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Physics 306
Chapter 10 – The Interstellar Medium
 Very hot stars can excite clouds of gas and dust to emit light and this reveals that
the clouds contain mostly hydrogen gas at very low densities.
 3 kinds of nebulae:
o Emission nebulae – produced when a star with temperature higher than
25,000K excites the gas near to produce an emission spectrum. Specific
pink color b/c of the blending of red, blue, and violet Balmer lines.
o Reflection nebulae – produced when starlight scatters from a dusty nebula.
Basically just the reflected absorption spectrum of starlight. The gas is not
excited to emit photons. The dust is very small and reflects a blue color for
the short wavelengths.
o Dark nebulae – dense clouds of gas and dust that obstruct the view of
more distant stars. All shapes and sizes hinting that even if there are no
nearby stars to ionize the gas there must be breezes/currents pushing
through the interstellar medium.
 Forbidden lines – proof of an emission nebulae has a very low density.
 Evidence of Interstellar dust:
o makes distant stars appear even fainter (interstellar extinction) ~ 2 mag per
1000 pc. (ex: star 1000 pc from Earth, then it looks 2mag fainter than it
really is)
o effect on colors of stars (interstellar reddening) – dust particles scattering
light . Scatter blue photons more than red photons.
 Interstellar Absorption lines evident by:
o Ionization – Ex: a very hot star, O, should not contain any ionized calcium
b/c at that high temp, the ions can’t exist, but in fact a lot of O stars do
show calcium lines. So, these lines apparently weren’t produced in the
star, but in the interstellar medium.
o Widths – Interstellar lines are narrow and sharp, which means the
interstellar matter is cold and has a low density. If it was hot, then we
know that Doppler broadening would have smeared the lines. If the gas
was dense, then we know that the atoms would collide often, so the lines
would be wide.
o Multiple Components – the absorption lines produced have different
wavelengths b/c they form when light passes through different clouds of
gas, and seeing how different clouds have different radial velocities, then
the lines have different wavelengths.
 4 Components of the Interstellar Medium:
o HI clouds – clouds of neutral gas, 50-150 pc in diameter, few solar
masses, 100K temp (low), 10- few hundred atom/cubic cm (high density)
not ionized! *make up about 25% of interstellar mass
o Intercloud medium – few thousand K, .1 atom/cubic cm (low density),
IONIZED hydrogen (HII), in approx. equilibrium with HI clouds so
relatively same pressure. *make up about 50% of interstellar mass
o Molecular clouds – very dense, 60 pc in diameter, few million solar
masses, very cold. *makes up about 25% of interstellar mass
o Coronal gas – million degrees K (very hot), low density; thought to form
by supernovae exploding. *makes up about 5% of interstellar mass
 Wavelength Observations:
o 21 cm observations – can map the distribution of neutral hydrogen (HI)
not ionized hydrogen b/c it lacks an electron and can’t emit 21 cm
radiation.
o Radio observations – important molecules (CO, OH) that can be detected
with radio energy. Molecules can’t exist outside dense clouds. This shows
that if we can detect these molecules then the molecular clouds are dense.
o Infrared Radiation – The interstellar dust is very cold, but it has a large
surface area which gives off a lot of infrared radiation.
o X rays – come from high temp area which is the coronal gas. Remember
probably caused from exploding supernovae or gas flowing away from hot
stars.
o UV Observations – reveal that while some parts of interstellar medium are
cold and empty, some are hot. (ex: sun lies in hot regions with ionized
hydrogen, but down a few light yrs lies a cool neutral hydrogen cloud)
 Interstellar Cycle
o Star group forms, hottest stars begin ionizing gas to produce emission
nebulae
o Pressure of starlight and gas flowing away from the star push the gas
outward into the cloud
o Interstellar dust formed in the atmosphere of cool stars (low temps so
atoms can condense into solid matter)
o Pressure of starlight pushes dust out of the stars’ atmosphere and into the
interstellar medium to be replenished. (exploding supernovae add to the
interstellar medium as well)
o Interstellar gas and dust continue to build and form massive clouds
o Dust protects the interior of the cloud from uv photons, so molecules can
form
o Giant molecular clouds are formed and giving birth to new stars