The Milky Way Galaxy
Objectives
• Determine the size and shape of the Milky Way, as well
as Earth’s location within it.
• Describe how the Milky Way formed.
Vocabulary
– variable star
– RR Lyrae variable
– Cepheid variable
– halo
– spiral density wave
The Milky Way Galaxy
The Milky Way Galaxy
• The Milky Way is a great disk made of stars
orbiting a central point in the disk.
• Our Sun is just one of perhaps 100 billion stars
that make up the Milky Way.
The Milky Way Galaxy
Discovering the Milky Way
• It is difficult to see the Milky Way’s size and
shape not only because are we too close, but
we are also inside the galaxy.
• We also cannot tell where its center is, or what
Earth’s location is within it.
• Astronomers are still refining their measurements.
The Milky Way Galaxy
Discovering the Milky Way
Variable Stars
– Astronomers estimated the distances to globular
clusters by identifying variable stars in them.
– Variable stars are stars in the giant branch of the
Hertzsprung-Russell diagram that pulsate in brightness
because of the expansion and contraction of their
outer layers.
– RR Lyrae variables have periods of pulsation between
1.5 hours and 1 day, and on average, they have the
same luminosity.
– Cepheid variables have pulsation periods between 1
and 100 days, the longer the period of pulsation, the
greater the luminosity.
The Milky Way Galaxy
Discovering the Milky Way
Variable Stars
– By measuring a star’s period of pulsation, astronomers
can determine the star’s luminosity.
– The star’s luminosity, or absolute magnitude, can be
compared to its apparent magnitude to calculate the
distance to the star.
The Milky Way Galaxy
Discovering the Milky Way
The Galactic Center
– Astronomers used RR Lyrae variables to determine that
the globular clusters are distributed in space centered on
a point, 28 000 ly away.
– Astronomers reasoned that the globular clusters were
orbiting the center of the Milky Way which is a region of
very high star density.
– The direction of the galactic center is toward the
constellation Sagittarius.
The Milky Way Galaxy
The Shape of the Milky Way
• Astronomers have been able to determine the
galaxy’s shape by mapping it with radio waves.
• The galactic center, also called the nucleus, is
surrounded by a nuclear bulge, which sticks out
of the galactic disk.
• The halo is a spherical region, around the
nuclear bulge and disk, where globular clusters
are located.
The Milky Way Galaxy
The Shape of the Milky Way
The Milky Way
consists of a
nuclear bulge in
the center of a
disk. The disk
and bulge are
surrounded by a
spherical region
called the halo.
(not to scale)
The Milky Way Galaxy
The Shape of the Milky Way
Spiral Arms
– Hydrogen atoms in a very low-density gas can emit
radiation at a wavelength of 21 cm.
– Using this emission as a guide, astronomers have
identified four major spiral arms and numerous minor
arms in the Milky Way.
– The Sun is located in the minor arm Orion at a distance
of about 28 000 ly from the galactic center.
– The Sun’s orbital speed is about 220 km/s, and thus its
orbital period is about 240 million years.
The Milky Way Galaxy
The Shape of the Milky Way
Spiral Arms
The Milky Way Galaxy
Mass of the Milky Way
• The mass located within the circle of the Sun’s
orbit through the galaxy is about 100 billion
times the mass of the Sun.
• Because the Sun is about average in mass,
astronomers have concluded that the galaxy
contains about 100 billion stars within its disk.
• Evidence indicates that as much as 90 percent
of the galaxy’s mass is contained in the halo.
• Some of this unseen matter, or dark matter, is
probably in the form of dim stellar remnants such
as white dwarfs, neutron stars, or black holes.
The Milky Way Galaxy
Mass of the Milky Way
A Galactic Black Hole
– Sagittarius A*, the center of the galaxy, has about
2.6 million times the mass of the Sun but is smaller
than our solar system.
– Data gathered by the Chandra X-Ray Observatory
reveal intense X-ray emissions as well.
– Astronomers believe that Sagittarius A* is a
supermassive black hole that probably formed at the
time when the galaxy’s disk was forming.
The Milky Way Galaxy
Stars in the Milky Way
• Globular clusters in the halo of the Milky Way,
which are estimated to be as old as 12 to
14 billion years, are the oldest-known objects
in the galaxy.
