Lecture 22
1
PRS: According to modern ideas and observations, what can be said
about the location of the center of the Universe?
1. The Sun is at the center.
2. The Milky Way is at the center.
3. An unknown, distant galaxy is at the center.
4. A black hole is at the center.
5. The Universe does not have a center.
3
Recession Velocity
Hubble’s Diagram
500
km/s
0
1 Mpc
v = c × ∆λ ⁄ λ
In the early 1900’s it was discovered that most
galaxies appeared to have large Doppler shifts.
Cepheid Variables
2 Mpc
Note: nearly all galaxies are shifted to longer wavelengths,
although a few nearby ones are blueshifted.
5
PRS: Distance Uncertainties
Suppose we learned tomorrow that Cepheids
are more luminous than we had previously
thought. How would that affect our estimates
of the distances and sizes of galaxies? The
distances would be ____ and the sizes ____.
(1) farther; smaller
(2) farther; bigger
(3) nearer; smaller
(4) nearer; bigger
Answer: 2
4
• Used to calibrate Hubble’s
Law.
• Luminous yellow
supergiants with unstable
atmospheres that pulsate:
a “stable” instability
• Longer Periods of
oscillation happen in more
Luminous stars.
• Measure P, get L, and then
get the distance.
1000
km/s
Distance
2
Hubble discovered
most galaxies
seemed to be
moving away
from the Milky
Way…
The speed of recession
is from the
Doppler formula:
Hubble’s Law and
the Large Scale
Structure of the
Universe
SOME NEGATIVE EFFECTS OF THE
EXPANSION OF THE UNIVERSE….
0
The Cosmological Redshift
6
Standard Candles for distance
• A light source whose luminosity we know.
• Apply distance-brightness law:
L
4πB
• High L stars can be used to distances >10 Mpc.
• Also used to calibrate Hubble’s Law.
• Higher L supernovae can be used out to billions
of parsecs (remember Type Ia are all the same L).
d=
1
Hubble’s Law (see Lec 22, 23)
7
PRS: Redshift Distance Relationship
• Since galaxies follow a straight line in Hubble’s
diagram, we can write an equation for the slope:
Assuming a Hubble constant of 70 km/sec
per Mpc, how far away is a galaxy that is
traveling at 2100 km/sec?
v = Ho d
• Hubble’s Constant Ho is the slope of the line.
• We can determine a galaxy’s distance from its
redshift velocity: d = v/ Ho
• Note that individual galaxies may deviate from
the line if they have local gravitational influences.
The Cosmic Distance Ladder
8
9
At larger distances we have to find new approaches because
the previous techniques don’t work anymore.
(1)
(2)
(3)
(4)
(2100 x 70) km
(2100 / 70) km
(2100 x 70) Mpc
(2100 / 70) Mpc
Hubble' s law :
v = H0 × d
Answer: 4
Hubble’s observed redshift – is
not really a Doppler shift….
10
v = Ho x d
z=
At smaller
distances it is not always
possible to apply the techniques we use far away
If we see
a galaxy
with light
redshifted to
5× its original
wavelength, it
left the galaxy
when the universe was 5 × smaller!
z=
∆λ
λ
λ
In GR, space can move. In fact, GR predicts that
the space of the universe can expand.
Cause of the Cosmological Redshift 11
Light waves are stretched out as they travel
through expanding space.
∆λ
12
Are we at the center
of the universe?
It seems like the Milky Way occupies
a special position in the universe,
but not really.
Consider raisins in the expanding
dough of raisin bread. A raisin may
see a nearby neighbor traveling
away at 2 cm/hr. A raisin 5 times
farther away travels at 10 cm/hr, etc.
If the edge cannot be seen, it looks
to each raisin as if it is at the center!
2
Lookback Time
13
14
From the redshift of a
galaxy, and using a
cosmological model, the
time since the light left
the distant galaxy can be
determined. Since the
universe is expanding,
The Hubble Deep Field is a
Time Machine!
the true distance today is farther than just the
lookback time as light years (space expands
behind the photon). 14 billion ly 46 billion ly.
HDF Galaxies by Redshift—shows Galaxy evolution 15
What Are Quasars?
16
• The HST shows that
quasars are located in the
centers of galaxies.
• The galaxies often appear
to be interacting.
• Overall morphology is
like radio galaxies.
• Quasars are produced by
very active supermassive
black holes.
17
Computer Modeling of
Galaxy Interactions
Formation of an 18
Elliptical from a
Group of Galaxies
• Model individual particles interacting
gravitationally.
• Two galaxies near to each other will disturb the
orbits within each galaxy.
• “N-body simulations” are more and more realistic
as computers become faster.
Note all the other spiral, bar and irregular shapes
during the process of interaction.
3
Mergers in the Milky Way
19
20
The Milky Way and M31
• Analysis of stars
around the Milky
Way shows several
“streams”
• These appear to be
small galaxies that
have been
“cannibalized” by
the Milky Way
• The Milky Way is “falling” toward M31, so in
about 10 billion years…
21
What is our cosmic address?
UMass
Amherst
Massachusetts
USA
Earth
Our Place in the Milky Way
22
X
Solar system?
Spiral arm?
?
Outward from the Milky Way
We live on the inner edge of the “Orion” spiral arm about
2/3rds of the way out in the disk.
(photo is of a similar type of galaxy as the Milky Way)
23
Observing M31 Directly…
24
~50 kpc away
The 2-Micron All Sky Survey mapped
the whole sky at infrared wavelengths.
Note the Magellanic Clouds
M31 is the most distant object visible with the naked eye.
Galaxies look very dim because the stars are widely spaced.
The details we see in images are based on long exposures.
4
M31-Great Nebula in Andromeda
25
M31-Great Nebula in Andromeda
26
dwarf galaxies
orbiting M31
M31 is about 2 million
light years away and
bigger than the Milky Way.
The Cepheids measured by Hubble indicated that M31 is
~2 million light years away and bigger than the Milky Way.
D~700 kpc
R~20 kpc
The Local Group
It is the most distant object visible with the naked eye.
27
28
The Virgo Cluster
~3 dozen
D~700 kpc
R~5 kpc
D~50 kpc
R~2kpc
The Local Group is about 15 Mpc from a dense
cluster of galaxies in the constellation Virgo
Clusters of Galaxies... and of gas!
29
Clusters of Galaxies
30
• Galaxies are found clustered, from rich groups
with 100s to 1000s of galaxies, to poor groups
with only a few.
• Clusters are dynamic! Although we see only a
“snapshot” in time, different clusters show galaxy
mergers, acquisition of galaxies, interactions….
• Because the galaxies interact, clusters tend to
contain few spirals and mostly ellipticals.
• Composite Chandra X-ray / Kitt Peak optical image of
Abell 2029, showing intra-cluster gas.
• Gravity holds the galaxies together, and they can
fall through the center—in the process, gas is
stripped out of them into the intra-cluster medium.
5
Clusters can even cluster!
Redshift Surveys
31
32
• By measuring
redshifts of many
galaxies, we can
trace structure as in
an MRI scan.
• In the “slice” at
right each dot is a
galaxy with a
distance from its
redshift.
• Note the “voids”
and “superclusters”
Mapping Out Large
Scale Structure
33
Huge new surveys are revealing structure out
to billions of light years distance.
6