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The Sun

Visual (photosphere). The Sun. UV (chromosphere). (1) Basic Properties. Rotates once per month. Big : R sun ~ 100 R earth Massive : M sun ~ 3 x 10 5 M earth Ave. Density = Mass/Volume < ρ sun > = 1.4 gm/cm 3 (~ ρ water )

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The Sun

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  1. Visual (photosphere) The Sun UV (chromosphere)

  2. (1) Basic Properties • Rotates once per month • Big : Rsun ~ 100 Rearth • Massive : Msun ~ 3 x 105 Mearth • Ave. Density = Mass/Volume • < ρsun > = 1.4 gm/cm3 (~ ρwater) • (cf. < ρearth> = 5.5 gm/cm3) • Hot : Tsurf ~ 5800 K (yellow) • Tcenter ~ 14 x 106 K (X-ray) • Bright : Lsun = 4 x 1026 Watt • (1 sec = world power for 106 years)

  3. Three regions to consider : • Interior • – core (energy production) • – radiative/convective zones • 2. “Surface” (0.1mm / basket ball) • – photosphere : 0 – 400 km • – chromosphere : 400 – 3000 km • 3. Extended region • – corona : 3000 – 106 km • – wind : 106 km  past Pluto

  4. (2) Photosphere • Region where light comes from : • deeper: hidden by opacity (H– ion) • higher: too thin to give much light •  ~400 km deep • Hot thin gas : <T> ~ 5600 K • < ρ > ~ 10-3 ρair ; < P > ~ 10-2 Pair • Temp decreases : 8000  4000 K •  absorption lines formed here •  limb darkening

  5. Granulation : surface convection – heat rising from below. Size ~ 1000 km Rise/fall speeds ~500 m/s Lifetime ~ 10 – 20 min (cooler) (hot)

  6. (3) Chromosphere Thin & hot  faint & emission lines Pink : Balmer lines strong (chromo- ) Helium : discovered from spectra Study using “filter-grams” (e.g. Hα)

  7. (4) Corona Very thin & very hot ρ ~ 10-13 ρair T ~ 106 K very ionized (e.g. Mg8+) heated magnetically (?) Solar disk

  8. (5) Solar Wind • Flows out past Pluto at ~400 km/s •  pushes back comet tails • p+ & e– ; only 10-14 Msun per year • @ Earth: ~ 5 particles/cm3 •  aurora

  9. Umbra Pen-umbra (6) Solar Activity Complex & violent “weather” Unlike Earth’s weather,  magnetic fields important. • (a) Sunspots: • strong mag. fields inhibit convection •  gas cools by ~1500 K  darker • often pairs: magnetic N/S poles. Zeeman splitting Spectrograph Slit

  10. (b) Sunspot Cycle Number & distribution of sunspots goes through cycle Looks like 11 year cycle Actually 22 years to return to same magnetic polarity

  11. Babcock theory (1960s) : • differential rotation amplifies • magnetic fields • Stronger fields are buoyant • Rise & break surface •  sunspot Differential rotation

  12. (c) Prominences & Flares • Energetic outbursts linked to activity/sunspots • Prominences : expanding magnetic arches condense cooler gas • Flares : N-S fields reconnect & zero-out • rapid release of energy e.g. 109 megatons •  X-ray burst •  coronal mass ejections  aurora prominence Flare & CME

  13. (7) Helioseismology • Sound (pressure) waves move thru the Sun • Doppler imaging of surface shows waves • Find many “tones”, most ~5 min periods • “sound” of the Sun  interior properties • rotation/temp/density as f(r) • T(r) & ρ(r) agree well with theory

  14. (8) Sun’s Energy Source Msun = 2 x 1030 kg possible “fuel” resource Burns at Lsun = 4 x 1026 Watt (= Joules per second) For fuel with ‘X’ J/kg, how long before used up ? No No Yes Chemical (eg oil…) : ~104 years Gravity (slow contraction) : ~108 years Nuclear (transformation) : ~1010 years Clarification of Sun’s energy linked to geological estimates of the age of the Earth & life. ~1880 – 1910 became clear.

  15. (9) Energy in atomic nuclei • Protons & neutrons can stick with a very strong force • (cf overwhelms electric repulsion between protons) • Rearranging them within a nucleus can : • require or liberate energy, depending on the change • Typical binding energies ~ MeV per proton/neutron • cf ~ eV for electrons in atom  x106 less •  nuclear energy is huge, per kg, compared to chemical •  A & H bombs are 106 times more powerful, per kg Note: Nature has four forces : nuclear, electric, weak, gravity. Each can create/absorb energy when objects move closer or further.

  16. (10) Binding Energy Curve • Some nuclei are more • tightly packed than others • light : loosely packed • iron (26) : most tight • heavy : less packed • Energy is released when : • Fusion of light nuclei • Fission of heavy nuclei • On Earth : • Fusion : H bombs • Fission : A bombs & • nuclear reactors

  17. mass of fuel 2 x 1030 consumption rate 6 x 109 Lifetime 3 x 1019 s = 1011 yr = = = (11) Hydrogen fusion : energy release Overall, we know : 4p  4He + Energy How much energy ? Look for missing mass : 6.68 6.64 0.04 x10-27 kg x1038 in each second : 668 664 4 x109 kg E = mc2 = 4x109x (3x108)2 = 4 x 1026 J /s = Lsun In fact: only inner 10% used, so lifetime ~ 1010 years Compare efficiencies : H-fusion (0.7 %), chemical ~10-6% ( Black hole accretion ~ 50% )

  18. Net reaction: 4 1H  4He + 2e+ + 2υ +2γ Hydrogen Fusion : pp-chain Actual reactions not known until ~1930s Must involve series of 2-particle collisions: p-p chain Three stages, Twice repeated proton-proton chain Energy : KE & γ & υ e+ + e– 2γ KE & γ’s heat core υ’s escape core & sun (υ = neutrino)

  19. Hydrogen Fusion : gentle giant Need high temperatures – why ? Protons repel (+ve charges) = “Coulomb Barrier” Need high speed to collide & “stick”  high temperature Hence: Thermo-nuclear fusion } Tcore = 14 x 106 K ρcore = 150 gm/cm3 Pcore = 1011 atmospheres Fully ionized dense gas However: p+p  2Hweak/slow reaction, < treac> ~1010 yr  gentle reaction : Lsun / Vcore only 50 Watt/m3 (c.f. human ~5 kW/m3  100x greater !) the solar interior, while hot, is NOT like an H-bomb The Sun is powerful because it is HUGE.

  20. (12) Solar Neutrinos (υ’s) H-fusion creates neutrinos  come directly from nucleus Expect ~ 1014 cm-2 s-1 at the Earth (!) Difficult to detect (stopped by ~1 light-yr of lead) Need BIG targets & sensitive measurements υ’s are detected ! #s  confirm solar models Historically controversial : Too few  1/3 expected Now understood : three types of υ’s exist solar υ’s change into other types en-route.

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