• Stars in the globular clusters have extremely
small amounts of elements that are heavier than
hydrogen and helium.
• The nuclear bulge of the galaxy also contains
stars with compositions like those of globular
cluster stars.
The Milky Way Galaxy
Stars in the Milky Way
Stellar Populations
– Most of the young stars in the galaxy are located in the
spiral arms of the disk, where the interstellar gas and
dust are concentrated.
– The galaxy could be divided into two components:
• The round part made up of the halo and bulge,
where the stars are old and contain only traces of
heavy elements
• The disk, especially the spiral arms, where stars are
still forming
The Milky Way Galaxy
Stars in the Milky Way
Stellar Populations
– Astronomers divide stars in these two regions into
two classes.
• Population I stars, like the Sun, are those in the
disk and arms and have small amounts of
heavy elements.
• Population II stars are those in the halo and bulge
and contain only traces of heavy elements.
The Milky Way Galaxy
Stars in the Milky Way
Stellar Populations
Globular clusters
and the nuclear
bulge contain old
stars poor in
heavy elements.
The disk contains
young stars that
have higher heavy
element content.
(not to scale)
The Milky Way Galaxy
Formation and Evolution of the Milky Way
• The fact that the halo and bulge are made
exclusively of old stars suggests that these parts
of the galaxy formed first.
– The galaxy began as a spherical cloud in space with
the first stars forming while this cloud was round in
what is now the halo.
– The nuclear bulge represents the inner portion of the
original cloud.
– The cloud eventually collapsed under the force of its
own gravity, and rotation forced it into a disklike shape.
– Stars that formed after this time have orbits lying in the
plane of the disk.
The Milky Way Galaxy
Maintaining Spiral Arms
• The way in which the spiral arms are maintained
is not clearly understood.
• There are two different theories about how the
arms are maintained:
1. Spiral density waves, which can be thought of
alternating dense and less-dense regions that are
frozen in place and rotate as a rigid pattern, cause a
buildup of material as they move through space.
2. The spiral arms are not permanent structures but
instead, are continually forming as a result of
disturbances such as supernovae explosions.
The Milky Way Galaxy
Maintaining Spiral Arms
A slow truck on a
highway causing a
buildup of cars
around it is similar
to how spiral
density waves
create spiral arms
in a galaxy.
The Milky Way Galaxy
Maintaining Spiral Arms
• The Milky Way has a broken spiral-arm pattern,
which fits the disturbances model best.
• Some galaxies that have a prominent 2-armed
pattern are almost certainly created by
density waves.
The Milky Way Galaxy
Section Assessment
1. Match the following terms with their definitions.
___
B RR Lyrae
variable
___
D Cepheid
variable
___
C halo
___
A spiral density
wave
A. alternating dense and lessdense regions that are frozen
in place and rotate as a
rigid pattern
B. star with pulsation periods
between 1.5 hours and
1 day
C. a spherical region in the Milky
Way where globular clusters
are located
D. star with a pulsation period
between 1 to 100 days
The Milky Way Galaxy
Section Assessment
2. How did astronomers discover the arms of
the Milky Way?
Astronomers discovered the arms of the
Milky Way through observing the radiation
emitted by very-low density hydrogen gas.
The Milky Way Galaxy
Section Assessment
3. Identify whether the following statements are
true or false.
______
true The Sun has orbited the galaxy approximately
20 times in its history.
______
false Most of the Milky Way’s Mass is located in the
nuclear bulge.
______
true Population I stars are younger than
Population II stars.
______
false The Sun is located on the Perseus arm of the
Milky Way.
Other Galaxies in the Universe
Objectives
• Describe how astronomers classify galaxies.
• Identify how galaxies are organized into clusters and
superclusters.
• Describe the expansion of the universe.
Vocabulary
– supercluster
– active galactic nucleus
– Hubble constant
– quasar
– radio galaxy
Other Galaxies in the Universe
Other Galaxies in the Universe
• Our galaxy is just one of billions of galaxies in
the universe.
• By observing those galaxies that are farthest
away, astronomers get an idea of how the
universe looks as a whole.
• Because it takes so long for light to reach us
from remote galaxies, these far away galaxies
also provide an idea of what the universe was
like long ago.
