Transcript Scattering (and the blue sky)

The Interstellar
Medium
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
T~20K, n>1000/cc
T~100K, n~20/cc
T~5000K, n~0.1-1/cc
T~10000K, n~0.01-0.1/cc
T~1 million K, n~0.001/cc
HII regions
Ionizing radiation from hot
young stars makes
hydrogen clouds glow red
(other elements:
other colors)
Red, White, and Blue
Nebulae
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.
star
nebula
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.
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
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.
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.
Variable Stars –
A Standard Candle
How can we
get the scale
of the
Galaxy?
Parallaxes
won’t work.
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.
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…
Sources of Radio Emission -1
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
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.
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, there’s 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.
Optical and
Radio Sky
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.
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.
Tracers of Spiral Arms
In addition to radio maps, you can use HII regions or O&B stars to
try to locate spiral arms. The Sun is near the Orion-Cygnus arm,
but that is a recent”
occurrence. It’s
been around about
18 times.
21-cm radio map
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.
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
The Galactic Center
Infrared all-sky image
Central region
(X-rays)
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…
Recent adaptive optics pictures
in the infrared at the Galactic
Center show stars orbiting a
central invisible mass. Kepler’s
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…)
At the Core Lurks A Monster …
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 < 100 million years
Blue giants
Core collapse (Type II)
Open (e.g., Pleiades)
Yes
Yes
2%
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.
Population II
Bulge and halo
Random, high velocity
Only > 10 billion years
Red giants
White dwarf explosions (Type I)
Globular (e.g., M3)
No
No
0.1 - 1%
Galactic
Structure
Disk (Pop I)
(stars, ISM, open clusters)
Bulge (II) & Halo : Pop II
(stars, globular clusters)
Dark Matter Halo
Formation of the Galaxy