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Galaxies and the Universe
Galaxies
______________ galaxy
• Structure
• Determined by using ______ telescopes
• Large spiral galaxy
• About 100,000 light-years wide
• Thickness at the galactic nucleus is about 10,000
light-years
• _____ spiral arms of stars
• Sun is 30,000 light-years from the center
The Milky Way galaxy appears in our sky as a faint band of light.
Dusty gas
clouds obscure
our view
because they
absorb visible
light.
This is the
____________
________ that
makes new
star systems.
All-Sky View
Face-on view of the
Milk Way Galaxy
We see our galaxy edge-on.
Primary features: disk, bulge, halo, globular clusters
Galaxies
Milky Way galaxy
• Rotation
• Around the galactic nucleus
• Outermost stars move the slowest
• Sun rotates around the galactic nucleus once about
every 200 million years
• Halo surrounds the galactic disk
• Spherical
• Very tenuous gas
• Numerous globular clusters
If we could view the Milky Way from above the
disk, we would see its spiral arms.
Sun’s orbital
motion (radius
and velocity) tells
us mass within
Sun’s orbit:
1.0  1011 MSun
____________________
are found around shortlived high-mass stars,
signifying active star
formation.
___________ nebulae
scatter the light from
stars.
Why do reflection
nebulae look bluer than
the nearby stars?
For the same reason
that our sky is blue!
Halo: No ___________ nebulae, no blue stars
 no star formation
Disk: ___________ nebulae, blue stars  star formation
Much of star
formation in disk
happens in
________________
Whirlpool Galaxy
Our galaxy probably formed from a giant gas cloud.
Halo stars formed first as gravity caused the cloud to contract.
The remaining gas settled into a spinning disk.
Stars continuously form in the disk as the galaxy grows older.
Warning: This
model is
oversimplified.
Stars continuously form in the disk as the galaxy grows older.
Infrared light from
center
Radio emission from center
Radio emission from center
Swirling gas near center
Swirling gas near center
Orbiting star near center
Stars appear to
be ________
something
massive but
invisible …
a black hole?
Orbits of stars
indicate a mass
of about
4 million Msun.
X-ray flares
from galactic
center suggest
that tidal forces
of suspected
black hole
occasionally
tear apart
chunks of
matter about to
fall in.
Hubble Deep Field
• Our deepest images
of the universe show
a great variety of
galaxies, some of
them billions of
light-years away.
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
_________ —the
study of the structure
and evolution of the
universe.
Galaxies
Other galaxies
• Existence was first proposed in mid-1700s by
Immanuel Kant
• Four basic types of galaxies
• ______ galaxy
• _____ extending from nucleus
• About 30% of all galaxies
• Large diameter of 20,000 to 125,000 light years
• Contains both _______ and ___ stars
• e.g., Milky Way
halo
disk
bulge
Spiral Galaxy
Great Galaxy, a spiral galaxy, in
the constellation Andromeda
Galaxies
Other galaxies
• Four basic types of galaxies
• _____________ galaxy
• Stars arranged in the shape of a bar
• Generally quite large
• About 10% of all galaxies
• ___________ galaxy
• Ellipsoidal shape
• About 60% of all galaxies
• Most are smaller than spiral galaxies; however,
they are also the largest known galaxies
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
__________
component
Galaxies
Other galaxies
• Four basic types of galaxies
• ________ galaxy
• Lacks symmetry
• About 10% of all galaxies
• Contains mostly _______ stars
• e.g., Magellanic Clouds
Irregular Galaxy: Neither spiral
nor elliptical
Galaxies
Galactic cluster
• Group of galaxies
• Some contain thousands of galaxies
• ____________
• Our own group of galaxies
• Contains at least 28 galaxies
• Supercluster
• Huge swarm of galaxies
• May be the largest entity in the universe
Spiral
galaxies are
often found
in groups of
galaxies (up
to a few
dozen
galaxies per
group).
Elliptical
galaxies are
much more
common in
huge clusters
of galaxies
(hundreds to
thousands of
galaxies).
If the center of a
galaxy is
unusually bright,
we call it an
active galactic
nucleus.
_________ are
the most
luminous
examples.
Active Nucleus in M87
Quasars powerfully radiate energy over a very wide
range of wavelengths, indicating that they contain matter
with a wide range of temperatures.
