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The Sun - Our StarHow do we know what we know?Slide 3Slide 4Solar StructureSlide 6Solar GranulesSlide 8Hydrostatic EquilibriumThe Sun’s Energy SourceEinstein’s Mass-Energy RelationNuclear FusionSlide 13Studying the Solar InteriorSunspotsSolar Magnetic ActivityProminences & FlaresSlide 18Slide 19The Corona and Solar WindSlide 21Slide 22The Solar CycleMaunder's Sunspot DiagramSlide 25Slide 26Solar Cycle and ClimateSlide 28Recent Solar ObservatoriesSolar System by Voyager 1Extent of Solar WindBow Shock Around Star R Hydrae1The Sun - Our Star•Sun’s diameter 100 times the Earth’s•Sun’s mass is 700 times the mass of all the planets put together•The energy source of the Sun is the conversion of hydrogen atoms into helium atoms through nuclear fusion in the core of the Sun. Without this energy source the Sun would collapse.•Composition of the Sun is 71% Hydrogen, 27% Helium, 2% heavier elements2How do we know what we know?•Our understanding of the Sun is a combination of measurements of observed quantities (diameter, surface temperature, spectra) and computer models based on physical laws (internal temperature and density).•Models are correct so far as they can predict the observed properties of the Sun.3Solar Seismology•Solar seismology is the study of the Sun’s interior by analyzing wave motions on the Sun’s surface and atmosphere•P-mode (compression) waves only•The wave motion can be detected by the Doppler shift of the moving material–absorption line shifts in the photosphere are driven by processes in the convective region below the surface•The detected wave motion gives temperature and density profiles deep in the Sun’s interior•These profiles agree very well with current models4Solar Seismology5Solar Structure•The Sun is completely gaseous.•The atoms in this gas strongly absorb light. •Because of this absorption energy travels very slowly through the Sun. It takes about 100,000 years for energy to travel from the core to the solar surface.•Above a certain height however the gas is too thin to absorb light effectively. The point at which the gas becomes transparent is called the photosphere.•The photosphere is considered the “surface” of the Sun. Surface temperature about 6000 Kelvin.6Solar Structure•At the core of the Sun, where nuclear fusion occurs, temperatures are about 15 million degrees Kelvin. •As this energy propagates outwards it initially is carried by photons of light. This region of the Sun is called the radiative zone.•Near the photosphere this process of energy transfer becomes less efficient and energy begins moving outward through convective motions of the gas. As gas heats it rises to the surface. Once it reaches the surface it releases heat and sinks. This region is called the convective zone.7Solar Granules8Solar Granules•Caused by convection •Red regions show rising gas •Black regions show falling gas•White specks show regions of concentrated magnetic fields•White bar is 5000km long•Astronomy Picture of the day from 16 April 20109Hydrostatic Equilibrium•Requires that the pressure generated by the fusion reactions in the core of the Sun must be in exact balance with the weight of material falling inwards due to the Sun’s gravity.•Without this balance the Sun would either collapse (gravity wins) or explode (pressure wins).10•The Sun is not burning. If it were burning fuel like coal it would have exhausted its fuel long ago.•The slow collapse of the Sun was once thought to be the energy source but that wouldn’t have lasted more than a few million years.•It wasn’t until the 20th century that physicists understood the process behind what powers the Sun.The Sun’s Energy Source11Einstein’s Mass-Energy RelationAlbert Einstein (1879-1955)•In 1905 Albert Einstein recognized that mass and energy were related through the formula:E=mc2 (m =mass, E=energy, c=speed of light)•What this means is that a small amount of mass could be converted into an enormous amount of energy.•The means by which the Sun generates this energy is through nuclear fusion.12•Nuclear fusion involves combining two atomic nuclei together to form one, larger nucleus.•This can not occur under normal conditions because the electrical repulsion between protons prevents the nuclei from fusing.•At extremely high temperatures however this electrical repulsion is overcome and the nuclei fuse. Nuclear Fusion13Nuclear Fusion•The specific steps of nuclear fusion follow a process called the proton-proton chain•Through this process 2 neutrinos, 2 positrons, 2 1H and a 4He is created by the fusion of 6 1H.•The mass of all the particles created is less than the sum of the masses of colliding particles. This difference in mass was converted into energy through E=mc2.14Studying the Solar Interior•The neutrinos generated in the fusion process escape rapidly through the Sun.•These neutrinos are very difficult to detect but they can be detected. •This type of neutrino “telescope” can check whether our understanding of the solar interior is correct.•It seems that the number of neutrinos measured match computer models but only if– neutrinos have mass–neutrinos can oscillate among three different states or typesA neutrino detector in Japan.This is a giant tank of water buried deep underground. Tiny bursts of light due to neutrinos from the Sun are detected by light sensitive detectors on the walls of the tank.15SunspotsThis sunspot is twice the size of the Earth!•Sunspots are dark splotches on the photosphere of the Sun.•Sunspots are dark because they are cooler than their surroundings.•The reason they are cooler is because intense magnetic fields prevent hot, ionized gases from entering so the region cools forming a spot16Solar Magnetic Activity•Like several of the planets the Sun generates a magnetic field.•Unlike the planets the strength of the field strongly varies with time and with location on the Sun•The most easily detectable evidence for solar magnetic activity are the Sunspots17Prominences & Flares•Prominences are associated with pairs of sunspots are are arcs of magnetic field with glowing gas trapped inside.•Flares are explosive events where strong bursts of X-rays are emitted. These are due to rapid changes in the magnetic field.18The Sun’s Atmosphere•The extremely low-density gases that lie above the photosphere make up the Sun’s atmosphere19The


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PGCC PSC 1010 - Lecture Notes

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