Download Comparing Earth, Sun and Jupiter

Stars
 Stars have a range of intrinsic luminosities and
temperatures (colour)
 Correlations between colour and luminosity tell
the story of stellar evolution.
 The Solar luminosity is 3.8x1026 J/s
 This is enough energy to melt a block of ice
1AUx1kmx1km in only 0.1 s.
 If the luminosity of the Sun has been
approximately constant, the total energy
released since its formation would be
4.8x1043 J.
 The surface temperature of the sun is about
5700 K
 The central temperature is inferred to be
much higher: about 10 million K
 This means the Sun is a fairly ordinary, main
sequence star.
Stellar Structure
 Gas pressure in the stellar interior opposes
gravity to keep the star in equilibrium
 The equation of hydrostatic equilibrium equates
the pressure gradient to the gas density.
dP
GM r 

dr
r2
 The gas density and enclosed mass are related by
the conservation of mass:
dM r
 4r 2 
dr
 The density, pressure and temperature are
related by the equation of state, which can
usually be approximated as the ideal gas law.
kT
P  nkT 
where n is the number density
mH
and r is the mass density of the
gas and  is the mean molecular weight.
Stellar luminosity
 The energy production in main-sequence stars is
due to the fusion of hydrogen into helium in the
core
 This is the only process that can produce
sufficient energy
 Classically fusion would not be efficient
enough: requires quantum mechanics (barrier
tunneling).
 Energy is transported to the surface via either
radiation or convection.
 The Sun has a convective envelope and
radiative interior. Low mass main-sequence
stars are entirely convective.
 More massive stars are much more luminous (the
fusion reactions release much more energy)
 Therefore they burn their core mass more
quickly, and have shorter main sequence
lifetimes.
The Solar atmosphere
 The light we see from the Sun is emitted from
the photosphere. The atmosphere extends
thousands of kilometers above this.
 In general the atmosphere is very low density,
but much higher temperature.
 Different parts of the Sun are visible at
different wavelengths:
X-ray:
 All emitted from the million-degree corona
 Show a very stormy and dynamic atmosphere, the
source of the solar wind
Ultraviolet:
 UV radiation comes from the chromosphere,
between the photosphere and corona
Infrared:
 Long-wavelength radiation from the chromosphere
and corona
 Dark spots caused by absorption of light due to
denser gas in the atmosphere.
Radio:
 Emitted near the transition region between the
chromosphere and corona