The accretion of gas onto a supermassive black hole appears
to be the only way to explain all the properties of quasars.
Radio
galaxies
don’t appear
as quasars
because
dusty gas
clouds block
our view of
the accretion
disk.
Energy from a Black Hole
• The gravitational potential energy of matter falling
into a black hole turns into kinetic energy.
• Friction in the accretion disk turns kinetic energy
into thermal energy (heat).
• Heat produces thermal radiation (photons).
• This process can convert 10–40% of E = mc2 into
radiation.
Jets are thought to come from the twisting of a magnetic
field in the inner part of the accretion disk.
Do supermassive black holes
really exist?
____________
_____ at center
of Milky Way
stars indicate a
black hole with
mass of
4 million Msun.
Orbital speed and distance of gas orbiting center of M87
indicate a black hole with mass of 3 billion Msun.
Black Holes in Galaxies
• Many nearby galaxies—perhaps all of them—
have supermassive black holes at their centers.
• These black holes seem to be ________ active
galactic nuclei.
• All galaxies may have passed through a quasarlike stage earlier in time.
Hubble also knew that the spectral features of virtually
all galaxies are __________  they’re all moving
away from us.
Red shifts
Doppler effect
• Change in the wavelength of light emitted by an
object due to its motion
• Movement away stretches the wavelength
• Longer wavelength
• Light appears redder
• Movement toward “squeezes” the wavelength
• Shorter wavelength
• Light shifted toward the blue
Red shifts
Doppler effect
• Amount of the Doppler shift indicates the rate
of movement
• Large Doppler shift indicates a high velocity
• Small Doppler shift indicates a lower velocity
Expanding universe
• Most galaxies exhibit a red Doppler shift
• Moving _____
Raisin bread analogy of an
expanding universe
Red shifts
Expanding universe
• Most galaxies exhibit a red Doppler shift
• Far galaxies
• Exhibit the greatest shift
• Greater velocity
• Discovered in 1929 by Edwin Hubble
• _________ Law – the recessional speed of galaxies
is proportional to their distance
• Accounts for red shifts
By measuring
distances to
galaxies, Hubble
found that redshift
and distance are
related in a special
way.
Hubble’s law:
velocity = H0  distance
Redshift of a
galaxy tells us
its _________
through
Hubble’s law:
distance =
velocity
H0
We measure galaxy distances using
a chain of interdependent
techniques.
Cosmological Principle
The universe looks about the same no matter
where you are within it.
• Matter is ______ distributed on very large scales
in the universe
• No _______ and no _____
• Not proved but consistent with all observations to
date
• The expansion rate appears to be the same
everywhere in space.
• The universe has no center and no edge (as
far as we can tell).
One example of something that expands but has no center
or edge is the surface of a balloon.
Distances
between
faraway
galaxies
change while
light travels.
distance?
Astronomers
think in terms
of _________
time rather
than distance.
We still can’t directly observe the earliest galaxies.
______________ theory
Accounts for galaxies moving away from us
Universe was once confined to a "ball" that
was
• Supermassive
• Dense
• Hot
Big Bang theory
Big Bang marks the inception of the
universe
• Occurred about 15 billion years ago
• All matter and space was created
Matter is moving ________
Fate of the universe
• Two possibilities
• Universe will ____________
• Outward expansion will stop and gravitational
____________ will follow
The main question
• Does the universe have enough kinetic
energy to escape its own gravitational pull?
– What is the escape velocity of the Universe?
Big Bang theory
Fate of the universe
• Final fate depends on the average _______ of
the universe
• If the density is _____ than the critical density, then
the universe would ________
• Current estimates point to _____ then the critical
density and predict an _______________, or open,
universe
Fate of
universe
depends
on the
amount
of
_______
_______
Lots of
dark
matter
Critical
density of
matter
Not enough
dark matter
Amount of matter is
~25% of the critical
density, suggesting fate
is eternal expansion
Not enough
dark matter
But expansion
appears to be
speeding up!
____________?
Not enough
dark matter
old
older
oldest
Estimated age depends on both dark matter and dark energy.
Is the expansion of the universe
accelerating?
The brightness of distant white-dwarf supernovae tells us
how much the universe has expanded since they exploded.
An accelerating universe is the best fit to supernova data.
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