PTYS/ASTR 206. The Sun 3/1/07

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1 The

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

3 The : 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 1.55x10 7 K Temperature in the corona 2 x 10 6 K

4 The 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/m 3 Composition (by mass) 74% Hydrogen, 25% Helium, 1% other elements seen in Xrays

5 The s appearance from the ground Using a Baader solar filter Can see the sun s visible surface, or photosphere This is how the 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

6 The s appearance from the ground Hα filters Observes the at a particular wavelength The a line at 6563 angstroms (Å) which is the strongest of the 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 seen with an Hα filter

7 SOLAR CORONA SEEN DURING A TOTAL ECLIPSE

8 The s Appearance from Space Solar Corona As seen in x-rays Solar Corona As with a coronagraph

9 The s Energy Source is Thermonuclear Fusion 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 = mc 2 Thermonuclear fusion occurs only at the very high temperatures at the s core Fusion should not be confused with fission! Will continue to heat the for another 5 billion years

10 The Proton-Proton Chain

11 Detecting Solar Neutrinos Solar Neutrinos are the only direct probe of the s interior that we have hence, they are a valuable tool for understanding the 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.

12 What is the 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 spots (in the photosphere) are cool enough that simple molecules can form But other than this small region, the is almost entirely made of plasma!

13 Hydrogen fusion takes place in a core extending from the s center to about 0.25 solar radius The Structure of the s interior 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

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

15 Solar Granulation These are convection cells that are about 1000 km wide They are part of the s atmosphere known as the Photosphere

16 High-resolution images of granulation and a sunspot

17 Solar Oscillations Waves can propagate through the 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 The branch of science that studies solar oscillations is known as Helioseismology

18 The photosphere is the lowest of three main layers in the s atmosphere The 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, the photosphere, is the lowest layer in the solar atmosphere

19 Limb Darkening The edges of the 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

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

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

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23 Prominences are the same as filaments, except that they are seen from the side rather than straight on

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 Because of the high temperatures, the corona is best viewed in the X-ray part of the spectrum

25 X-Ray image of the

26 spots 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

27 Zeeman Effect: The splitting of spectral lines by a magnetic field

28 The 11-year spot Cycle Number of spots versus time they come and go every 11 years Number of spots versus latitude forms a butterfly pattern

29 These changes are caused by convection and the s differential rotation: The Solar Dynamo

30 Next Class: Solar Variability and its Effect on Earth and its Inhabitants