Other Galaxies in the Universe
Discovering Other Galaxies
• Astronomers were aware of galaxies outside the
Milky Way long before they knew what they were
or if they were part of our own galaxy.
• In 1924, Edwin Hubble discovered Cepheid
variable stars in the Great Nebula in the
Andromeda constellation.
• Measuring the distance to the nebula, Hubble
showed that they were much too far away to be
located in our own galaxy and it became known
as the Andromeda Galaxy.
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of Galaxies
– Hubble went on to study galaxies and sort them into
categories according to their shapes.
– The disklike galaxies with spiral arms were called spiral
galaxies and were divided into two subclasses: normal
spirals (S) and barred spirals (SB).
– These are further subdivided based on: (a) tightly
wound arms and a large, bright nucleus; (c) loosely
wound arms and a small, dim nucleus; (b) having
characteristics between a and c.
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of Galaxies
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of Galaxies
– Galaxies with flat disks that do not have spiral arms are
denoted S0.
– Elliptical galaxies are not flattened into disks, do not
have spiral arms, and are denoted E.
– Ellipticals are divided into subclasses (E0–7) based on
the apparent ratio of their major and minor axes: round
ellipticals are classified as E0, while very elongated
ellipticals are classified as E7.
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of Galaxies
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of Galaxies
– Some galaxies do not have distinct shapes, and thus
do not fit into either the spiral or elliptical classification.
– They are called irregular galaxies and are denoted
by Irr.
Other Galaxies in the Universe
Discovering Other Galaxies
Masses of Galaxies
– Dwarf elliptical galaxies have masses of perhaps one
million Suns.
– Large spirals, such as the Milky Way, have masses of
around 100 billion Suns.
– The largest galaxies, called giant ellipticals, have
masses as high as 100 trillion times the Sun’s mass.
Other Galaxies in the Universe
Groups and Clusters of Galaxies
• Most galaxies are located in groups, rather than
being spread uniformly throughout the universe.
– The Milky Way belongs to a small cluster of galaxies
called the Local Group which is roughly 2 million ly
in diameter.
– There are about 35 known members including the
Milky Way and Andromeda Galaxies, but most of are
dwarf ellipticals.
– Clusters larger than the Local Group may have
hundreds of members and diameters in the range of
about 5 to 30 million ly.
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Masses of Clusters
– For clusters of galaxies, the mass determined by
analyzing the motion of member galaxies is always
much larger than the sum of the visible masses of
the galaxies.
– This provides the strongest evidence that the universe
contains a great amount of dark matter, the nature of
which is still unknown.
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Superclusters
– Superclusters are clusters of galaxies organized into
even larger groups that are hundreds of millions of
light-years in size.
– These superclusters appear in sheetlike and
threadlike shapes.
Other Galaxies in the Universe
The Expanding Universe
• In 1929, Edwin Hubble, by measuring the
redshifts and distances of many galaxies, found
that the farther away from Earth a galaxy is, the
faster it is moving away.
• The universe is expanding.
• In a medium that is uniformly expanding, all
points are moving away from all other points,
and no point has to be at the center.
Other Galaxies in the Universe
The Expanding Universe
Hubble’s Law
– Hubble determined that the universe is expanding by
making a graph of the speed at which a galaxy is
moving versus its distance.
– The result is a straight line, which can be expressed as
a simple equation, v = Hd.
• v is the speed at which a galaxy is moving away
measured in kilometers per second
• d is the distance to the galaxy measured in
megaparsecs (Mpc), where 1 Mpc = 1 × 106 pc
• H is a number called the Hubble constant
Other Galaxies in the Universe
The Expanding Universe
Hubble’s Law
– The Hubble constant, which is the value (H) that is
used to calculate the rate that the universe is expanding,
is measured in kilometers per second per megaparsec.
– To measure H requires finding distances and speeds for
many galaxies, out to the largest possible distance, and
constructing a graph to find the slope.
– Currently, the best measurements indicate a value of
approximately 70 kilometers per second per
megaparsec.
– Once the value of H is known, it can be used to find
distances to far away galaxies.
Other Galaxies in the Universe
Active Galaxies
• Radio galaxies, often giant elliptical galaxies,
emit as much as or more energy in radio
wavelengths than they do in wavelengths of
visible light.
