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Chapter 30
Galaxies and the
Universe
Chapter 30:
Galaxies and the Universe
Chapter 30.1:
• Stars with varying light output
allowed astronomers to map
the Milky Way, which has a
halo, spiral arm, and a
massive galactic black hole
at its center.
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Chapter 30:
Galaxies and the Universe
Chapter 30.2:
• Finding galaxies with
different shapes reveals
the past, present, and
future of the universe.
Chapter 30:
Galaxies and the Universe
Chapter 30.3:
• The Big Bang Theory was
formulated by comparing
evidence and models to
describe the beginning of
the universe.
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Section 30.1:
The Milky Way Galaxy
Objectives:
• Determine the size and shape of
our galaxy.
• Distinguish the different kinds of
variable stars.
• Identify different kinds of stars in a
galaxy and their locations.
The Milky Way Galaxy
• Galaxy: any of the very large groups
of stars and associated matter found
throughout the universe.
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The Milky Way Galaxy
• Direct observation of the
Milky Way galaxy is difficult
because not only are we
too close, but we are also
inside the galaxy.
The Milky Way Galaxy
• When we observe the band
of stars stretching across the
sky, we are seeing the edge
of a disk from the inside of
the disk.
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Variable Stars
• Variable star: A star in the
giant branch of the H-R
Diagram that pulsates in
brightness due to its outer
layers expanding and
contracting.
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Variable Stars
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Variable Stars
• Astronomers can estimate
the distance of globular
clusters of stars within the
Milky Way by identifying
variable stars.
Variable Stars
RR Lyrae Variables:
• Stars that have periods
of pulsation between
1.5 hours to and 1 day.
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Variable Stars
Cepheid Variables:
• Stars that have periods
of pulsation between 1
and 100 days. (luminosity
as much as doubles)
Variable Stars
• By measuring the star’s
period of pulsation,
astronomers can calculate
how far away the star must
be to appear this dim or this
bright.
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The Shape of the Milky Way
• Halo: spherical region
where globular clusters
are located; surrounds
the Milky Way’s nuclear
bulge and disk.
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Spiral Arms
• Astronomers have
identified four major spiral
arms and numerous
minor arms of the Milky
Way using hydrogen
emissions.
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Spiral Arms
Orbital Period
• The orbital period of our sun
around the galactic center
is about 240 million years.
• In its 5 billion-year life, the
Sun has orbited the galaxy
approximately 20 times.
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Orbital Speed
• The Sun’s orbital speed is
roughly 220km/s
• 792,000 km/h
• ~ 500,000 miles per hour
Mass of the Milky Way
• The mass of the Milky Way is
about 100 billion times the
mass of our Sun.
• From this number, astronomers
have concluded there are
about 100 billion stars within
the Milky Way.
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A Galactic Black Hole
• Supermassive black holes
occupy the center of most
galaxies.
• These black holes are anywhere
from a few million to a few billion
times the mass of our Sun. (Much
larger than stellar black holes)
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Illustration:
Supermassive Black Hole
Stellar Populations in
the Milky Way
• The differences among stars
include differences in location,
motion, and age.
• Stars are categorized based on
their location:
• Disk: Population I
• Nuclear Bulge/Halo: Population II
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Population II
Population II
Population I
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Stellar Populations in
the Milky Way
•
•
•
•
•
Population I Stars:
Found in Disk Arms
98% H and He
<10 billion years old
Young sequence stars
Stellar Populations in
the Milky Way
•
•
•
•
•
Population II Stars:
Found in Bulge and Halo
99.9% H and He
>10 billion years old
Old main sequence stars.
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Evolution of the
Milky Way
• The fact that the halo and
nuclear bulge are made
exclusively of old stars
suggests that these parts of the
galaxy formed first.
Evolution of the
Milky Way
• Our galaxy began as a
spherical cloud in space.
• The first stars formed while this
cloud was round. (PII)
• The young stars in the disk
formed after the cloud
collapsed by its own gravity.
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Spiral Arms
• Most of the main features of
the galaxy are understood by
astronomers, except for the
way in which the spiral arms
are retained.
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Spiral Arms
• One hypothesis is that a kind of
wave called a spiral density
wave is responsible.
• A second hypothesis is that the
spiral arms are not permanent
but are continually forming.
