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Galaxies and the Foundation of
Modern Cosmology
Islands of Stars
Our goals for learning:
• How are the lives of galaxies connected
with the history of the universe?
• What are the three major types of galaxies?
• How are galaxies grouped together?
Recall: The Sun is only one star among well over 100 billion
stars in our own galaxy, the Milky Way, which has the form of
a highly flattened “disk” of stars some 100,000 light-years in
diameter. Galaxies are the “building blocks” of the Universe.
Andromeda Galaxy
• https://www.youtube.com/watch?v=udAL4
8P5NJU
How are the lives of galaxies connected
with the history of the universe?
• Our deepest images of the universe show a great
variety of galaxies, some of them billions of lightyears away
This is the Hubble Deep Field,
a small, apparently blank, piece
of sky near the Big Dipper and
well out of the Galactic Plane.
Hubble Space
Telescope image
Big Dipper
Galaxies and Cosmology
• A galaxy’s age, its
distance, and the age
of the universe are all
closely related
• The study of galaxies
is thus intimately
connected with
cosmology—the
study of the structure
and evolution of the
universe
What are the three
major types of galaxies?
Hubble
Ultra
Deep
Field
Hubble
Ultra
Deep
Field
Hubble
Ultra
Deep
Field
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SPIRAL GALAXY
Disk Component:
stars of all ages,
many gas clouds
Spheroidal Component:
bulge & halo, old stars,
few gas clouds
Another view of a
spiral galaxy
Disk
Component:
stars of all
ages,
many gas
clouds
Spheroidal
Component:
bulge & halo,
old stars,
few gas
clouds
Thought Question
Why does ongoing star formation lead to a bluewhite appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light
Thought Question
Why does ongoing star formation lead to a bluewhite appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light
Barred Spiral Galaxy: Has a bar of stars across the bulge
Lenticular
Galaxy:
Has a disk like
a spiral galaxy
but much less
dusty gas
(intermediate
between spiral
and elliptical)
Elliptical
Galaxy:
All spheroidal
component,
virtually no
disk
component
Red-yellow
color indicates
older star
population
Irregular Galaxy
Blue-white color indicates ongoing star formation
Edwin Hubble’s galaxy classes
Spheroid
Dominates
Disk
Dominates
How are galaxies grouped together?
Spiral galaxies
are often found
in groups of
galaxies
(up to a few
dozen galaxies)
Elliptical
galaxies are
much more
common in
huge clusters
of galaxies
(hundreds to
thousands of
galaxies)
What have we learned?
• How are the lives of galaxies connected
with the history of the universe?
– Galaxies generally formed when the universe
was young and have aged along with the
universe
• What are the three major types of
galaxies?
– Spiral galaxies, elliptical galaxies, and
irregular galaxies
– Spirals have both disk and spheroidal
components; ellipticals have no disk
What have we learned?
• How are galaxies grouped together?
– Spiral galaxies tend to collect into groups of
up to a few dozen galaxies
– Elliptical galaxies are more common in large
clusters containing hundreds to thousands of
galaxies
Measuring Galactic Distances
Our goals for learning:
• How do we measure the distances to
galaxies?
Recall:
Brightness
alone does not
provide
enough
information to
measure
distance.
We must know
the luminosity
(power output)
of the object.
Distances to
astronomical
objects can be
determined using
a series of overlapping steps in
what is called the
DISTANCE
LADDER.
Step 1
Determine size
of solar system
using radar.
Step 2
p
Determine
distances of stars
out to a few
hundred lightyears using
parallax
(trigonometry)
d
d = 1/p
Distance in parsecs
= 1/parallax angle
in seconds of arc;
1pc = 3.26 ly
Relationship between apparent brightness
and luminosity depends on distance:
Brightness =
Luminosity
4π (distance)2
We can determine a star’s distance if we know its
luminosity and can measure its apparent brightness:
Distance =
Luminosity
4π x Brightness
A standard candle is an object whose luminosity we
can determine without measuring its distance
Step 3
The apparent
brightness of a
star cluster’s
Main Sequence
tells us its
distance
Then, knowing a star cluster’s distance, we can determine
the luminosity of each type of star within it.
