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Exploring the Sun: Our Star's Structure and Energy

Delve into the fascinating world of the Sun, discovering its immense size, structure, and energy generation processes. Uncover the solar cycle, core temperatures, and the Sun's radiant energy output. Explore the layers of the Sun, from the photosphere to the corona, and learn about convective currents and energy generation through nuclear fusion in the core. Engage with the complexities of the Sun's energy-producing mechanisms and the sheer power it generates, shaping our solar system and sustaining life on Earth.

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Exploring the Sun: Our Star's Structure and Energy

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  1. Introduction to Astronomy • Announcements

  2. The Sun, Our Star Size & Structure Energy Generation Magnetic Activity The Solar Cycle

  3. Size & Structure Basic properties of the Sun…

  4. DSun ~ 100 Dearth • A million Earths could fit inside the Sun • MSun ~ 300,000 MEarth • I weigh 225 lbs on Earth. • On the Sun, I would weigh ~ 3 tons! • Distance to Earth = 150 million km = 93 million miles = 1 AU • Driving at 75 miles/hour, you would reach the Sun in 141½ years!

  5. Surface Temperature ~ 6000 K • Measured from color & Wien’s Law • Core Temperature ~ 15 million K ~ 27 million °F • from modeling/calculations & indirect measurements

  6. Composition • 71% Hydrogen • 27% Helium • 2% vaporized heavy elements • Oxygen, Silicon, Calcium, Iron, Strontium, Titanium, etc… • Radiant Energy ~ 4 x 1026 Watts • Compare this to your 60 Watt household light bulbs

  7. Solar Structure • Photosphere (top-most, only directly visible layer) • Convective Zone (pot of boiling water) • Radiative Zone (direct energy radiation) • Core (powerhouse)

  8. Core • Fusion here to produce energy • More on this in a minute…

  9. Radiative Zone • Energy (photons of light) produced in core must travel outward • In this zone, energy moved by direct radiation • Campfire analogy: energy from fire travels outward by RADIATION • You can feel the heat from the fire w/o putting your hands directly into the flames

  10. Incredibly dense, so photons can travel only ~ 1 inch before they are absorbed by atoms in the gas • Re-radiated, but then can only travel another inch before re-absorbed • “Random Walk” • Photons re-radiated in random directions • These photons take an average of 16 million years to reach the surface! “Today’s sunshine was born in the Sun’s core before we even existed as a species!”

  11. The distance traveled is much greater than the net displacement

  12. Convective Zone • Further from core, so gas is much cooler… • Lower temps mean gas atoms can “hang on” to more of their electrons • This causes much more absorption at the base • Therefore, heat piles up at the base, starts the gas “boiling”

  13. Photosphere Convective Zone Radiative Zone Core Not to scale

  14. “convective currents” • Hot gas near base is less dense, so it starts to float to surface • Gets to surface, cools down, becomes more dense, sinks back below • Continue ad infinitum… • Granulation • Convection cells • Move at ~ 1 km/sec Cooler here Hotter here

  15. Photosphere • Name = “light sphere” (visible surface) • Has density similar to liquid water • Below this surface, gas density increases rapidly • The Sun is gaseous throughout • No solid core • Temperatures too high to form molecular bonds required to have solids and/or liquids • Despite incredibly huge densities

  16. Solar atmosphere • Everything above the visible surface (photosphere) • Very low density gases • Chromosphere • Corona

  17. Chromosphere • Thin layer • Typical temperature ~ 25,000 K • Name = “color sphere” • Most emission is red light from Hydrogen • Spicules • Vertical jets of hot gas • Formed from shock waves rising through photosphere (from convection)

  18. Mostly vertical… Due to many overlapping spherical shock waves

  19. Corona • Typical temperature 1 – 10 million K • Such low density, we look right through it without noticing it • Only visible during solar eclipse • Or with special coronagraphic telescopes • Coronal Streamers (X-Ray) • Coronal Holes (source of solar wind)

  20. chromosphere Streamers Corona in visible light

  21. Coronal Holes Corona in X-Rays

  22. Energy Generation What makes the Sun shine? How?

  23. Hydrostatic Equilibrium • Outward pressure balances inward force of gravity • Ex: tire pressure • Air pressure in tire balances weight of car • Let air out = no pressure = flat tire = weight of car not balanced = car sinks down • Ex: floating • Buoyancy forces balance your weight in pool

  24. In a gas: • Pressure increase comes from squeezing atoms closer together...(density increase) • …or making them move faster…(temperature increase)

  25. Increase density • More atoms in a given volume = more frequent collisions • Increase temperature • Atoms move faster = harder, more frequent collisions • Therefore, PRESSURE = DENSITY x TEMPERATURE x A CONSTANT

  26. So to balance Sun’s enormous gravity, need high densities and high temperatures in the interior…

  27. Powering the Sun • Early theories 1. Sun made of coal • if true, Sun would only shine for few thousand years 2. Sun NOT in H.E., but gravitationally compressing & heating gases • If true, Sun would only shine for 10 million years • If true, Sun would be shrinking enough to observe over the last 100 years or so…

  28. Bogus. • New theory (late 1920s – early 1930s) • Nuclear fusion in Sun’s core (heavy elements could be forged by smashing light elements together) • E = mc2 • 1 gram of matter “contains” energy equal to that released by small nuclear device!

  29. In Sun’s core, have incredibly high temperatures • Atoms move incredibly fast • Normally, 2 protons moving toward each other would repel • Recall opposite charges attract, like charges repel • But, if moving fast enough, protons (nuclei) are driven extremely close together • Strong force overcomes electrical repulsion • Protons bound together to form heavy nucleus

  30. “Collision” of like-charged particles

  31. Proton-Proton Chain (PP Chain) • 1H + 1H 2H + e+ + neutrino + extra energy • 1H + 2H  3He + photon + extra energy • 3He + 3He  4He + 1H + 1H + extra energy • Note that a little bit of energy is liberated at each step of this process… • Note also that the final step regenerates the reactants needed for the initial step!

  32. 4 protons (1H) needed to produce each 4He • Mass difference: • 4 x mass of proton > mass of 4He • “extra” mass converted to extra energy (E=mc2)

  33. 0.048 x 10-27 kg = 4.3 x 10-12 J of energy • Pretty small, but… • ~ 30 MeV (1/40 eV = room temperature) • Many, many, many of these reactions happen every second! • Total rate equivalent to 100 billion 1-megaton nuclear devices exploding per second! • Hiroshima Bomb ~ 15 kilotons = 0.015 megatons

  34. The PP-Chain is the dominant fusion reaction in stars that are roughly the same size/age as our sun… • Other reactions are possible (triple-α process, CNO-cycle) • These occur mostly in heavier, older stars

  35. Introduction to Astronomy • Announcements • HW #5 due monday

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