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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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1 basic properties
(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)
slide3

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
2 photosphere
(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
slide5

Granulation : surface convection – heat rising from below.

Size ~ 1000 km

Rise/fall speeds ~500 m/s

Lifetime ~ 10 – 20 min

(cooler)

(hot)

3 chromosphere
(3) Chromosphere

Thin & hot  faint & emission lines

Pink : Balmer lines strong (chromo- )

Helium : discovered from spectra

Study using “filter-grams” (e.g. Hα)

4 corona
(4) Corona

Very thin & very hot

ρ ~ 10-13 ρair T ~ 106 K

very ionized (e.g. Mg8+)

heated magnetically (?)

Solar

disk

5 solar wind
(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
6 solar activity

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

slide10

(b) Sunspot Cycle

Number & distribution of sunspots goes through cycle

Looks like 11 year cycle

Actually 22 years to return to same magnetic polarity

slide11

Babcock theory (1960s) :

    • differential rotation amplifies
      • magnetic fields
    • Stronger fields are buoyant
    • Rise & break surface
    •  sunspot

Differential rotation

slide12

(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

7 helioseismology
(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
8 sun s energy source
(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.

9 energy in atomic nuclei
(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.

10 binding energy curve
(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
11 hydrogen fusion energy release

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% )

hydrogen fusion pp chain

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)

hydrogen fusion gentle giant
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.

12 solar neutrinos s
(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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