•Earlier in the course, I
told you stellar spectra
are black bodies
•Why are there all
these features?
Spectral Lines
High resolution solar spectrum
•In dense inner part, light gets
randomized – black body spectrum
•Light passes through cooler stellar
atmosphere
•But some wavelengths get
absorbed!
+-
Stellar spectra
•Approximate black body spectrum – color tells you temperature
•Red is cool, blue is hot
•Spectral lines tell you a lot about the star
•Different lines for each element (and ionization state)
•Strength tells you about composition and temperature
More about stars
•Stars are formed from clouds of cool gas
•They come in different sizes:
•High mass stars tend to burn hotter (bluer) and don’t last long
•Low mass stars tend to burn cooler (redder) and last a long time
•The color of a collection of stars (clusters) tells you about them:
•Young clusters look bluish
•Old clusters look reddish
Doppler Effect Revisited
f0
 
f 
 1  v cos  c 
1 v c
f  f0
1 v c
•In astronomy, most of the motion is towards you
f0  f  1  vr c 
or away from you
•Let vr = - v cos be the velocity away from you  f  c  0 f 0
•In astronomy, we work with wavelength instead of frequency
•Except they actually use z, the fractional increase in wavelength
2

f0
1

z
1

vr 
1 z 

  1  vr c 

0
f
c 1  z 2  1
•For non-relativistic:
z  vr c
 0
f  f0
1 v c
1 v c
Orbits
•When a low mass object orbits a high mass object, there is a simple
relationship between the distance and the velocity:
2
GMm mv

