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The Sun Surface of Sun the Chromosphere of the Sun The The Sun The Surface of the Sun • The Sun is huge compared to the Earth • Visible surface is – 110 times bigger in diameter photosphere – 300,000 times as massive • Temp range 6500K at – H & He (<2% of mass in heavier base to 4400K at top of elements photosphere – Slightly denser than water 1400 kg/m3 – Teffective ~ 5700K – Contains 99.999% of the mass in the • Granular structure - gas solar system motion carry’s energy to – Emits 3.9x1026 Watts - State of surface Kansas absorbs 10x world energy usage • Dark cool regions are • G 6 Main Sequence Star called sunspots Surface of Sun The Chromosphere of the Sun • Outside photosphere • Temperature starts to rise – 4,400K to 25,000K – Emission lines • Supergranulation • Pierced by gas jets called Spicules The Corona of the Sun • Corona extends far out – Visible only during eclipses – Very tenuous • Much hotter – 500,000 to 2,000,000 K – Emits mostly in X-rays • How does it get so hot? – Puzzle - Heat only flows from From C. L. Selhorst1 - A. V. R. hot to cold Silva2 - J. E. R. Costa1 – Clue - hottest near sunspots A&A 433, 365-374 (2005) 1 S Solar Activity U N • Sunspots have an 11 year cycle S – Number P of spots changes O – Position T of spots S changes Solar Activity Sunspot Formation • Sunspots are regions where the solar magnetic field breaks through surface • Occur in pairs - one is a north pole, the other a south pole • Polarity switches every 11 years http://sohowww.nascom.nasa.gov/gallery/Movies Sunspot Cycle • Cycle could be due to twisting of magnetic field – Equator rotates much faster than pole – Fields twist up like rubber bands – When fields cross they connect – Field flips every 11 years http://sohowww.nascom.nasa.gov/gallery/Movies 2 Sunspots and the Earth The Maunder Minimum • From 1650-1700 almost no sunspots • The amount of energy occurred the Sun releases is not • Known as “Little Ice Age in Europe” perfectly constant – Thames river in London froze 11 times • The number of – Ice seen off the coast of England sunspots may be related to the amount • Another similar minimum may have of energy the Sun occurred in the 1400s emits – More spots = more energy – 1% more UV at solar Max – Not understood why Solar Activity • Flares and prominences erupt from disturbed regions near sunspots. • Sun's magnetic field can hold them in place for Skating in the Town Moat of Brussels - Robert van den days. Hoecke (1649 during the Maunder Minimum) Solar Activity Solar Activity Images from TRACE Satellite TRACE Satellite 3 Solar Activity Solar Activity SOHO SOHO Solar Activity Solar Wind • stream of particles from • Xray the sun's surface emission • coronal mass ejections • Yokoh send large numbers of Satellite particles toward the earth • can cause dramatic aurorae, disrupt radio communications • (From SOHO, LASCO) Solar Wind The Aurora • These particles are trapped -14 -1 • 3x10 M yr in the earth’s magnetic field • Highly ionized • The field channels them Comet Hyakutake near the pole • When they hit the Comet Hale-Bopp Comet West atmosphere it causes the air to glow 4 The Structure of the Stars The Aurora • Stars arrange themselves to balance the forces within them – Gravity tries to pull all matter together – Pressure differential of the gas – Pressure of escaping radiation • Not enough pressure – Star will contract until pressure increases to balance gravity • Too much luminosity – Star will expand until pressure drops Pressure & Matter Interior Structure of the Sun • The pressure of a gas is due to motion of gas atoms • Pressure increases as you – Higher temperature - faster speeds - higher pressure move deeper into the sun – Higher density - more atoms - higher pressure – Must hold up all the overlaying • Compressing a gas material increases pressure – Density and temperature • Heating a gas increase increases pressure • Only in core is it hot and dense enough for nuclear fusion – All energy is generated in core Energy Escaping From the Sun Random Walking • Energy always flows from Hot to Cold • Even in the transparent interior of the sun a photon – Heat flows from core into space only travels about 1 cm before it is absorbed • Two Major ways to transport energy • A photon is absorbed and remitted billions of times out before it wanders out of the sun – Photons (Radiation Transport) • Deep interior relatively transparent to – Re-emitted in random directions each time light • It takes about 1 million years for a photon to work its • Photons can carry the energy out ways out – Currents of hot gas (Convection) • Outer parts are opaque to light • Gas motion must move the energy out 5 Question Announcements • In the Star Trek movie “Generations”, • No Class next Tuesday The evil Dr. Zorron launches a missile • Projects should be in into a star. The missile stops fusion • Read Chapter 9 inside the star so it blows up during a – Completed reading Chapters 1- 6 life and death battle between Zorron, – Skip Chapters 7 & 8 Kirk and Picard. How long would they need to be fighting before they would • Homework Due next Thursday notice fusion had stopped in the star? Where Does the Sun Get Its Energy? Nuclear Fusion • Several light nuclei (H) collide and • Life time is related to source of energy combine to form a heavier – life time = (energy available) / (luminosity) nucleus (He) • Gravitational Contraction? • Energy is released – Contraction due to gravity releases energy • Need high temperature and • Total energy due to contraction 1049 ergs from Newton’s Laws densities – life time = (1049 ergs)/(4x1033 erg/sec) = 108 years – Must overcome electrostatic • Radioactive decay, similar to heating of Earth? repulsion of like charged atomic nuclei – Mostly H and He which do not decay, need heavy nuclei – Even if solid uranium it would produce 1/2 of energy seen – Core of Sun is 15,000,000 K – Density is 150,000 kg/m3 Where Does the Energy Come From? Energy From Fusion • Mass is converted directly into energy E=mc2 • Converting Mass directly into energy is – Einstein’s Famous relationship the most efficient way to get energy – Mass and energy are the same thing! – The speed of light is very big so you get a lot of from matter energy from a little bit of matter • If 10% of solar Hydrogen is converted • 4 H → 1 He – Mass of H = 1.67252x10-24 gm into He the Sun will shine at its current – Mass of He = 6.64258x10-24 gm rate for 10 Billion Years – 4xH - He = 0.04750x10-24 gm – E = mc2 = 4.75x10-26 gm (3x1010cm/sec)2 = 4.5x10-5 ergs 6 Question The Proton-Proton Chain • Remember that the luminosity of a star was found be to closely related to its mass (L∝M4). Now we know that stars get their energy by converting their mass directly into energy so the total amount of energy a star has is proportional to its mass (E∝M). Will a massive star live a longer or shorter time than a low mass star? Solar Neutrinos Solar Neutrinos • Neutrinos • 3 different neutrino – Very low mass particles produced as a side product of nuclear fusion experiments are sensitive – They hardly interact with matter so to neutrinos from different they can travel completely out of the nuclear reactions Sun undisturbed • The measured points do • Detection of Neutrinos not agree very well with the – Difficult since they interact so weakly with matter predicted number of each – Takes very large detectors type of neutrino – Several have been built to detect • There is a problem with the different types of neutrinos from inside Standard Model the sun The Main Sequence Revisited • Like the Sun all stars arrange themselves to balance the force of gravity and their interior pressure – As mass increases gravity increases • Pressure increases inside stars • Energy generation + luminosity increases • Temperature + size increase to let increased energy out • This equilibrium sequence of mass is the Main Sequence 7.
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