• The radio emission usually comes from two huge
lobes of very hot gas located on opposite sides of
the visible galaxy, linked by jets of gas.
• Active galactic nuclei, or AGNs, in some unusual
galaxies, contain a highly energetic object or
activity that emits as much or more energy than
the rest of the galaxy.
Other Galaxies in the Universe
Quasars
• In the 1960s, astronomers discovered another
new type of object that looked like ordinary stars,
but were strong radio emitters.
– Quasars are starlike objects with emission lines in
their spectra.
– The emission lines in the spectra of quasars are
shifted very far toward longer wavelengths.
– The redshifts of quasars indicate that they are very
far away.
– Most quasars appear to be extra-bright active galactic
nuclei of very dim galaxies.
Other Galaxies in the Universe
Quasars
Looking Back in Time
– Because many quasars are far away, it takes their light
a long time to reach Earth.
– The most remote quasars are several billion light-years
away, which indicates that they existed billions of
years ago.
– This suggests that many galaxies went through a
quasar stage when they were young.
– Today’s active galactic nuclei might be former quasars
that are not quite as energetic as they were long ago.
Other Galaxies in the Universe
Quasars
Source of Power
– The AGNs and quasars emit far more energy than
ordinary galaxies, but they are as small as solar
systems which suggests that all of these objects are
supermassive black holes.
– The beams of charged particles that stream out of the
cores of radio galaxies and form jets are probably
created by the magnetic forces of black holes.
– Radio-lobed quasars have jets that are essentially
related to radio galaxies.
Other Galaxies in the Universe
Section Assessment
1. Match the following terms with their definitions.
___
B supercluster
___
A radio galaxy
___
D active galactic
nucleus
___
C quasar
A. galaxies that emit as much or
more energy in radio
wavelengths than they do in
visible light
B. organizations of clusters of
galaxies that are hundreds of
millions of light years in size
C. starlike objects with emission
lines in their spectra
D. galaxy cores that emit as much
or more energy than the rest of
the galaxy
Other Galaxies in the Universe
Section Assessment
2. What two important discoveries did
Edwin Hubble make?
Edwin Hubble discovered that there were other
galaxies outside of the Milky Way and that the
universe is expanding.
Other Galaxies in the Universe
Section Assessment
3. Identify whether the following statements are
true or false.
______
true We see the Sun as it was about eight
minutes ago.
______
false There are about 15 known members of the
Local Group.
______
true The universe will appear to be expanding from
any location within it.
______
true Radio galaxies are often elliptical.
Cosmology
Objectives
• Explain the different theories about the formation
of the universe.
• Describe the possible outcomes of universal expansion.
Vocabulary
– cosmology
– Big Bang theory
– steady-state theory
– cosmic background
radiation
– inflationary universe
Cosmology
Cosmology
• Cosmology is the study of the universe, its
current nature, and its origin and evolution.
• Astronomers use a combination of observations
and theoretical models in cosmology.
• In cosmology, objects that have a range of
properties cannot be compared, as there is only
one universe to consider and nothing to
compare it with.
Cosmology
Models of the Universe
• The fact that the universe is expanding implies
that it had a beginning.
– The Big Bang theory is the theory that the universe
began as a point and has been expanding ever since.
– The steady-state theory proposes that the universe
is the same as it has always been and new matter is
being created as the universe expands maintaining
its density.
• The evidence from observational tests weighs
in favor of the Big Bang.
Cosmology
Models of the Universe
Without the
creation of new
matter, the area
within the dotted
box would not
contain 3 galaxies
after a time.
The steady-state
theory requires
new matter to be
added so that the
area within the
dotted box always
contains 3 galaxies.
Cosmology
Models of the Universe
Cosmic Background Radiation
– If the universe began in a highly compressed state,
as the Big Bang theory suggests, it would be filled
with radiation.
– As the universe expanded and cooled, the radiation
would have been Doppler shifted to lower energies and
longer wavelengths.
– Cosmic background radiation, which was discovered
in 1965, is background noise caused by weak radiation
that comes from all directions in space.
– This radiation is interpreted to be from the beginning of
the Big Bang.
Cosmology
Models of the Universe
Mapping the Radiation
– Observations have confirmed that cosmic background
radiation matches the properties of the predicted
leftover radiation.