Cepheid
variable stars:
are very
luminous and
can therefore be
seen over great
distances
These stars are
excellent standard
candles because of
a relationship
between their
average luminosity
and their period of
variation, thus …
Step 4
Observe the variations in
apparent magnitude of a
Cepheid-type star and find
its period. Read off the
luminosity corresponding
to this period.
Calculate the distance.
Cepheid variable stars with longer periods
have greater luminosities. Edwin Hubble
found Cepheid’s beyond the Milky Way
in nearby galaxies. The Hubble Space
Telescope Key Project extended this to
even more distant galaxies.
White-dwarf
supernovae
can also be
used as
standard
candles.
Incredibly
luminous!
Implies very
large distances
These supernovae occur when a white dwarf in a binary system
reaches the 1.4 MSun limit – same mass for all, same Luminosity.
Step 5
Apparent
brightness of
white-dwarf
supernova tells
us the distance
to its host
galaxy. This
technique
works for up to
10 billion lightyears!
Tully-Fisher
Relation
Entire galaxies
can also be
used as
standard
candles
because galaxy
luminosity is
related to
rotation speed
THE DISTANCE LADDER
We measure galaxy distances using
a chain of interdependent
techniques
What have we learned?
• How do we measure the distances to
galaxies?
– The distance-measurement chain begins with
parallax measurements that build on radar
ranging in our solar system
– Using parallax and the relationship between
luminosity, distance, and brightness, we can
calibrate a series of standard candles
– We can measure distances greater than 10
billion light years using white dwarf
supernovae as standard candles
Hubble’s Law
Our goals for learning:
• How did Hubble prove that galaxies lie far beyond
the Milky Way?
• What is Hubble’s Law?
• How do distance measurements tell us the age of
the universe?
• How does the universe’s expansion affect our
distance measurements?
How did Hubble prove that galaxies
lie far beyond the Milky Way?
The Puzzle of “Spiral Nebulae”
• Before Edwin Hubble, some
scientists argued that “spiral
nebulae” were entire galaxies like
our Milky Way, while others
maintained they were smaller
collections of stars within the Milky
Way
• Hubble settled the debate by
measuring the distance to the
Andromeda Galaxy (M31) using
Cepheid variables as standard
candles; ~2.5 million light-years
The spectral features of all galaxies beyond the Local Group
are redshifted  they are all moving away from us!
Hubble’s Law:
By measuring
distances to
galaxies, Edwin
Hubble found
that redshift (the
amount by which
spectral lines
were shifted) and
distance are
related in a
special way.
The redshift
gives a velocity.
Using galaxies
of known
distance (from
Cepheid
Variables) one
gets the slope
of the line (Hnaught) ,
known as
Hubble’s
constant.
Hubble’s Law:
velocity = H0 x distance
Inverting Hubble’s Law
For an unknown
distance, the spectrum
redshift of a galaxy
tells us its distance
through Hubble’s Law:
distance =
velocity
H0
Distances of farthest galaxies are measured from redshifts
How do distance measurements
tell us the age of the universe?
The more distant a galaxy, the faster it is receding from us.
Hubble interpreted this as evidence for expansion of the universe.
Thought Question
Your friend leaves your house. She later calls
you on her cell phone, saying that she’s been
driving at 60 miles an hour directly away from
you the whole time and is now 60 miles away.
How long has she been gone?
A.
B.
C.
D.
1 minute
30 minutes
60 minutes
120 minutes
Thought Question
Your friend leaves your house. She later calls you on her
cell phone, saying that she’s been driving at 60 miles an
hour directly away from you the whole time and is now
60 miles away. How long has she been gone?
A.
B.
C.
D.
1 minute
30 minutes
60 minutes
120 minutes
Interpreting the
Hubble Law
Does the fact that all
galaxies are receding
from us mean that we
are at the center of an
explosion?
