ph0021 astronomy lecture 19 the sun 021203v7 the sun in all its glory
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PH0021 Astronomy Lecture 19 (The Sun) 021203v7 The Sun, in all its glory!. Prof Rick Gaitskell Department of Physics Brown University See course pages for source Sun Data. The Sun - Photosphere. Size Scale Composition Compare terrestrial planets

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ph0021 astronomy lecture 19 the sun 021203v7 the sun in all its glory

PH0021AstronomyLecture 19 (The Sun)021203v7The Sun, in all its glory!

Prof Rick Gaitskell

Department of PhysicsBrown University

See course pages for source

the sun photosphere
The Sun - Photosphere
  • Size
    • Scale
    • Composition
      • Compare terrestrial planets
      • Compare Jovian planets
      • Sun
        • GAS/PLASMA - No solids, even at core, despite pressure…
  • Why does it appear as a well defined “ball”?
    • (i) Gas distribution is nearly spherical
    • (ii) Only seeing gas from thin layer of gas
    • Thin layer of gas 400 km
    • 1/2000th radius (Rsun~0.7x106 km)
    • “Limb darkening”
the sun structure of photosphere
The Sun - Structure of Photosphere

Typical grain ~1000 km (hires solar images)

[DEMO Silicon Oil & Al flakes]

  • Above Photosphere (rPhoto~0.01% of rEarth atmosphere)
  • Is even less dense Chromosphere (rChromo~0.01% of rPhoto )
    • Difficult to see, except during real or artificial eclipse
    • Pink in colour! Why pink??
      • Emission Spectrum [BOARD]
  • Outermost layer of Sun is the Corona (“Crown”)
    • Requires eclipse (or coronagraph to observe)
    • Not sphereical
      • Streamers/projections
    • Emission lines [see next] indicate hotter than Chromosph.
      • Not just Fe+ but Fe13+
      • => T~2x106
    • Intensity? (Flux ~ sT4 )
coronal mass ejection
Coronal Mass Ejection
  • SOHO Satellite observation
    • C3 coronagraph
      • (obs range 3.5-30 solar corona radii)
  • Coronal Mass Ejection
    • 18 Feb, 2000 (Medium Sized)
    • Follows onset of solar flare previous day (but not necessarily linked)
    • Takes ~2 days to arrive (this one is heading direct for Eath, hence “halo” like appearance
    • Up to 109 tonnes plasma @ 0.01 c

Like solar flares, they occur whenever there's a rapid, large-scale change in the sun's magnetic field. Solar flares and CMEs often occur together, but not necessarily because the flare triggers the CME or vice versa. One can happen without the other and frequently during solar maximum we see CMEs without an associated flare

sun spots



Sun Spots
  • And the Brightest and Hottest parts of Corona are…
    • …directly above another conspicuous feature: SUN SPOTS


differential rotation of sun
26 days

36 days

Differential Rotation of Sun
  • Sun Spot Observation
    • Clear that different horizontal bands (latitudes) rotate at different rates
      • Gallileo made first rot obs
        • (Live ~2 months)
      • Richard Carrington, 1859
        • Differential rotation
  • Helioseismology
    • 1980’s able to determine how bulk is behaving
sun spot cycle of sun spots position
Sun Spot Cycle (# of Sun Spots & Position)
  • 11 year cycle
    • i.e. ~11 years between max #
    • max -> min (no sun spots) -> max
  • Position migrates over cycle
    • Min -> Starts ~30 deg lat
    • Max as moves toward equator
    • Next Min as Spots annihilate at equator


zeeman spliting of abs lines in sun spots
Zeeman Spliting of Abs. Lines in Sun Spots
  • George Hale 1908
  • Permit determination of magnetic field strength and polarity
magnetic dynamo model explain 22 year cycle
Magnetic-Dynamo Model (explain 22-year cycle)
  • Horace Babcock 1960
    • Proposes Magnetic-Dynamo Model to explain 22-year cycle (with polarity flips)
    • Makes use of
      • Differential Rotation
      • Convection

See for dynamo discussion and figs

sun spot movie
Sun Spot Movie
  • First part
    • Differential Rotation of Sun Spots
    • Sphere is then mapped onto sheet
  • Second Part
    • Again see differential rotation
      • Makes higher latitudes move slower than equator
    • Evolution of Sun Spots over 22 year cycle
      • 1980 max
        • Top Hemisph: Yellow leads Blue
        • Bot Hemisph: Blue leads Yellow
      • 1986 minima
      • 1991 max (Poles Reversed)
        • Top Hemisph: Blue leads Yellow
        • Bot Hemisph: Yellow leads Blue
      • 1997 minima

hydrostatic equilibrium
Hydrostatic Equilibrium
  • [DEMO - Balloons]
regions of sun
Regions of Sun
  • What happens at radius~0.7?
    • Inside - radiation
    • Outside - convection
  • T is low enough that neutral hydrogen forms
    • H absorbs visible light much better
    • Opacity increases
    • ->Convection
  • Starting at the top…
  • Corona (Flares/CME)
    • Very high temperature gas/plasma
    • Flares (hot ionized gas from sun spot) and CME (larger amounts of coronal gas)
  • Chromasphere
  • Photosphere (Sun Spots)
    • 400 km thick, 5800 K (Blackbody appears Yellow)
    • Sun spots show 22-year cycle (magnetic behaviour of Sun)
  • Convection Zone [cell structure]
  • Radiation Zone [r<0.7]
  • Core (Fusion) [r<0.25]
    • Has it gone out, no! We see neutrinos!
    • But not as many as we expected…particle physics!