– An orbiting observatory called the Cosmic Background
Explorer (COBE), launched by NASA in 1989, mapped
the radiation in detail.
– Proponents of the
steady-state universe
theory have not
succeeded in explaining
the cosmic background
radiation.
Cosmology
The Big Bang Model
• In the Big Bang
model, there is a
competition between
the outward
momentum of the
expansion of the
universe and the
inward force of
gravity as the matter
in the universe acts to
slow the expansion.
Cosmology
The Big Bang Model
• Three possible outcomes for the universe include:
– an open universe, in
which the expansion
will never stop
– a closed universe, in
which the expansion
will stop and turn into
a contraction
– a flat universe, in
which the expansion
will slow to a halt in
time—but it will never
contract
Cosmology
The Big Bang Model
The Critical Density
– The total amount of matter in the universe is one of the
factors that will determine whether or not the expansion
will stop.
– The critical density, about 10-26 kg/m3, is the dividing
point between a closed or open universe.
• If the average density is higher than the critical
density, the universe is closed.
• If the average density is lower than the critical
density, the universe is open.
• If the density equals the critical density, the universe
is flat.
Cosmology
The Big Bang Model
Expansion Rate
– Another approach to determining the fate of the
universe is to measure how much slowing has
occurred so far in its expansion.
– By measuring the redshifts of the most remote
galaxies, it is possible for astronomers to determine the
expansion rate long ago.
– Astronomers have found that the rate of expansion is
speeding up.
– The only explanation offered so far is that a previously
unknown force is acting to push the galaxies apart.
Cosmology
The Big Bang Model
The Inflationary Model
– The combination of the observed density of the
universe, including an allowance for dark matter, and
the apparent acceleration of the expansion, fit together
into a model in which the universe is flat.
– The inflationary universe model proposes that the
universe began as a fluctuation in a vacuum and
expanded very rapidly for a fraction of a second before
settling into a more orderly expansion.
– When the rate of expansion of the universe is known, it
is possible to calculate the time since the expansion
started, or the age of the universe.
Cosmology
The Big Bang Model
The Inflationary Model
– A correction is needed to allow for the fact that the
expansion was more rapid at the beginning and to take
into account the acceleration of the expansion that is
occurring now.
– Based on the best value for H that has been
calculated, as well as the appropriate corrections, the
age of the universe is hypothesized to be about
13 billion years.
Cosmology
The Big Bang Model
The Inflationary Model
According to the
inflationary model,
the universe
expanded very
quickly in the early
portion of the Big
Bang. The time in
which inflation
occurs is the
shaded portion of
the graph, where
the size of the
universe increases
drastically.
Cosmology
Section Assessment
1. Match the following terms with their definitions.
___
C Big Bang
theory
___
D steady-state
theory
___
A cosmic
background
radiation
___
B inflationary
universe
A. background noise caused by weak
radiation that comes from all
directions in space
B. model that proposes that the
universe began as a fluctuation in a
vacuum and expanded very rapidly
for a fraction of a second
C. theory that the universe began as
a point and has been expanding
ever since
D. theory that the universe
is the same as it always has been
Cosmology
Section Assessment
2. What are the three possible outcomes for the
universe according to the Big Bang theory?
The three possible outcomes include: an open
universe, in which the expansion will never stop;
a closed universe, in which the expansion will
stop and turn into a contraction; and a flat
universe, in which the expansion will slow to a
halt in time—but it will never contract.
Cosmology
Section Assessment
3. Identify whether the following statements are
true or false.
______
false The inflationary epoch, as proposed in the
inflationary universe model, lasted for several
years.
______
true Expansion of the universe is currently
speeding up.
______
true If H is known, the age of the universe can be
determined.
______
false The steady-state theory is currently the leading
theory of universal origin.
Chapter Resources Menu
Study Guide
Section 31.1
Section 31.2
Section 31.3
Chapter Assessment
Image Bank
Section 31.1 Study Guide
Section 31.1 Main Ideas
• Because our solar system is inside the Milky Way galaxy,
it was difficult at first for astronomers to determine the
size and shape of our galaxy, and where Earth is located
within it.
• The Milky Way consists of a nuclear bulge, a disk, and a
halo. Much of the mass of the galaxy is not visible.
