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The Sun SESAME Astronomy Week 4 Our star • Not special: typical mass, typical temperature, typical size, typical planetary system • about halfway through its 10 billion year lifespan Vital statistics • Radius: 7 x 105 km = R⊙ • Mass: 2 x 1030 kg = 1 M⊙ • 109 R 332,000 M Luminosity: 3 x 1026 Watts = 1 L⊙ • Temperature: 5800* K (surface) • “day” 25 earth days at equator, 33 earth days at poles • surface gravity: 28 times stronger than Earth’s * Depending on which source you choose, you will find a range of values from 5500-6000 a little info about light light is a wave blackbody radiation • a hot object glows • the hotter the object, the brighter it glows • the hotter the object, the bluer it glows Structure • core - where the fusion takes place • not really differentiated like the planets (although core is slowly going from mostly Hydrogen to mostly Helium) fusion proton-proton chain (pp chain) • 2 protons get close • if one happens to turn into a neutron at the moment they are close, the neutron and other proton stick together forming Deuterium (when proton turns into neutron, it emits a positron) • Deuterium gets hit by another proton forming Helium-3 and emitting energy • 2 Helium-3 nuclei collide forming a Helium-4 nucleus and emitting 2 protons convection • heat from bottom makes material just above it expand and rise • this material leaves empty space that is filled from the sides • this material leaves empty space that is filled from above • heated material cools when it reaches the top • previously heated material is pushed aside by rising newly heated material from below • previously heated, now cooled material that has been pushed aside sinks back to bottom • and the whole thing starts again... convection • it happens in the sun • result is called “granules” Sun’s magnetic field • rotating faster at equator than at poles • magnetic field gets pulled along with charged particles • field gets tangled up and pops out in places prominence - where the magnetic field sticks out of surface of sun charged particles spiral around field lines flare - when the field lines break • particles that had been trapped are released into space at high speeds • sometimes flare is in direction of Earth Sunspots • magnetic field prevents hot gas from rising to surface where it sticks out of surface • places where hot gas can’t rise to surface cool off a little (still VERY hot) • sunspots only look dark compared to even brighter surface of sun • always come in pairs (why?) sunspots sunspot cycle • know WHAT happens but not WHY • every 11 years@ or so magnetic field of sun flips • just after a flip, few sunspots, sunspots at middle latitudes • at solar maximum, lots of spots and close to equator, lots of prominences and flares* • then field flips and sunspots few and at middle latitudes * we’ll talk about flares on a later slide @ranges from 7-15, average is 11 years The sunspot cycle solar maximum • more sunspots • more prominences • more flares solar minimum • fewer sunspots • fewer prominences • fewer flares • number of sunspots varies from cycle to cycle How does the energy get out? • random walk - photons (the packets of energy released during fusion) bounce around bumping into atoms and nuclei for 30,000-1,000,000* years until they finally wander out * depends on who you ask and what assumptions he/she made Summary • • • • the sun is an average star the sun is about halfway through its life • Sun converts Hydrogen to Helium in its core (and only in its core) via the proton-proton chain (fusion) light takes up to a million years to make its way from the core to the surface fun fact: Helium was first discovered in the sun magnetic field: • • • light is a wave objects glow when they’re hot, and how bright and what color depends on temperature: hotter = brighter and bluer, cooler = dimmer and redder ✴sun’s magnetic field gets tangled up and flips every 11 or so years ✴magnetic field sticks out of surface (in prominences) and causes prominences and cooler places called sunspots ✦ flares happen when field lines in a prominence breaks