Introduction to Astronomy

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!

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

8. 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

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

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

13. 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

14. 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!”

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

17. 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”

18. Photosphere Convective Zone Radiative Zone Core Not to scale

19. “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

21. 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

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

25. 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)

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

29. 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)

30. chromosphere Streamers Corona in visible light

31. Coronal Holes Corona in X-Rays

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

33. 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

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

36. 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

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

39. 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…

40. 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!

41. 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

42. “Collision” of like-charged particles

43. 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!

47. 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)

48. 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

49. 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

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