  44. The Solar Corona • In contrast, some of the light from the corona produces a continuous spectrum that lacks absorption lines. • That happens when sunlight from the photosphere is scattered off free electrons in the ionized coronal gas. • As the coronal gas has a temperature over 1 million K, the electrons travel very fast. • The reflected photons suffer large, random Doppler shifts that smear out solar absorption lines to produce a continuous spectrum.

  45. The Solar Corona • Superimposed on the corona’s continuous spectrum are emission lines of highly ionized gases. • In the lower corona, the atoms are not as highly ionized as at higher altitudes. • This tells you that the temperature of the corona rises with altitude.

  46. The Solar Corona • Just above the chromosphere, the temperature is about 500,000 K. • In the outer corona, it can be as high as 2 million K or more.

  47. The Solar Corona • The corona is very hot, but it is not very bright. • Its density is very low, only 106 atoms/cm3 in its lower regions. • That is about a trillion times less dense than the air you breathe. • In its outer layers, the corona contains only 1 to 10 atoms/cm3, less dense than the best vacuum on Earth. • Due to this low density, the hot gas does not emit much radiation.

  48. The Solar Corona • Astronomers have wondered for years how the corona and chromosphere can be so hot. • Heat flows from hot regions to cool regions. • So, how can the heat from the photosphere—with a temperature of only 5,800 K—flow out into the much hotter chromosphere and corona?

  49. The Solar Corona • Observations made by the Solar and Heliospheric Observatory (SOHO) satellite have mapped a magnetic carpet of looped magnetic fields extending up through the photosphere. • Unseen turbulence below the surface may be whipping these fields about and churning the low density gases of the chromosphere and corona, heating the gas. • In this instance, energy appears to flow outward as the agitation of the magnetic fields.

  50. The Solar Corona • Gas follows the magnetic fields pointing outward and flows away from the sun in a breeze of ionized atoms called the solar wind. • Like an extension of the corona, the low-density gases of the solar wind blow past Earth at 300 to 800 km/s, with gusts as high as 1,000 km/s. • Earth is bathed in the corona’s hot breath.

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