Sun
The photosphere: surface temperature 5800K has darker edge and spots (darker=cooler)
Re was the creator of our world. In the beginning of time, an egg rose from the water. Once out of his shell, Re had two children who became the atmosphere and clouds. They in turn had more children,Geb and Nut, who became the Earth and the stars. They in turn had two sons, Seth and Osiris, the father of Horus. Re cried one day, and humans were made from his tears.
Limb darkening
Sun is spherical: R=696,000 km centre hotter, denser than surface photons scatter repeatedly; last scattering surface far from centre neutrinos scatter much less!
Energy source
expected life = E /L = E /(3.86 1026J/s)
Gravitational contraction? (Kelvin-Helmholtz) EKH = GM2 /R = M c2 (GM /c2)/R = M c2 (1.5/696,000) = 2.15 106 M c2
Chemical energy? Echem = (M /mproton)atoms(1019 J/atom) = 1.2 1057 1019J 1038J = M c2 (1019J/mprotonc2) = 0.66 109 M c2
Nuclear energy? Efusion = 0.007 M c2 if 4H He + neutrinos + photons
Energy for fusion
collision if de Broglie waves overlap: h/(mpv) electrostatic energy: e2/ = (e2/hc) mpv c
fusion if kinetic electrostatic i.e. mpv 2 (e2/hc)mpv c so v e2/h or mpv 2 (e2/hc)2 mpc2 in terms of temperature: mpv 2 kT so kT (e2/hc)2 mpc2 so kT (103)2 1.6 1010J i.e. T (1.6/1.38) 107K
require temperatures of million degrees! (gas is ionized plasma)
Thermal and hydrostatic equilibrium
Temperature, pressure, density, higher in core than at surface
Energy generated in core must be transported to surface: conduction, convection, radiation
Pressure
Pressure (GM2 /R2 )/(4R2 )
Ideal gas: P = (N/V) kT set N M /mp kT = PV/N = (GM2 /R4 ) R3 (mp/M ) = (GM /R ) mp M = 1.989 1030 kg (from orbits) R = 6.96 108 m (from angular size) T = 1.9 (1.67/1.38) 107 K
P, T and density almost certainly even higher in core: Fusion is possible!
Stars shine because GRAVITY SUCKS
if a ball of self-gravitating gas loses energy (e.g. by radiation) and is forced to contract, then, if it has no compensating energy source, it gets hotter
Radiative Transfer: Diusion
central regions hot, dense: opaque photons scatter repeatedly: approximate as random walk with typical step
xn+1 x2 n+1 x2 n x2 n+1
xn = . . . = x 1 = 0 1 1 = (xn + )2 + (xn )2 2 2 = x2 + 2 n = =
2 x2 n1 +
=
x2 + 2 2 = (n + 1) 2 n1
2 2 we want R2 = x2 + yn + zn = 3n 2 n in 3d it takes n = 3(R / )2 steps
total ight time t = n /c = 3(R / )(R /c)
Convection
centre hot dense; all H ionized outer regions cooler; H atomic or H H much more opaque; radiative transport cannot move energy eciently to surface energy balance maintained by convection; hotter regions rise, expand, cool colder regions sink, shrink, heat-up
The model Sun
Granulation
individual yellow blobs are granules convection cells in Suns outer layers yellow=hot, rising; black=cold, sinking each granule about 1000 km across (radius of earth is 6370 km)
The Chromosphere
emission (H line = 656.3 nm) spectrum
spicules: jets of cool gas that surge upward into warmer regions of outer atmosphere.
spicules are located along irregularly shaped boundaries between large organized cells called supergranules
The Corona
emission lines from highly ionized atoms high temperature T 106K, low density (corona 1011atom/m3; photo 1023) streamers extend millions of kilometers above surface solar wind
The SOHO Sun
outer image, light from highly ionized oxygen atoms, material from corona streaming outward to become solar wind. inner image, UV light from highly ionized iron atoms, shows origin of these streamers in lower regions of corona.
blob of hot, glowing gas ejected to right expanded to the size of Sun in a few hours (yellow circle shows size of Sun)!
hot gases appeared on other side of Sun at same time
convulsions of the corona that extend all the way around the Suns equator?
Oh Yohkoh!
X-ray emission from the corona (false color)
bright high-temperature regions change appearance on time scales of days; often above sunspots in the photosphere
Sunspots
typical sunspot same diameter as Earth dark center is umbra; bordered by less dark, feathery penumbra; note granulation of surrounding photosphere
Sun rotates about once every four weeks sunspot groups last about 2 months
dierential rotation: faster at equator (25 days) than near poles (35 days)
number of sunspots varies with a period of about 11 years early in cycle sunspots form near 30; later, form closer to equator
rotation rate varies with latitude, depth colors represent rotation periods: dierential rotation (25 days at equator, 35 days near poles), persists throughout convective envelope
radiative core rotates like rigid body
The M...