Announcements
• Reading for next class: Chapters 22.6, 23
• Cosmos Assignment 4,
Due Wednesday, April 21, Angel Quiz
• Monday, April 26
Quiz 3 & Review, chapters 16-23
• Wednesday, April 28,
Midterm 3: chapters 16-23
1
What are Galaxies?
Galaxies are vast collections of stars (~1011)
and sometimes gas and dust as well
3
Universe is Expanding
•
•
•
•
•
You and I are NOT expanding
The solar system is NOT expanding
The Milky Way Galaxy is NOT expanding
Our local group of Galaxies is NOT expanding
Nothing that is bound together by a force is
expanding
• SPACE between groups of galaxies IS expanding
4
Hubble’s Law
Velocity = Hubble’s Constant x Distance
V = HD
If you are twice as far away,
you are moving away twice as fast,
so you started moving away at the same
time!
How long ago was that?
5
Age of the Universe
• V=HD
• D = VT = V/H
• T = D/V = 1/H
Age if expansion not accelerated or
decelerated
6
Fig 22.18
7
Questions:
How did galaxies form?
Why are there different types of Galaxies?
8
Galaxy Formation
Similar to star formation
• H & He gas filled space almost uniformly
• Where density slightly greater, gravity slightly
greater
• Matter falls into gravitational potential well,
increases gravity
• Matter pulled in by more gravity, density excess
grows
• Densest cores became 1st generation massive
stars
9
Galaxy Formation Models
Assumptions:
• Matter originally
filled all of space
almost uniformly
• Gravity of denser
regions pulled in
surrounding
matter
10
Galaxy Formation:
Gravitational Instability
11
Clues to Galaxy Formation
Halo stars are old, have randomly oriented orbits
Disk has young stars with orbits nearly in plane
Initially gravity pulled in matter from all
directions. Stars formed during this stage have
random orbits passing close to center
Later, rotation made any remaining gas flatten
into disk. Stars forming after this have orbits in
disk.
12
Spiral or Elliptical Galaxy?
Possible explanations
1. Rate of star formation
2. Amount of Rotation
3. Collisions & mergers
13
Density Excess?
Higher density proto-galactic clouds form stars more rapidly,
use up all their gas before it can form a disk.
14
Rotation?
Larger rotation produces more disk-like distribution of matter.
15
Collisions & Mergers
Question 1: If the Milky Way were the size of
a grapefruit, where would the Andromeda
galaxy (nearest comparable size galaxy)
be?
A.
B.
C.
D.
E.
F.
About 1 cm away
About 1 m away
About 1 km away
About San Francisco
About the Moon
About the Sun
16
Collisions & Mergers
Question 1: If the Milky Way were the size of
a grapefruit, where would the Andromeda
galaxy (nearest comparable size galaxy)
be?
A.
B.
C.
D.
E.
F.
About 1 cm away
About 1 m away (~3 m)
About 1 km away
About San Francisco
About the Moon
About the Sun
17
Galaxies are Close together
simulation of galaxy interaction via gravity
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
18
Galaxies are close together
Evidence of galaxy interactions via gravity
Burst of
star
formation
19
Evidence of Galaxy mergers
Super massive galaxies in the centers of clusters of
galaxies
Merged
galaxies
See also
Fig 21.11
22
Galaxy Merger Simulation
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
23
Elliptical
galaxies are
much more
common in
huge clusters
of galaxies
1. Denser cloud
2. More collisions
(hundreds to
thousands of
galaxies)
24
How do we know?
When we
look
farther out
in space
we are
looking
farther
back in
time. See
galaxies at
different
stages
in their
25
evolution
Expansion of the Universe
26
Problem!
• In an expanding universe, gravity takes
longer to pull matter together.
Need stronger gravity
Need more mass
Don’t see any more mass
Postulate existence of DARK MATTER
27
Other evidence for Dark Matter
1. Rotation of galaxies
2. Motions in clusters of galaxies
3. Hot x-ray emitting gas in clusters of
galaxies
4. Gravitational lensing
28
Rotation
Curve
A plot of orbital
velocity versus
orbital radius
Solar system’s
rotation curve
declines because
Sun has almost all
the mass &
gravity gets
weaker with
Increasing 29
distance
Rotation
Curve
of Milky Way
stays flat with
distance
Mass must be
more spread out
than in solar
system
30
Milky Way
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Significant mass exists outside radius of most stars!
31
Mass of
Milky
Way
Mass within
Sun’s orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun
-> Dark Matter
32
Motions of Galaxies in Clusters
• Galaxies are moving too fast in clusters of
galaxies to be held together by the gravity
of the visible stellar material
Total Mass ~ 10x visible mass
33
Hot Gas in Galaxy Clusters
• Galaxy clusters are filled with hot gas that
emits x-rays. Temperature ~ 100 million K
• Pressure of hot gas must be balanced by
gravity to hold it together in the cluster.
Total Mass ~ 10x visible mass
34
Gravitational Lensing
• Mass produces gravity
• Gravity bends light
• Gravity can distort the
image of an object
behind the mass
35
Galaxy
cluster
acts as
gravitational
lens.
Focuses
image of
galaxy
behind
it into
blue arcs.
36
What is Dark Matter?
• We DON’T KNOW!!!
• We only know what it is not
 It is not ordinary matter composed of
protons, neutrons, electrons, etc.
37
Large Scale Structure of the Universe
38
Galaxies
are
arranged
like soap
bubbles
with
voids
devoid of
galaxies
inside
39