Download The Galaxies

Lecture 23
Galaxies and the Universe
FINAL EXAM NEXT WEDNESDAY!
► The
Final is next Wednesday, May 9th.
 5 to 7 pm!!!!
► Required
 Pencil/pen
 Equation sheet
► Recommended
 Calculator
 Scratch paper
TEST DETAILS
► Covers
Primarily Lectures 18 - 23
 18 multiple-choice questions (2 points each)
 12 True/False questions (2 points each)
 6 Short answer/problem questions (5 points
each)
 Equation sheet (10 points)
► Mostly
conceptual
 2/3 new material
 1/3 previous material (basic level)
Discovery Of The Galaxies
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The discovery of the nature of
the galaxies happened only in
the last century.
During the 1920's, Heber
Curtis and Harlow Shapley
debated the nature and size of
Milky Way Galaxy.
Shapley thought that the Milky
Way was very large and the
"spiral nebulas" were smaller
and within it.
Curtis argued that the Milky
Way was considerably smaller
and that the galaxies were
island universes comparable in
size to the Milky Way and very
far away.
The Galaxy Takes Shape
► By
the 18th century, astronomers think the
stars are distant suns.
► But how are they distributed through space?
► Herschel and others attempt to make the
first map.
Here’s Herschel’s Map:
By Early 20th Century:
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Astronomers have an estimate for the size of
the Milky Way.
Estimate based on careful study of star
brightness and motions.
Shaped like a flattened disk
Sun is near the
center.
Empty void
outside of the star
system.
Galaxy is 32,000 LY across
But That’s Not Right!
► Modern
View:
 Milky Way is 100,000 LY across!
 Surrounded by countless other galaxies!
► Why
were we wrong?
► How did we find the correct answer?
Why Were We Wrong?
► Based
on one incorrect assumption:
► Space was totally empty. No dust or gas to
obscure our view. We can see to great
distances.
Harlow Shapley
► Studied
two types of common star clusters.
 Open clusters
 Globular clusters
Open Clusters
► Young
stars.
► Hundreds to
Thousands of stars.
► Lie all along the Milky
Way’s band.
Globular Clusters
► Old
stars
► 100,000s or millions of
stars.
► Very dense.
► About half are
clustered around
Sagittarius.
Shapley’s Assumption
► Star
clusters orbit center of the Galaxy.
► Globulars centered around Sagittarius, so they
must orbit something in that direction.
► The Galaxy’s center is NOT near the sun, it’s far
away, in the direction of Sagittarius.
But How Far Away?
► Shapley
found
the clusters were
centered on a
point about
25,000 LY away,
in the direction
of Sagittarius.
But How Far Away?
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Notice how small
Herschel’s Milky Way
Map is when put onto
Shapley’s Map.
In fact, if Hershel’s
map represents one
end of the Milky Way,
Shapley’s implies the
Milky Way is over
100,000 LY across!
Turning Point
► Major
turning point in astronomy.
► It was discovered that the sun was not at
the center of a small cloud of stars.
► It was at the edge of a HUGE galaxy of
stars.
Cepheids In The Andromeda
Galaxy
Edwin Hubble used the
2.5 meter (100") telescope
on Mt. Wilson in 1925 to
observe Cepheid variable
stars in the Andromeda
Galaxy and showed it was
over 2 million light years
away - well beyond the limit
of the Milky Way Galaxy,
proving Shapley wrong.
► On the other hand, the
Galaxy was much larger
than Curtis imagined.
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But What Is Our Galaxy Like?
► The
Milky Way is an
example of a special
type of spiral galaxy
called a barred
spiral.
But What Is Our Galaxy Like?
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1. Many of the Milky
Way’s stars are in the disk
of the galaxy. This disk is
100,000 LY across, but
very thin (~1,000 LY).
The disk contains the
spiral arms.
But What Is Our Galaxy Like?
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2. In the center of the
disk is the nuclear bulge,
which has a foot-ball like
shape. This elongated
shape is often called a
bar, which is why the
Milky Way is called a
barred spiral.
But What Is Our Galaxy Like?
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3. Surrounding the galaxy
is the halo, which
contains the globular
clusters, a low
concentration of
wandering stars, and very
little dust and gas.
The Nuclear Bulge
► The
most crowded
portion of our galaxy.
► Rather old – little gas
and dust for new star
formation.
The Mass of the Milky Way
If all mass were concentrated in the
center, the rotation curve would follow a
modified version of Kepler’s 3rd law
rotation curve = orbital velocity
as function of radius
Dark Matter?
