11ASTR 1020: Stars & GalaxiesJanuary 30, 2008• Reading: Chapter 14, section 14.2.• MasteringAstronomy Homework onLight and Spectroscopy is due Feb. 4th.•Planetarium on Jan. 31 – Elin Deeb presents “Cassini Mission Update”.• Guest lectures on Monday & Wednesday of next week.2Measuring velocities without a stopwatch: the Doppler Shift• Familiar shift in pitch of SOUND: higher when approaching, lower when receding• Similar shift in frequency of light: higher (blueshift) when approaching, lower (redshift) when recedingDoppler Movie3• Clicker Question: A brave student volunteer swings the “Doppler ball” in a circle directly over her head. What does she hear?a) A changing pitch, higher and lower, with each circular swingb) no change in pitchc) nothing at all4(c) Nothing at all!If the ball is swung directly over her head, the ball is never moving TOWARDS or AWAY from her, only tangentially around her. There should be no doppler change in pitch.Doppler effect is limited to only motions towards or away from the listener/viewer. No Doppler shift if object is moving tangentially (across or in a perfect circle around)5Today’s Class: The Sun• The Sun is a mass of incandescent gas• A gigantic nuclear furnace• Where hydrogen is built into helium• At temperatures of millions of degrees6Clicker Homework Question:Is the Sun hotter in its interior or at its surface?A) InteriorB) At the surfaceC) The Sun is the same temperature throughout27Clicker Homework Question:Is the Sun hotter in its interior or at its surface?A) InteriorB) At the surfaceC) The Sun is the same temperature throughout8Sun facts• Mass = 300,000 times Earth• Radius = 100 times Earth• 70% hydrogen by mass, 28% helium• Traces of other elements• Energy released is 1 second is 3,000,000 times yearly energy consumption of entire USA.9Structure of the Sun- from the core outwards• Core temp 15 million K,hot & dense from the gravitational weight of all that mass.• Hydrogen fuses into heliium, releasing energy (gamma rays). Much more on this next class!10• Energy generation increase pressure pushing outwards, balancing gravity.• Gravitational equilibrium:Outward force (pressure) balanced by inwards force (gravity).11• Radiation moves through the interior of the Sun = Radiation Zone.• Absorption and re-radiation in cooler layers converts gamma-rays to less energetic photons.12• Outer third: gas becomes turbulent.• Convection zone:hotter regions rise, cool, sink.• Energy continues to work its way out- 1 million years to get out.313• Convection: up and down movement of heating and cooling gas.• Granulation: appearance of convection patterns.• Darker areas are cooler, sinking on outside of pattern.• Videos of convection.14Convection on the Sun15• “Visible surface” of the Sun: photosphere.• T = only 5800 K.• Photons free to fly-seen at Earth 8 min later.• Thermal spectrum, T= 5800 K plus absorption from cooler gasses just on top.1617Solar spectrum (shown in tiers instead of one very long spectrum)18Clicker Question: The Sun suddenly stops burning hydrogen and loses its energy source. Which is true?A) The core will start to collapse. B) The Sun will appear fainter to us after 8 minutes.C) The core will become cooler.419• (A) only!• It will take about 1 million years+ 8 min for the radiation change to be seen at Earth.• The collapse of the Sun’s core will actually cause it to heat up, probably increasing fusion reactions again!20Magnetic Fields in the Sun• Magnetic fields entrain gas in huge bubbling loops• Cooler areas at “liftoff” cause dark “sunspots”21Movie22• Outer regions are hotter:• Chromosphere, • T= 10,000 K, Hydrogen alpha emission from thin gas (pink!)• Heated by energy twisting and spilling around magnetic field lines?23• Energy deposited by big flares and ejected gas?24Corona =outermost partsT = 1 million K =>X-rays!525• Solar eclipse by the Moon:chromosphereand corona become clearer in visible light without glare from the rest of the Sun26• Solar wind: particles (electrons, protons etc.) streaming into space at 500