Hubble’s law tells us
that the Universe is
expanding and that it
has no center and no
edge (as far as we can
tell).
ANALOGY FOR THE
EXPANSION OF THE
UNIVERSE
Fixed size
dots on the
surface of
a balloon.
Dots are
galaxies
held
together
by gravity.
One example of something that expands but has no center
or edge is the surface of a balloon.
Cosmological Principle
The universe looks about the same no matter
where you are within it
• Matter is evenly distributed on very large scales
in the universe
• No center & no edges
• Not proved but consistent with all observations to
date
Thought Question
Your observe a galaxy moving away from you at
0.1 light-years per year, and it is now 1.4 billion
light-years away from you. How long has it
taken to get there?
A.
B.
C.
D.
1 million years
14 million years
10 billion years
14 billion years
Thought Question
Your observe a galaxy moving away from you at
0.1 light-years per year, and it is now 1.4 billion
light-years away from you. How long has it
taken to get there?
A.
B.
C.
D.
1 million years
14 million years
10 billion years
14 billion years (t = D/V = 1.4 billion ly divided by
0.1 ly per yr)
Hubble’s
constant tells us
age of universe
because it
relates
velocities and
distances of all
galaxies
Age =
Distance
Velocity
~ 1 / H0
How does the universe’s expansion
affect our distance measurements?
Distances
between
faraway
galaxies
change while
light travels
distance?
Astronomers
think in terms of
lookback time
rather than
distance
Cosmological Redshift
Cosmological Horizon
Maximum lookback time of
about 14 billion years limits
how far we can “see”
Expansion stretches photon wavelengths, causing a
cosmological redshift directly related to lookback time
What have we learned?
• How did Hubble prove that galaxies lie far
beyond the Milky Way?
– He measured the distance to the Andromeda
galaxy using Cepheid variable stars as
standard candles
• What is Hubble’s Law?
– The faster a galaxy is moving away from us,
the greater its distance:
velocity = H0 x distance
What have we learned?
• How do distance measurements tell us the
age of the universe?
– Measuring a galaxy’s distance and speed
allows us to figure out how long the galaxy
took to reach its current distance
– Measuring Hubble’s constant tells us that
amount of time: about 14 billion years
• How does the universe’s expansion affect
our distance measurements?
– Lookback time is easier to define than
distance for objects whose distances grow
while their light travels to Earth
Galaxy Evolution
Looking Back Through Time
Our goals for learning:
• How do we observe the life histories of
galaxies?
• How did galaxies form?
Deep images
(meaning a long
exposure to
reveal faint
objects) show
us very distant
galaxies
We see those
galaxies as they
were much
earlier in time!
Old light from
young galaxies
Observing galaxies at different distances shows how they age; this
works because of the finite speed of light and the enormous distances.
By looking back in time we can begin to see how galaxies form
We still can’t directly observe the earliest galaxies
Our best models for
galaxy formation
assume:
• Matter originally
filled all of space
almost uniformly
• Gravity of denser
regions pulled in
surrounding
matter - clumps
Denser regions contracted,
forming protogalactic clouds
H and He gases in these clouds
formed the very first stars
Supernova explosions from first
stars kept much of the gas from
forming stars
Leftover gas settled into spinning
disk - Conservation of angular
momentum
But why do some galaxies end up looking so different?
NGC 4414
M87
Look back at the Hubble Space Telescope pictures and we
notice that the youngest galaxies are more irregular and
closer together – galaxies merge and change over time
What have we learned?
• How do we observe the life histories of
galaxies?
– Deep observations of the universe are
showing us the history of galaxies because we
are seeing galaxies as they were at different
ages
• How did galaxies form?
– Our best models for galaxy formation assume
that gravity made galaxies out of regions of
the early universe that were slightly denser
than their surroundings
The Lives of Galaxies
Our goals for learning:
• Why do galaxies differ?
