The Sun

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The Sun
Visual (photosphere)
UV (chromosphere)
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(1) Basic Properties

• Rotates once per month

• Big Rsun 100 Rearth
• Massive Msun 3 x 105 Mearth
• Ave. Density Mass/Volume
• lt ?sun gt 1.4 gm/cm3 ( ?water)
• (cf. lt ?earthgt 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)

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• 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
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(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 ltTgt 5600 K
• lt ? gt 10-3 ?air lt P gt 10-2 Pair

• Temp decreases 8000 ? 4000 K
• ? absorption lines formed here
• ? limb darkening

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• Granulation surface convection heat rising
  from below.

Size 1000 km Rise/fall speeds 500 m/s Lifetime
10 20 min
(hot)
(cooler)
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(3) Chromosphere
Thin hot ? faint emission lines Pink
Balmer lines strong (chromo- ) Helium
discovered from spectra Study using
filter-grams (e.g. Ha)
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(4) Corona
Very thin very hot ? 10-13 ?air
T 106 K very ionized (e.g. Mg8)
heated magnetically (?)
Solar disk
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(5) Solar Wind

• Flows out past Pluto at 400 km/s
• ? pushes back comet tails
• p e only 10-14 Msun per year
• _at_ Earth 5 particles/cm3
• ? aurora

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(6) Solar Activity
Umbra Pen-umbra
Complex violent weather Unlike Earths
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
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(b) Sunspot Cycle
Number distribution of sunspots goes through
cycle Looks like 11 year cycle Actually 22 years
to return to same magnetic polarity
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• Babcock theory (1960s)
• differential rotation amplifies
• magnetic fields
• Stronger fields are buoyant
• Rise break surface
• ? sunspot

Differential rotation
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(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
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(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) ?(r) agree well with theory

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(8) Suns 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 Suns energy linked to
geological estimates of the age of the Earth
life. 1880 1910 became clear.
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(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.
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(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

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(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
4x109 x (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 )
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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 ? ? e e ?
2?
KE ?s heat core ?s escape core sun (?
neutrino)
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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 ?core 150 gm/cm3
Pcore 1011 atmospheres Fully ionized dense gas
However pp ? 2H weak/slow reaction, lt treacgt
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.
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(12) Solar Neutrinos (?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

• ?s are detected !
• s ? confirm solar models
• Historically controversial
• Too few ? 1/3 expected
• Now understood
• three types of ?s exist
• solar ?s change into other
• types en-route.