The Sun: A Star of Our Own ASTR 2110 Sarazin
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
The Sun: A Star of Our Own ASTR 2110 Sarazin
Sun = A Very Average Star R 8 = 7 x 10 10 cm M 8 = 2 x 10 33 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)
Solar Composition Element Abundance by mass Hydrogen 73.5% Helium 24.8% Oxygen 0.788% Carbon 0.326% Nitrogen 0.118% Iron 0.162%
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
Solar Rotation
Solar Rotation Rotation period Equator: 25 days Poles: 30 days Differential rotation Equator plane of planetary orbits Direction same as planetary orbits
Heat Transport Radiation: important in the inner 86% of the Sun Convection: boiling important in the outer part of the Sun
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)
Solar Atmosphere Photosphere Where most light comes from Granulation due to convection
Granulation
Movie of Granulation
Photospheric Spectrum
Photospheric Spectrum Continuum emission (all wavelengths) ~ black body Absorption lines
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
Photospheric Spectrum General result Stellar spectra = continuum emission + absorption lines
Photosphere Chromosphere Solar Atmosphere Hotter 10,000 K Corona Very hot, millions K Extends out very far
Solar Corona
Photosphere Chromosphere Corona Solar Wind Solar Atmosphere
What heats the corona and chromosphere? Not completely understood, but almost certainly due to magnetic fields
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
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
Magnetic Field Lines Magnetic field lines magnetic field B
Magnetic Field Lines No magnetic charges (monopoles) è magnetic field lines never end
Magnetic Field Lines No magnetic charges (monopoles) è magnetic field lines never end Magnetic field lines are loops
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
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
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
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
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)
Sunspots Solar Activity Galileo 1613 Dark Cool (4000K, not 6000K) Pairs (east-west)
Sunspots
Sunspots
Sunspots Prominences Solar Activity
Solar Prominence Sep. 14, 1999
Sunspots Prominences Flares Solar Activity Giant explosions on Sun Shoot out dangerous particles
Solar Flare
Solar Flare
Solar Flare
Solar Activity Sunspots Prominences Flares Occur together in active region
Solar Magnetic Field Sun s magnetic fields stronger than Earth Causes solar activity Field ~1000x stronger in sunspots, active regions
S N
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
Magnetic Field, Corona, and Solar Wind Solar Flares: Magnetic loops break, reconnect B B Solar flare Sun surface Sun surface
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
Coronal Hole
Sun Spots and Magnetic Loops
Sunspot pairs opposite magnetic polarity equator
S N
N S S N Magnetic field reverses every 11 years Electromagnetic dynamo field
Sunspots and Solar Cycle
Last Solar Maximum Last Solar Minimum
Solar Cycle Number of sunspots, other activity maximum every 11 years
Solar Cycle Number of sunspots, other activity maximum every 11 years Magnetic reversal 22 year solar cycle
Butterfly Pattern
Butterfly Pattern
Solar Dynamo Magnetic Field