The Interstellar Medium

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The Interstellar Medium
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Components of the ISM

• Gas (hydrogen and helium)
• Clouds
• Molecular Clouds T20K, ngt1000/cc
• Cold HI (neutral H) T100K, n20/cc
• Warm HI T5000K, n0.1-1/cc
• Diffuse gas
• HII regions (ionized H) T10000K, n0.01-0.1/cc
• Hot intercloud medium T1 million K, n0.001/cc
• (most of the volume)
• Dust (silicates, graphites, ices)
• Dark Clouds
• Cirrus

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HII regions
Ionizing radiation from hot young stars makes
hydrogen clouds glow red (other elements other
colors)
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Red, White, and Blue Nebulae
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Scattering (and the blue sky)
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Reflection Nebulae
Blue light is scattered by dust more efficiently
than red light, so dust seen in scattered light
looks bluish.
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Dark Clouds
Associated with dense gas is about 1 (by mass)
of rocky/icy grains that could eventually make
terrestrial planets.
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Visible and Infrared Extinction
The dark dust clouds are very opaque in the
visible, but we can see through them better and
better, the longer the wavelength of light that
is used. Looking through the galactic plane has
the same effect to see to the heart of the
Galaxy you must use infrared or radio (or
X-rays!).
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Emission, Extinction, Scattering, and Reddening
Emission nebula
HII region
Reflection nebula
Balmer emission
Ionizing radiation
extinction reddening
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Kirchoffs Laws

1. An opaque object emits a continuous (blackbody)
   spectrum.
2. An thin gas cloud produces an emission line
   spectrum.
3. A thin gas cloud in front of a blackbody source
   usually produces an absorption line spectrum.

star
nebula
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Emission and Absorption Spectra
More accurately, a gas cloud is only opaque
within spectral lines, while a star is opaque at
all wavelengths. The brightness of each depends
on the usual T4 relation. If, as is usually the
case, the cloud is colder than the star (or the
stars atmosphere is colder than its surface),
then an absorption line spectrum is produced. If
one looks only at the cloud, the background
(empty space) is even colder, so you always get
an emission line spectrum. If you look at a cloud
through a hotter cloud of gas, you will get an
emission line spectrum which includes a continuum.
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Astro Quiz
Suppose the thin cloud of gas had the same
temperature as the hot solid object. The
spectrum would look like

1. A continuous spectrum
2. An absorption spectrum
3. An emission spectrum

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The Milky Way
A whole sky view on a dark clear night shows a
band of light running across the sky. It has some
kind of structure running through the middle of
it.
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Discovery of the Galaxy
Democritus (400 BC) Milky Way is unresolved
stars? Galileo (1610) thats right! Wright,
Kant (1750) it must have a slab-like
arrangement Herschel (1773) we can map the
Galaxy by counting stars (assume all are same
luminosity and no absorption)
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Shape of the Milky Way
To be surrounded by a band of stars in the sky
implies that most stars are in one plane (and we
are in it ourselves). Because it is brighter in
one direction, that implies we are not at the
center.
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Variable Stars A Standard Candle
How can we get the scale of the Galaxy?
Parallaxes wont work.
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The Shapley-Curtis Debate
Shapley Curtis
In 1920, 2 astronomers debated the nature of the
Galaxy and spiral nebulae before the National
Academy of Science. They were from S. and N.
California (Mt. Wilson Lick). They also wrote
papers about it. Here are their arguments which
are a good example of how science actually works
in the process of discovery.
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Mapping the Galaxy Radio Astronomy
We can only see our local neighborhood because of
interstellar dust.
To penetrate this, we can use radio wavelengths
(much longer than the size of dust particles). Of
course, something has to be producing radio
emission
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Sources of Radio Emission -1

• Thermal emission from cold interstellar clouds
• At a few 10s of K, blackbody emission will be in
  the radio, or somewhat hotter clouds have a long
  wavelength tail

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Sources of Radio Emission -2
2) In a strong magnetic field, spiraling
electrons will produce non-thermal synchrotron
radiation. This can happen near stars or compact
objects, or from cosmic rays in the galactic
field.
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Sources of Radio Emission 21 cm radiation
Neutral hydrogen has a very weak radio spectral
transition. So the Galaxy is transparent to it.
On the other hand, theres a lot of neutral
hydrogen. So we can see it everywhere. There are
also molecular lines from CO and other molecules.
The transition occurs because electrons and
protons have spin. Having the spins aligned is
a higher energy state. So in about 10 million
years it will decay to the ground state
(anti-aligned). Or a 21-cm photon can be absorbed
and align the spins. Because the Galaxy is
transparent, it is hard to tell where the
emission is coming from along the line-of-sight.
But because we know its precise wavelength,
Doppler shifts in this line can tell us how the
gas is moving.
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Optical and Radio Sky
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Spiral Arms in Galaxies
Since inner orbits are faster than outer orbits,
you might think that is why one sees spiral arms.
But these would rapidly wind tightly galaxies
have had 100 rotations since they formed.
Instead,
the spiral arms are density waves apparent
patterns where stars are denser due to slowing
down from mutual gravity.
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Density Waves
Traffic jams are good examples of density waves.
Certain parts of the freeway may have a high
density of cars, yet individual cars do not stay
with the pattern, but flow through it. They move
slowly when at high density, and move quickly
when at low density. The site of an accident
might produce a stationary density wave (but
again, cars are always moving through it).
Thus, the spiral arms of a galaxy are just a
pattern that may rotate slowly or not at all
individual stars will be passing through it all
the time.
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Tracers of Spiral Arms
In addition to radio maps, you can use HII
regions or OB stars to try to locate spiral
arms. The Sun is near the Orion-Cygnus arm,
but that is a recent occurrence. Its been
around about 18 times.
21-cm radio map
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Spiral Arms and Star Formation
When the ISM passes through it, it gets
compressed, and star formation is enhanced. This
makes bright hot young stars, and the pattern
stands out.
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Spiral Tracers from Outside
In other galaxies, the arms are easy to see
because their ISM does not hide optical
diagnostics from us. There are always only a few
arms (often 2), and they are never too tightly
wound.
O B stars HII
regions 21-cm radiation
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The Galactic Center
Infrared all-sky image
Central region (X-rays)
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The Heart of the Galaxy
To see to the center, we must use infrared or
radio telescopes. A strange mini-spiral swirls,
casting off a ring of molecular gas. Magnetic
fields are produced, and pervade the Galaxy.
AO infrared image of true center
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At the Core Lurks A Monster
Recent adaptive optics pictures in the infrared
at the Galactic Center show stars orbiting a
central invisible mass. Keplers Laws yield a
mass inside one light year of 2.7 million solar
masses! It has to be a black hole (but apparently
it is napping at the moment)
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Stellar Populations
Stellar Population Location Star motions Ages
of stars Brightest stars Supernovae Star
clusters Association with gas and dust? Active
star formation? Abundance of heavy elements (mass)
Population I Disk and spiral arms Circular, low
velocity Some lt 100 million years Blue giants
Core collapse (Type II) Open (e.g., Pleiades)
Yes Yes 2
Population II Bulge and halo Random, high
velocity Only gt 10 billion years Red giants
White dwarf explosions (Type I) Globular (e.g.,
M3) No No 0.1 - 1
Population II stars are old and metal poor, found
in large orbits in a random spherical
distribution. Population I stars are young and
metal rich (including hot stars), all orbiting in
the disk in the same direction.
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Galactic Structure
Disk (Pop I) (stars, ISM, open clusters) Bulge
(II) Halo Pop II (stars, globular
clusters) Dark Matter Halo
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Formation of the Galaxy