• What are starbursts?
Edwin Hubble’s Tuning Fork Diagram
Why don’t all galaxies have similar disks of
stars?
Conditions in Protogalactic Cloud?
Spin: Initial angular momentum of protogalactic
cloud could determine size of resulting disk
Conditions in Protogalactic Cloud?
Density: Elliptical galaxies could come from dense
protogalactic clouds that were able to cool and
form stars before gas settled into a disk
Distant Red Ellipticals
• Observations of
some distant red
elliptical galaxies
support the idea that
most of their stars
formed very early in
the history of the
universe
• All the stars in this
galaxy are old (red)
and any young blue
stars have died
We must also consider the effects of collisions
Collisions were much more likely early in time, because
galaxies were closer together–remember space is expanding
Many of the galaxies we see at great distances (and early
times) indeed look violently disturbed–evidence of merging
The galaxy collisions that we observe in nearby systems
clearly trigger bursts of star formation–hot blue stars
What is the end result when galaxies collide?
Modeling such collisions on a computer shows that two
spiral galaxies can merge to make an elliptical
Collisions
may explain
why elliptical
galaxies tend
to be found
where
galaxies are
closer
together, as
in this large
cluster of
galaxies
WHAT ARE STARBURSTS
Starburst
galaxies are
forming stars
so quickly
they would
use up all
their gas in
less than a
billion years
These
galaxies tend
to be very
bright in the
infrared
The intensity of supernova explosions in starburst galaxies can
drive galactic winds that expel material far above the galaxy
This X-ray image
from the Chandra
Space
Telescope shows
individual
supernovae
and bubbles of hot
gas.
Most of the gas
can be driven out
of the galaxy!
Intensity of supernova explosions in
starburst galaxies can drive galactic winds
What have we learned?
• Why do galaxies differ?
– Some of the differences between galaxies may
arise from the conditions in their protogalactic
clouds
– Collisions can play a major role because they
can transform two spiral galaxies into an
elliptical galaxy
• What are starbursts?
– A starburst galaxy is transforming its gas into
stars much more rapidly than a normal galaxy
Quasars and Other Active Galactic
Nuclei
Our goals for learning:
• What are quasars?
• What is the power source for quasars and
other active galactic nuclei?
• Do supermassive black holes really exist?
• How do quasars let us study gas between
the galaxies?
If the center of a
galaxy is unusually
bright we call it an
active galactic
nucleus
Quasars are the
most luminous
examples
Active Nucleus in M87
The word stands for
quasi-stellar objects
associated with
radio sources …
Mid-1960s very intense, star-like radio sources discovered. Associated with faint
stars but spectra with 200-inch (5m) Palomar telescope unlike any star. Maarten
Schmidt (Caltech) realized the spectrum was highly redshifted hydrogen emission.
•The highly redshifted spectra of quasars indicate large
distances according to Hubble’s Law
•From brightness and distance we find that luminosities of
some quasars are >1012 LSun (10x the entire Milky Way)
Redshift & Variability
• Redshift is defined by
z = change in wavelength divided by original wavelength
Usually, z = v/c and so z = 0.2  20% speed of light
But quasars with z = 6 have been found. This does NOT mean a
velocity = 6x the speed of light!!!
Why? Because the redshift is cosmological – due to expansion of
space – NOT due to the Doppler Effect, so z is not equal to v/c
In fact, 1/1+z is the size of the universe compared to now; z = 6
means that we see the object at a time when the universe was
1/7 of its present size
• Quasar light is variable over a time of a few hours. All the quasar
energy must come from a region smaller than solar system.
Thought Question
What can you conclude from the fact that quasars
usually have very large redshifts?
A.
B.
C.
D.
They are generally very distant
They were more common early in time
Galaxy collisions might turn them on
Nearby galaxies might hold dead quasars
Thought Question
What can you conclude from the fact that quasars
usually have very large redshifts?
A.
B.
C.
D.
