The Sun: A Star of Our Own ASTR 2110 Sarazin

Transcription

1 The Sun: A Star of Our Own ASTR 2110 Sarazin

2 Sarazin Travel Wednesday, September 19 afternoon Friday, September 21 Will miss class Friday, September 21 TA Molly Finn will be guest lecturer Cancel Office Hours on Thursday, September 20

3 The Sun: A Star of Our Own ASTR 2110 Sarazin

4 Sun = A Very Average Star R 8 = 7 x cm M 8 = 2 x g Average Density <ρ> = 1.41 g/cm 3 (less than Earth) Sun is mainly hydrogen Composition: 74% Hydrogen, 24% Helium, 2% everything else (by mass)

5 Solar Composition Element Abundance by mass Hydrogen 73.5% Helium 24.8% Oxygen 0.788% Carbon 0.326% Nitrogen 0.118% Iron 0.162%

6 Age of Sun Radioactive dating Oldest materials in Solar System 4.6 billion years Consistent with age of Sun from stellar theory Age of Sun = 4.6 billion years

7 Solar Rotation

8 Solar Rotation Rotation period Equator: 25 days Poles: 30 days Differential rotation Equator plane of planetary orbits Direction same as planetary orbits

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10 Heat Transport Radiation: important in the inner 86% of the Sun Convection: boiling important in the outer part of the Sun

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12 Where Does Light Come From? Mean-free-path of photons in the Sun ~ 1 cm in the center ~ 400 km at surface Light that we see comes only from surface (from narrow layer) (doesn t tell us where energy ultimately comes from)

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14 Solar Atmosphere Photosphere Where most light comes from Granulation due to convection

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16 Granulation

17 Movie of Granulation

18 Photospheric Spectrum

19 Photospheric Spectrum Continuum emission (all wavelengths) ~ black body Absorption lines

20 Photospheric Spectrum Wavelengths of lines bigger opacity, smaller mean-free-path Don t see in as far Sun gets hotter inside cooler absorption lines hotter continuum emission center

21 Photospheric Spectrum General result Stellar spectra = continuum emission + absorption lines

22 Photosphere Chromosphere Solar Atmosphere Hotter 10,000 K Corona Very hot, millions K Extends out very far

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24 Solar Corona

25 Photosphere Chromosphere Corona Solar Wind Solar Atmosphere

26 What heats the corona and chromosphere? Not completely understood, but almost certainly due to magnetic fields

27 Magnetic Fields and Plasmas Sun made of ionized gas = plasma True of much of materials in astrophysics Free electrons and ions è forces due to magnetic fields Free electrons è plasmas are very good electrical conductors Plasma can have currents è make magnetic field

28 Maxwell s Equations Complete theory of electricity and magnetism Electric charges make electric field No magnetic charges (magnetic monopoles) charges E no magnetic charges B

29 Magnetic Field Lines Magnetic field lines magnetic field B

30 Magnetic Field Lines No magnetic charges (monopoles) è magnetic field lines never end

31 Magnetic Field Lines No magnetic charges (monopoles) è magnetic field lines never end Magnetic field lines are loops

32 Charged Particles in Magnetic Fields Helical motion F = q c v B ( ) q = charge Work = F dr = F v dt = 0 E = constant, thus KE = constant v = constant In plane B, circle orbit r g = mv c qb gyro radius v = constant, v = constant Helical motion B

33 Maxwell s Equations Complete theory of electricity and magnetism Electric charges make electric field No magnetic charges (magnetic monopoles) Moving charges (currents) make magnetic fields Changing magnet fields make electric fields Charge is conserved charges E changing B no magnetic charges currents B changing E

34 Bulk Properties of Plasma with Magnetic Field Faraday s Law: changing magnetic field electric field current (if conductor) opposite sign Ampere s Law: current magnetic field Acts to prevent change in magnetic field B

35 Bulk Properties of Plasma with Magnetic Field Can t pull wire from B field Plasma = like wires in all directions Frozen-In Condition Plasma and Magnetic Field are locked together B B wire plasma

36 Bulk Properties of Plasma with Magnetic Field Frozen-In Condition plasma and magnetic field tied Who is master and who is slave? Bigger pressure wins. Gas pressure P gas = n k T Magnetic pressure P B = B 2 /(8π)

37 Sunspots Solar Activity Galileo 1613 Dark Cool (4000K, not 6000K) Pairs (east-west)

38 Sunspots

39 Sunspots

40 Sunspots Prominences Solar Activity

41 Solar Prominence Sep. 14, 1999

42 Sunspots Prominences Flares Solar Activity Giant explosions on Sun Shoot out dangerous particles

43 Solar Flare

44 Solar Flare

45 Solar Flare

46 Solar Activity Sunspots Prominences Flares Occur together in active region

47 Solar Magnetic Field Sun s magnetic fields stronger than Earth Causes solar activity Field ~1000x stronger in sunspots, active regions

48 S N

49 Magnetic Field, Corona, and Solar Wind Deep in Sun, density, temperature, pressure high, P gas big magnetic field anchored in Sun Further out, pressure low, magnetic field controls gas No magnetic monopoles field lines never end Closed Loops: hold plasma = corona B Sun surface

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52 Magnetic Field, Corona, and Solar Wind Solar Flares: Magnetic loops break, reconnect B B Solar flare Sun surface Sun surface

53 Magnetic Field, Corona, and Closed Loops: Solar Wind Open Loops: hold plasma = corona gas leaves = no corona, solar wind Solar Wind comes from coronal holes B B Sun surface

54 Coronal Hole

55 Sun Spots and Magnetic Loops

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58 Sunspot pairs opposite magnetic polarity equator

59 S N

60 N S S N Magnetic field reverses every 11 years Electromagnetic dynamo field

61 Sunspots and Solar Cycle

62 Last Solar Maximum Last Solar Minimum

63 Solar Cycle Number of sunspots, other activity maximum every 11 years

64 Solar Cycle Number of sunspots, other activity maximum every 11 years Magnetic reversal 22 year solar cycle

65 Butterfly Pattern

66 Butterfly Pattern

67 Solar Dynamo Magnetic Field