• It is hypothesized that the Milky Way began as a spherical
cloud of gas that eventually collapsed into a disk.
• Population I stars contain small amounts of heavy
elements and are located in the arms and disk. Population
II stars are located in the bulge and halo, and contain only
trace amounts of heavy elements.
Section 31.2 Study Guide
Section 31.2 Main Ideas
• Galaxies are classified according to their shapes as
normal spirals, barred spirals, ellipticals, irregulars, or
dwarf ellipticals.
• Most galaxies occur in clusters, which are further
organized into superclusters.
• The universe is expanding, and the Hubble constant, H,
measures the rate of expansion.
• Because it takes the light from very distant galaxies so
long to reach Earth, when astronomers observe these
galaxies, they are looking back in time.
• Many galaxies have energetic objects or activities at
their cores called active galactic nuclei.
Section 31.3 Study Guide
Section 31.3 Main Ideas
• The Big Bang theory proposes that the universe began
as a single point and has been expanding ever since.
• The steady-state theory proposes that the universe is the
same as it always has been.
• The universe is filled with cosmic background radiation
that is left over from the early, hot stages in the Big Bang
expansion of the universe.
• In the Big Bang model, the universe could be open,
closed, or flat.
Section 31.3 Study Guide
Section 31.3 Main Ideas
• The inflationary model can explain the walls and voids of
the distribution of galaxies. It also predicts that the
universe is flat, which is supported by observations.
• Current observations indicate that an unknown force is
accelerating the expansion of the universe.
Chapter Assessment
Multiple Choice
1. How far is the Sun from the galactic center?
a. 5 000 ly
c. 28 000 ly
b. 17 000 ly
d. 55 000 ly
The Sun is located about 28 000 ly from the galactic
center on the minor arm Orion.
Chapter Assessment
Multiple Choice
2. How would a galaxy that is a normal spiral
with tightly wound arms and a large, bright
nucleus be classified?
a. SBb
c. Sc
b. S0
d. Sa
Normal spiral galaxies are denoted with the letter S. The
denotations a–c indicate how tightly wound the arms are
as well as the size and brightness of the nucleus. SB
indicates a barred spiral galaxy.
Chapter Assessment
Multiple Choice
3. According to the Big Bang theory, a(n) ____
universe will expand forever.
a. open
c. inflationary
b. closed
d. flat
In an closed universe, the expansion will stop and turn
into a contraction. In a flat universe, the expansion will
slow to a halt in an infinite amount of time—but will never
contract. The inflationary universe model is a variation of
the flat universe theory.
Chapter Assessment
Multiple Choice
4. Quasars are identified through ____.
a. an absorption spectra
b. visible light
c. an emission spectra
d. blueshifts
Quasars are star-like objects that have mostly emission
lines in their spectra. Most normal stars have spectra with
absorption lines.
Chapter Assessment
Multiple Choice
5. Which equation represents Hubble’s law?
a. H = vd
c. E = mc2
b. D2 = Hv
d. v = Hd
In the above equation, v is the speed at which a galaxy is
moving away measured in kilometers per second, d is the
distance to the galaxy measured in megaparsecs (Mpc),
where 1 Mpc = 1 × 106 pc, and H is a number called the
Hubble constant.
Chapter Assessment
Short Answer
6. What is the difference between Population I stars
and Population II stars?
Population I stars are younger stars that are
located in the disk of the Milky Way. They have
a higher, though still small, concentration of
heavy elements. Population II stars are older
stars including globular clusters that are located
in the halo or nuclear bulge. They contain very
small amounts of heavy elements.
Chapter Assessment
Short Answer
7. How is the distribution of galaxies in the
universe organized?
Individual galaxies are organized into groups
or larger clusters. The groups and clusters
are then organized into even larger
superclusters which can bee hundreds of
millions of light years across.
Chapter Assessment
True or False
8. Identify whether the following statements are true
or false.
______
false Edwin Hubble discovered that the universe
is contracting.
______
true Variable stars are in the giant branch of the
H-R diagram.
______
false Most astronomers think that spiral density
waves are responsible for the arms of the
Milky Way.
______
false The inflationary universe model fits into the
steady-state theory.
Image Bank
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Image Bank
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Image Bank
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