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The Interstellar
Medium
Red, White, and Blue :
Nebulae
Components of the ISM
• Gas (hydrogen and helium)
– Clouds
• Molecular Clouds
• Cold HI (neutral H)
• Warm HI
– Diffuse gas
• HII regions (ionized H)
• Hot intercloud medium
(most of the volume)
• Dust (silicates, graphites, ices)
– Dark Clouds
– Cirrus
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T~1 million K, n~0.001/cc
The Orion Region – Visible and Infrared
Molecular Clouds
Optical
Infrared
HII regions
Ionizing radiation from hot
young stars makes
hydrogen clouds glow red
(other elements:
other colors)
Scattering (and the blue sky)
Reflection Nebulae
Blue light is scattered by dust more
efficiently than red light, so dust seen
in scattered light looks bluish.
Dark Clouds
Associated with dense gas is about 1%
(by mass) of “rocky/icy” grains that
could eventually make terrestrial
planets.
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!).
Emission, Extinction, Scattering, and Reddening
Emission
nebula
“HII region”
Reflection
nebula
Balmer emission
Ionizing
radiation
extinction & reddening
Kirchoff’s 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.
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
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 star’s 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.
The Milky
Way
Discovery of the Galaxy
Democritus (400 BC)
Milky Way is unresolved stars?
Galileo (1610)
that’s 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)
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.
Models of the Galaxy
The distribution of stars was
used to infer the shape.
Another issue was: how big is
the Galaxy. You need to be
able to know the luminosity
of stars – even very distant
stars…
Or perhaps you can make use
of globular clusters.
Then there is the question of
whether there is any
absorption between the stars.
Spiral Nebulae and the Zone of Avoidance
Another strange distribution was
found for the spiral nebulae. It
was unclear what these were or
how far away they were. They
could be forming solar systems,
or other galaxies (of course, we
weren’t sure how big ours was).
It was also found
that some of
them had rather
large radial
velocities (and
maybe proper
motions).
Variable Stars –
A Standard Candle
The ShapleyCurtis Debate
Shapley
Curtis
In 1920, 2 astronomers debated
the nature of the Galaxy before
the National Academy of Science.
They were from N. and S.
California (Lick and Mt. Wilson).
They also wrote papers. Here are
their arguments which are a good
example of how science actually
works in the process of discovery.
Basic Structure of
the Galaxy