► By
studying how the stars in our galaxy move, we
can tell that most of the galaxy’s mass is in the
corona, in a form that doesn’t give off light.
► It is called dark matter.
► We’re not sure what it is, although there are
several very good ideas.
Galaxy Diversity
► Even
seemingly
empty regions
of the sky
contain
thousands of
very faint, very
distant galaxies
of various
different types.
Galaxies
► Galaxies
are classified based on:
 Size
 Morphology – What the galaxy looks like
Galaxy Size
► There
are two basic
galaxy sizes.
► Giant galaxies are
those like the Milky
Way.
► 10,000’s of LY across.
► Have billions of stars.
Galaxy Size
► Dwarf
galaxies are
very common.
► For every giant there
are dozens of dwarf
galaxies.
► A few thousand light
years in diameter.
► Contain millions of
stars.
Morphology
► There
are three basic galaxy
morphologies.
This is a Spiral galaxy
78% of all galaxies are spiral galaxies
What Are The Spiral Arms?
Our best theory is the
density wave theory.
► Spiral arms are huge
shockwaves that travel
around the galaxy.
► Trigger new star formation
in their path.
► “Lit up” by lots of newly
formed O and B stars.
Makes them bright and
look blue.
► O and B stars quickly die
as the density wave moves
on to another part of the
Galaxy.
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This is an Elliptical galaxy
18% of all galaxies are elliptical galaxies
Elliptical Galaxies
► Ellipticals
are much like a spiral’s central bulge
with no disk.
► Very little dust and gas to form stars.
► Filled with old stars: yellow and red giants.
► HUGE range in sizes!
► Ellipticals range is size from the smallest known
galaxies (1,000 LY across and about a million
stars) to the largest known galaxies (nearly a
million LY across with tens of trillions of stars).
This is an Irregular galaxy
4% of all galaxies are irregular galaxies
Irregular Galaxies
► Irregulars
do not
have a well-defined
shape.
► Very, very rich in dust
and gas.
► Lots of new star
formation.
Dark Matter In Galaxies
► Based
on studies of
how galaxies move in
clusters:
 All galaxies contain
dark matter.
 Much of this dark
matter is in a halo
surrounding the galaxy.
Evidence For Dark Matter
► Galaxies
rotate as though most of their mass were
outside the luminous part of the galaxy.
► In galaxy clusters, galaxies move much faster than
expected. Something other than luminous matter
must be creating gravity.
► Gravitational Lenses – gravity from dark matter
bends light, acting as a lens to magnify and distort
the images of galaxies behind the dark matter.
Here’s Gravitational Lensing:
These arcs are the
distorted, magnified
images of
background
galaxies.
The distortion is
caused by the
huge amount of
dark matter in the
galaxy cluster in the
foreground.
Where Do Galaxies Hang Out?
► Galaxies
gather into galaxy clusters.
Loose Groups
Most common type of
galaxy cluster is a Loose
Group.
► Typically contain less than
10 giant galaxies, and a
few dozen satellite dwarfs.
► Low space density.
► Infrequent encounters
between giant galaxies.
► We are in a Loose Group
called the Local Group.
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Our Galaxy Cluster: The Local Group
Milky Way
Small Magellanic
Cloud
Large Magellanic
Cloud
Andromeda
galaxy
Superclusters: Clusters of
Clusters
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Individual galaxy
clusters group together
to form superclusters.
Typical supercluster:
 Rich, massive galaxy
cluster at center.
 Surrounded by dozens of
“satellite” loose groups.
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Our supercluster is
centered on the Virgo
Cluster. Called the
Virgo Supercluster.
Filaments, Sheets, and Voids
► One
final “level” of
structure:
 Superclusters
themselves cluster into
filaments and sheets
separated by vast voids.
 Appears to be very little
of anything inside the
voids.
The Expanding Universe
► In
any direction one observes, the clusters of
galaxies appear to be moving away from
the Earth.
► Their spectra are all red shifted.
► The farther away the galaxy is, the greater is
its red shift (and therefore velocity).
► The Earth only appears to be in the center of
the expansion.
► This is just what one would expect if we
lived in a uniformly expanding Universe.
Static Universe
Expanding Universe
Cosmology
► In
modern science Cosmology is the
overall study of the universe. It includes:
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Overall structure of the universe.
Overall properties of the universe.
Origin and early history of the universe.
Ultimate fate of the universe.
Cosmology Tries To Answer Some
Very Basic Questions
► How
big is our
universe?
