Visual (photosphere) The Sun
UV (chromosphere)
(1) Basic Properties
• Rotates once per month
• Big : Rsun ~ 100 Rearth
• Massive : Msun ~ 3 x 105 Mearth
• Ave. Density = Mass/Volume
= 1.4 gm/cm3 (~ ρwater)
(cf. = 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)
Three regions to consider :
1. 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
• Region where light comes from :
deeper: hidden by opacity (H– ion)
higher: too thin to give much light
~400 km deep
• Hot thin gas : ~ 5600 K
~ 10-3 ρair ; ~ 10-2 Pair
• Temp decreases : 8000 4000 K
absorption lines formed here
limb darkening
• Granulation : surface convection – heat rising from below.
Size ~ 1000 km
Rise/fall speeds ~500 m/s
Lifetime ~ 10 – 20 min
(cooler) (hot)
(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
Very thin & very hot
ρ ~ 10-13 ρair T ~ 106 K
very ionized (e.g. Mg8+)
heated magnetically (?)
Solar
disk
(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
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 Spectrograph Zeeman
Slit splitting
• often pairs: magnetic N/S poles.
(b) Sunspot Cycle
Number & distribution of sunspots goes through cycle
Looks like 11 year cycle
Actually 22 years to return to same magnetic polarity
Babcock theory (1960s) :
• differential rotation amplifies
magnetic fields
• Stronger fields are buoyant
• Rise & break surface
sunspot
Differential rotation
(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 prominence
X-ray burst
coronal mass ejections aurora
Flare &
CME
(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
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 ?
Chemical (eg oil…) : ~104 years No
Gravity (slow contraction) : ~108 years No
Nuclear (transformation) : ~1010 years Yes
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
• 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
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
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
Lifetime = mass of fuel = 2 x 1030 = 3 x 1019 s = 1011 yr
consumption rate 6 x 109
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
Actual reactions not known until ~1930s
Must involve series of 2-particle collisions: p-p chain
Three stages, proton-proton chain
Twice repeated
Net reaction:
4 1H 4He + 2e+ + 2υ +2γ
Energy : KE & γ & υ
e+ + e– 2γ
KE & γ‟s heat core
υ‟s escape core & sun
(υ = neutrino)
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 Pcore = 1011 atmospheres
ρcore = 150 gm/cm3
} Fully ionized dense gas
However: p+p 2H weak/slow reaction, ~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 (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.