F
2
R
R
GM
v
R
Distance
1 AU = Earth-Sun distance
= 1.496  1011 m
1 ly = cy = 9.46  1015 m
1 pc = 3.086  1016 m
50
Mean Velocity (km/s)
45
40
Planets
35
Fit to formula
30
25
20
15
10
5
0
0
5
10
15
20
25
Mean distance (AU)
30
35
40
Mass
1 MSun = 1.989  1030 kg
The Milky Way: Visible Light
•Obscured by gas
and dust
Center of
Galaxy
Galactic Plane
The Milky Way
The central portion
Baade’s Window
The Milky Way – From Outside
The Milky Way – From Outside
The Milky Way – Edge On
The Milky Way – Artist’s Conception
•Our galaxy is hard to study
because we are inside it
•We are in the galactic
plane, filled with
obscuring dust
•Other galaxies are hard
to study because they
are far away
•Generalize from far away
to nearby and vice versa
You
are here
The Milky Way – Basic Structure
•Galaxies and larger
structures are incredibly
large compared to
anything we have
studied up to now
•We need new
units to describe it
Distances
ly = 0.931016 m
kly = 103 ly
Mly = 106 ly
Gly = 109 ly
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
The
Sun
The Disk – Dimensions and Structure
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
•A large, flat disk, shaped like a pancake
•About 100 kly in diameter
•About 3 kly thick
•We are about half way out
•Has prominent spiral structure
27 kly
The
Sun
100 kly
3 kly
The Disk - Composition
•Stars, mixture of young and old
•Circular orbits in plane of galaxy
•From 0 to about 10 Gyr
•Open clusters
•The interstellar medium
•Hot bubbles
•Atomic hydrogen clouds
•Molecular clouds
•Ionization nebulae
•Dust
•Obscures and reddens things
•Causes “reflection nebulae”
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Open Clusters
M35
NGC 290
NGC 2158
M6
M36
Pleiades
Hot Bubbles
•Gas heated by supernovae and other violent events
•Gas is very thin
•Gas is very hot and ionized
•Temperatures up to 106 K
•Can be
traced out
by X-rays
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Hot Bubbles - Images
Atomic Hydrogen Clouds
Slightly cooler regions of gas
•Hydrogen atoms produce 21 cm line
•Electron and proton are spinning and
have magnetic interactions
•When electron spin flips over, 21 cm
radio emission is seen
•21 cm line used to map out
our disk
•Can also get accurate Doppler
shift
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Radio
waves
The 21 cm line
Approximate Map of Galaxy
Molecular Clouds
Coldest and densest regions
•Atoms join together to make molecules
•Principally hydrogen (H2), but this is difficult to detect
•Other molecules vibrate to produce characteristic radio
waves
•These regions are where new stars can form
Carbon monoxide (CO)
emissions from cool
clouds in our galaxy
Molecular Clouds – Eagle Nebula
Molecular Clouds
Molecular Clouds – Keyhole and Orion
Molecular Clouds
Molecular Clouds – Horsehead Nebula
Ionization (Emission) Nebulae
•Light from hot stars ionizes hydrogen
•Hot thin gas
•When it recombines, light is made
Light
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Ionization (Emission) Nebulae
Ionization (Emission) Nebulae
Labeled Eagle Nebula
Dust
•Lots of dust in spiral galaxies
•Dust absorbs and scatters short
wavelength light
•The color of objects is reddened if
we look through thin dust clouds
•Dust clouds look blue
•You can peek through dust in near
infrared light
Dust cloud
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Dust
•Dust is warmed by absorbing light
from stars
•It glows in the infrared
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
A reflection nebula – the Pleiades
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
A reflection nebula – the Merope Nebula
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Reflection Nebulae
IC 349
Witch Head Nebula
Spiral Arms
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Spiral Arms – Signs of Rotation
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
•Bulge is flattened - rotation
•Disk - rotation vs. gravity
•Spiral arms - what causes them?
•Simple winding? No!
•Density waves? Yes! Complicated!
Simple Winding – The Wrong Theory
•Spiral arms
wind up in one
cycle
•20 cycles since
beginning of
galaxy
•Something else
is going on
An Analogy from Driving
•Knot in traffic
causes
slowdown
•Slowdown
causes other
cars to slow
down
•Knot in traffic
moves
•Different cars
at different
times
Density Waves – The explanation
•Gas is in a disk
•Inevitably, some
parts are more dense
than others
•Gravity/rotation
causes dense parts to
become denser
•Different gas
goes in/out
•Dense parts form
new/bright stars
The Bulge
•Our view of it is (mostly) blocked by dust
•We can see through the dust in near infrared
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
The Bulge
The Sun
•Flattened sphere approximately 20 kly across
•Composition
•Mostly older stars
•Little gas and dust
•Recent evidence indicates it
is bar shaped, with left side
closer to us than right side.
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
The Bulge – Color and Appearance
•Older
stars give
it a redder
appearance
The Nucleus
•The disk
The Sun
•The bulge
•The nucleus
•The halo
•At the center of our galaxy lies a complex region
•Globular
•Fast star formation
clusters
•Recent supernovae remnants
•Hot gas
•Fast motion
•Density of stars is very high here
•Intense radio sources can penetrate the gas and dust
The Nucleus in Radio
•Close in, we see streamers
of gas apparently flowing in
•At the heart is an intense
radio source called
Sagittarius A*
What is that thing in the Nucleus?
•Copious quantities of X-rays close in
•Stars orbit this source very quickly
•Kepler’s Laws tell us mass
X-ray image
•It appears to be a 4 million
solar mass black hole
The Monster in the Middle
•Radio waves can’t come from black hole itself
•Gas from nearby attracted by gravity
•Accelerates to near light speed
•Friction creates heat/X-rays/etc.
•More efficient than any other power source
•Black hole
•4.0 million MSun
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
New Discovery?
•Gamma Rays coming
from pair of lobes
•Outburst of energy from
nucleus?
•Was our Galaxy
formerly active?
The Halo
•Roughly spherical shape
•At least 2  size of disk
•Stars:
•Old stars (about 10 Gyr)
•Orbits well out of plane
•Globular Clusters
•Up to a million stars each
•Oldest stars (up to about 13 Gyr)
•Little gas or dust
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Total Mass in the Galaxy
Object
Mass (MSun )
•Total of a few hundred billion stars
Disk Stars 60 billion
•Total mass about 100 billion  Sun
•Stars mostly concentrated near center Disk Gas ~10 billiion
20 billion
•Treat gravity as if it all comes from a Bulge
Halo Stars 1 billion
point source in the center
Nucleus 4 million
•Use Kepler’s Third
If all the mass is in the center, how would
law: MP2 = a3
we expect the velocity of the stars to
d 2 a
M change as we move away from the center?
v 
 2
t
P
a A) Fastest for stars near the center
B) Fastest for stars far out
C) Roughly same speed for all
Where is the Mass?
•Measure
rotational
velocities
using Doppler
shift of 21 cm
line
•Plot vs.
distance from
center of
galaxy
Dark Matter
•85% of mass is not concentrated in the center
•It does not glow, it is dark
•We do not know what it is
•We know a lot of things it isn’t:
•Living Stars
•Gas
What is the dark matter?
•Dust
A) Neutron Stars
•Could it be
B) Black Holes
MACHOs?
C) White Dwarfs
D) Planets without stars
E) Something else
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
Massive
Compact
Halo
Objects
MACHOs
How to catch a MACHO
•All of these objects are dark
•But they do have gravity!
•Einstein says they can bend light
•They can magnify a distant star
MACHO:
MACHOs
Neutron Stars
Black Holes
White Dwarfs
Planets without stars
We see:
MACHO’s: What the Data Tells Us
•MACHO’s do exist
•Substantial fraction
of stars
•But not the
majority
•There are not enough
of them to account for
the dark matter
•They are probably
mostly white dwarfs
•The disk
•The bulge
•The nucleus
•The halo
•Globular
clusters
•Most likely
dark matter
is some
weird new
particle