What does our galaxy look like?
The Milky Way galaxy appears in our sky as a faint band of light
We see our galaxy edge-on
Primary features: disk, bulge, halo, globular clusters
Halo: No ionization nebulae, no blue stars
 no star formation
Disk: Ionization nebulae, blue stars  star formation
If we could view the Milky Way from above the
disk, we would see its spiral arms
How do stars orbit in our galaxy?
Stars in the disk all orbit in the same direction with a
little up-and-down motion
Orbits of stars
in the bulge
and halo have
random
orientations
Sun’s orbital
motion (radius
and velocity) tells
us mass within
Sun’s orbit:
1.0 x 1011 MSun
Much of star
formation in disk
happens in spiral
arms
Ionization Nebulae
Blue Stars
Gas Clouds
Whirlpool Galaxy
Spiral arms are waves
of star formation
1. Gas clouds get
squeezed as they
move into spiral
arms
2. Squeezing of clouds
triggers star
formation
3. Young stars flow
out of spiral arms
Saturn’s Rings: Natural Color
15km/s
< 1cm/s
15km/s
Unseen Influences
Dark Matter: An undetected form of mass that emits little or
no light but whose existence we infer from its gravitational
influence
Dark Energy: An unknown form of energy that seems to be
the source of a repulsive force causing the expansion of the
universe to accelerate
Mass within Sun’s
orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun
What is the evidence for dark
matter in galaxies?
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
Contents of Universe
• “Normal” Matter:
~ 4.4%
– Normal Matter inside stars:
– Normal Matter outside stars:
• Dark Matter:
• Dark Energy
~ 0.6%
~ 3.8%
~ 25%
~ 71%
We can
measure
rotation
curves of
other spiral
galaxies
using the
Doppler
shift of the
21-cm line
of atomic H
Gravitational lensing, the bending of light rays by
gravity, can also tell us a cluster’s mass
All three methods of measuring cluster mass indicate
similar amounts of dark matter
Clusters contain
large amounts of Xray emitting hot gas
Temperature of hot
gas (particle
motions) tells us
cluster mass:
85% dark matter
13% hot gas
2% stars
Our Options
1. Dark matter really exists, and we are observing
the effects of its gravitational attraction
2. Something is wrong with our understanding of
gravity, causing us to mistakenly infer the
existence of dark matter
How dark is it?
… not as bright as a star.
Two Basic Options
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
Halo stars formed first as gravity caused cloud to contract
Remaining gas settled into spinning disk
Detailed studies: Halo stars formed in clumps that later
merged
Our Galaxy
Earth
Sun
Earth orbit
Solar system
Nearest star
=
=
=
=
=
100 nm
10 μm
¼ cm
20 cm
250 m
=
=
=
=
=
virus
cell
pin head
saucer
lawn
Jewel Box
Cluster
Naked eye
stars
Eagle
Nebula
Galaxy Center
Crab
Nebula
Solar system
NGC 3310
Spiral Galaxy
NGC 5194
Spiral Galaxy
NGC 1365
Barred Spiral Galaxy
Interacting
galaxies