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

PTYS/ASTR 206Sun 3/1/07 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.

PTYS/ASTR 206Sun 3/1/07 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 –1.55x10 7 K Temperature in the corona –2 x 10 6 K

PTYS/ASTR 206Sun 3/1/07 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/m 3 Composition (by mass) –74% Hydrogen, 25% Helium, 1% other elements Sun seen in Xrays

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 SOLAR CORONA – SEEN DURING A TOTAL ECLIPSE

PTYS/ASTR 206Sun 3/1/07 The Sun’s Appearance from Space Solar Corona As seen in x-rays Solar Corona As with a coronagraph

PTYS/ASTR 206Sun 3/1/07 The Sun’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 Sun’s core Fusion should not be confused with fission ! Will continue to heat the Sun for another 5 billion years

PTYS/ASTR 206Sun 3/1/07 The Proton-Proton Chain

PTYS/ASTR 206Sun 3/1/07 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.

PTYS/ASTR 206Sun 3/1/07 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!

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 These are convection cells that are about 1000 km wide They are part of the Sun’s atmosphere known as the Photosphere Solar Granulation

PTYS/ASTR 206Sun 3/1/07 High-resolution images of granulation and a sunspot

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07

PTYS/ASTR 206Sun 3/1/07 Prominences are the same as filaments, except that they are seen from the side rather than “straight on”

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 X-Ray image of the Sun

PTYS/ASTR 206Sun 3/1/07 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

PTYS/ASTR 206Sun 3/1/07 Zeeman Effect: The splitting of spectral lines by a magnetic field

PTYS/ASTR 206Sun 3/1/07 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”

PTYS/ASTR 206Sun 3/1/07 These changes are caused by convection and the Sun’s differential rotation: The Solar Dynamo

PTYS/ASTR 206Sun 3/1/07 Next Class: Solar Variability and its Effect on Earth and its Inhabitants