 We don’t know for sure.
 Visible universe is
almost 30 billion light
years in diameter.
 Universe is much bigger
than that, but how big
exactly isn’t known.
Olbers’ Paradox
Why is the sky dark at night?
If the universe is infinite and filled
with stars, then every line of sight
should end on the surface of a star
at some point.
 The night sky should be as
bright as the surface of stars!
Solution to
Olbers’ Paradox:
If the universe had a beginning, then we
can only see light from galaxies that has
had time to travel to us since the
beginning of the universe.
 The visible universe is finite!
Cosmology Tries To Answer Some
Very Basic Questions
► Is
there a center or an edge to the
universe?
 There is no center in space, although in one
sense there is a center in time.
 There is no edge to the universe, although that
doesn’t necessarily mean it is infinite.
What Is The Evidence?
► It
starts with the discovery that the universe
is expanding.
► First observed by Edwin Hubble in the
1920s:
 All but the closest galaxies are moving away
from Earth.
 The more distant the galaxy, the faster it moves
away.
The Expanding Universe
This does not mean that we are at the center of the
universe!
You have the same impression
from any other galaxy as well.
Finite, But Without Edge?
2-dimensional analogy
Surface of a sphere
Surface is finite, but has no edge.
For a creature living on the
sphere, having no sense of the
third dimension, there’s no center
(on the sphere!): All points are
equal.
Alternative: Any point on the
surface can be defined as the
center of a coordinate system.
The Necessity of a Big Bang
If galaxies are moving away from each other
with a speed proportional to distance, there
must have been a beginning, when everything
was concentrated in one single point:
The Big Bang!
?
Looking Back Towards the Early Universe
The more distant the objects we
observe, the further back into the
past of the universe we are looking.
Universe cools down as time passes
The History of the Universe
Universe expands as time passes
The Cosmic Background Radiation
The radiation from the
very early phase of the
universe should still be
detectable today
R. Wilson & A. Penzias
Was, in fact, discovered in
mid-1960s as the Cosmic
Microwave Background:
Blackbody radiation
with a temperature of T
= 2.73 K
Cosmology with the Cosmic Microwave
Background
If the universe were perfectly homogeneous on all
scales at the time of recombination (z = 1000), then
the CMB should be perfectly isotropic over the sky.
Instead, it shows small-scale fluctuations:
Measuring the Deceleration of the Universe
By observing
type Ia
supernovae,
astronomers
can measure
the recession
speed at large
distances
It was expected that this would measure the deceleration
of the universe, but …
Apparent Magnitude of
Type Ia Supernovae
The Accelerating Universe!
Flat decelerating Universe
Flat accelerating Universe
Red Shift z
In fact, SN Ia measurements showed that the
universe is accelerating!
Confirmation of the Acceleration
Observation of the
high-red-shift (z = 1.7)
SN Ia SN1997ff seems
to confirm the
acceleration of the
universe.
What Is Dark Energy?
► Here’s
what we know:
 Somehow it drives a repulsive force that acts on
all matter.
 It gets stronger the more space there is
between two globs of matter.
 It is extremely weak on “local” scales (millions
of light years), but gets overwhelmingly strong
on “cosmological” scales (tens of billions of light
years).
What Is Dark Energy?
► Here’s
what we don’t know:
 Dang near everything else about it.
► How
did we discover it?
 The most distant galaxies are not moving away
as fast as we would expect.
 Something is providing a repulsive force that
shoves them away from us.
 But what?!
What is our universe made of?
► Atomic
Matter: 4%
 Hydrogen/Helium 98%
 Other stuff ~ 2%
► Dark
Matter: 23%
 Unknown composition.
 Not atomic.
► Dark
Energy: 73%
 Very mysterious.
Fluctuations in the Cosmic Microwave
Background
Angular size of the CMB fluctuations allows us to
probe the geometry of space-time!
CMB fluctuations have a characteristic size of 1 degree.
Analysis of the Cosmic Background
Fluctuations
Analyze frequency
of occurrence of
fluctuations on a
particular angular
scale
 Universe
has a
flat geometry
The Future of Cosmology
► We
have learned a great deal about the
Universe in just the last few years, and many
decades-old problems have been solved by
Hubble and WMAP.
► However, many fundamental questions such as
the nature of dark mater and dark energy
remain unanswered. It is likely that significant
progress will be made in our lifetime, so keep an
eye out for new breakthroughs in the years
ahead.
Next Time
► Have
a Good Summer