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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Size ~ 1000 km
Rise/fall speeds ~500 m/s
Lifetime ~ 10 – 20 min
Thin & hot faint & emission lines
Pink : Balmer lines strong (chromo- )
Helium : discovered from spectra
Study using “filter-grams” (e.g. Hα)
Very thin & very hot
ρ ~ 10-13 ρair T ~ 106 K
very ionized (e.g. Mg8+)
heated magnetically (?)
Complex & violent “weather”
Unlike Earth’s weather,
magnetic fields important.
(b) Sunspot Cycle
Number & distribution of sunspots goes through cycle
Looks like 11 year cycle
Actually 22 years to return to same magnetic polarity
(c) Prominences & Flares
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
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.
Note: Nature has four forces : nuclear, electric, weak, gravity.
Each can create/absorb energy when objects move closer or further.
mass of fuel 2 x 1030
consumption rate 6 x 109
3 x 1019 s = 1011 yr
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
In fact: only inner 10% used, so lifetime ~ 1010 years
Compare efficiencies : H-fusion (0.7 %), chemical ~10-6%
( Black hole accretion ~ 50% )
4 1H 4He + 2e+ + 2υ +2γ
Actual reactions not known until ~1930s
Must involve series of 2-particle collisions: p-p chain
Energy : KE & γ & υ
e+ + e– 2γ
KE & γ’s heat core
υ’s escape core & sun
(υ = neutrino)
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.
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