They are generally very distant
They were more common early in time
Galaxy collisions might turn them on
Nearby galaxies might hold dead quasars
All of the above!
Galaxies
around
quasars
sometimes
appear
disturbed by
collisions
Quasars powerfully radiate energy over a very wide
range of wavelengths, indicating that they contain
matter with a wide range of temperatures
RADIO IMAGE
Radio galaxies contain active nuclei shooting out vast jets
of plasma that emits radio waves coming from electrons
moving at near light speed. Note the scale of the radio
“lobes” – over 100,000 light-years!
But the lobes of some radio galaxies can extend over
hundreds of millions of light years! What drives such
enormous outflows of matter from a galaxy?
An active galactic
nucleus can shoot
out blobs of
plasma (ionized
gas) moving at
nearly the speed
of light!
False color images of an active galaxy
Speed of ejection
suggests that a
black hole is
present …
Quasars and
radio galaxies
are related:
Radio galaxies
don’t appear
as quasars
because dusty
gas clouds
block our
view of the
hot accretion
disk, but we
can see the jet
and the radio
lobes.
Characteristics of Active Galaxies
• Luminosity can be enormous (>1012 LSun)
• Luminosity can rapidly vary (comes from a
space smaller than solar system)
• Emit energy over a wide range of wavelengths
(contain matter with wide temperature range)
• Some drive jets of plasma at near light speed
What is the power source for
quasars and other active galactic
nuclei?
A truly enormous black hole
Quasars are super
massive Black Holes at
the centers of early
galaxies.
Accretion of gas onto a supermassive black hole appears
to be the only way to explain all the properties of quasars
Energy from a Black Hole
• Gravitational potential energy of matter falling
into black hole turns into kinetic energy – the
speeds are very high
• Friction in the accretion disk turns kinetic energy
into thermal energy (heat) – temperatures are very
high (X-rays emitted)
• Heat produces thermal radiation (photons)
• This process can convert 10-40% of the rest-mass
energy (E = mc2) into radiation – hence the high
luminosity
Jets are thought to come from twisting of magnetic fields
in the inner part of the accretion disk
Do supermassive black holes
really exist?
What is the evidence?
Recall:
The orbits of
stars at center
of Milky Way
indicate a black
hole with a
mass of
4 million MSun
Can we use
motions to derive
the mass of a
quasar black
hole?
Yes! The orbital speed and distance of gas orbiting the
center of the galaxy M87 indicate a black hole with mass
of 3 billion MSun – that is almost 1000x the mass of our
black hole. This is enormous!
Black Holes in Galaxies
• Many nearby galaxies – perhaps all of them –
have supermassive black holes at their centers
• These black holes seem to be dormant active
galactic nuclei
– The Milky Way’s black hole is very quiet (dormant)
• All galaxies may have passed through a quasarlike stage earlier in time
– The earliest galaxies were bigger (more merging)
– Their black holes are much bigger (quasars)
Galaxies and Black Holes
• The mass of a
galaxy’s
central black
hole is closely
related to the
mass of its
bulge of stars
• The larger the
bulge mass the
greater the
black hole mass
The development of the central black hole must be somehow
related to galaxy evolution.
How do quasars let us study gas between the galaxies?
Intergalactic gas clouds between a quasar and Earth absorb
some of a quasar’s light
We can learn about protogalactic clouds by studying the
absorption lines they produce in quasar spectra
What have we learned?
• What are quasars?
– Active galactic nuclei are very bright objects
seen in the centers of some galaxies, and
quasars are the most luminous type
• What is the power source for quasars and
other active galactic nuclei?
– The only model that adequately explains the
observations holds that supermassive black
holes are the power source
What have we learned?
• Do supermassive black holes really exist?
– Observations of stars and gas clouds orbiting
at the centers of galaxies indicate that many
galaxies, and perhaps all of them, have
supermassive black holes
• How do quasars let us study gas between
the galaxies?
– Absorption lines in the spectra of quasars tell
us about intergalactic clouds between those
quasars and Earth