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Section 30.2: Other
Galaxies in the Universe
• Objectives:
o How astronomers classify galaxies
o How galaxies organized into clusters
and superclusters
o The universe is expanding
Section 30.2: Other
Galaxies in the Universe
• Vocabulary:
o Dark matter
o Supercluster
o Hubble constant
o Radio galaxy
o Active galactic nucleus
o Quasar
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Discovering Other Galaxies
• In 1924, Edwin Hubble
found evidence to
suggest there were
other galaxies in the
universe.
• The first galaxy he
discovered is now
called the Andromeda
galaxy.
Properties of Galaxies
• Masses of galaxies have a wide
range from 1 million to 1 trillion
times the mass of our Sun.
• Dwarf ellipticals = 1 million x Sun’s mass
• Large spirals = 100 billion x Sun’s mass
• Giant ellipticals = 1 trillion x Sun’s mass
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Properties of Galaxies
• Luminosities of galaxies also vary
over a wide range.
• Dwarf spheroidals = not much
brighter than a globular cluster of
stars
• Supergiant ellipticals = more than
100 times more luminous than our
Milky Way galaxy.
Classification of Galaxies
• Hubble went on to study
galaxies and categorize them
according to their shape.
• The Hubble Space Telescope
was named after Edwin Hubble
for his contributions to
astronomy.
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Disklike Galaxies
• Hubble classified disklike
galaxies with spiral arms as
spiral galaxies.
• Spiral galaxies are subdivided
into normal spirals and barred
spirals.
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Disklike Galaxies
• A normal spiral galaxy is
denoted by the letter S
• A barred spiral galaxy is
denoted by the letters SB
• The letter a represents tightly
wound arms, while the letter c
represents loosely wound arms.
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Sb
SBb
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Disklike Galaxies
• Disklike galaxies that do not
have spiral arms are denoted
as S0 or SB0
• These types of galaxies are also
called lenticular galaxies.
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NCG 4866 Lenticular
S0
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Elliptical Galaxies
• Galaxies that are not flattened
into a disk and do not have
spiral arms are called elliptical
galaxies.
• Round ellipticals are denoted E0
• Elongated ellipticals are denoted E7
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E0
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Irregular Galaxies
• Some galaxies do not have
distinct shapes. These irregular
galaxies are denote Irr
• The Large and Small Magellanic
Clouds are examples of
irregular galaxies.
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Bell Ringer
• Categorize the following
galaxies using the
classification system of
galaxies.
1.
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2.
3.
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Groups and Clusters of
Galaxies
• The Milky Way belongs to a
small cluster of galaxies
called the Local Group.
• Diameter = 2 million ly
• There are about 40 known
galaxies in the Local Group.
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Local Galactic Group
• The largest galaxies in the
Local Group are the Milky
Way and the Andromeda
Galaxy
• The closest Galaxies to us
are the Small and Large
Megellanic Clouds.
Large Clusters
• Galaxy clusters larger than the
Local Group may have hundreds
or thousands of members with
diameters up to 30 million ly
across.
• Galaxies close together often
collide to form strangely shaped
galaxies or galaxies with more
than one nucleus (Andromeda)
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Superclusters
• Clusters of galaxies are
organized into even larger
groups called superclusters.
• These gigantic formations
can be hundreds of millions
of light-years in size.
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Earth’s Address
Earth
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Solar System
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Milky Way Galaxy
Milky Way Galaxy
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The Expanding Universe
• In 1929, Edwin Hubble made
another dramatic discovery.
• He found that all galaxies
have redshifts.
• The redshift depends on its
distance from Earth.
• In other words, the universe is
expanding.
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The Expanding Universe
• We can make an inference that if
the Universe is expanding now, it
must have been smaller and
denser in the past.
• If we put this expansion in rewind,
we can go back to a single point
in the history of the Universe;
The Big Bang.
The Expanding Universe
• Hubble determined that the
universe was expanding by
making a chart of distance
over time.
• The speed at which the
Universe is expanding is
called Hubble’s constant.
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Active Galaxies
• Active galaxies are galaxies
that emit large amounts of
energy from their core; the
active galactic nucleus.
• Around 10% of all known
galaxies are active.
Active Galaxies
• There are two main forms of active
galaxies:
• Radio Galaxies: elliptical galaxies
that emit massive amounts of radio
energy.
• Quasars: Small active galaxies
around the size of our solar system
that can emit thousands of times
more energy than our entire galaxy.
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Cosmology
• The study of the universe – its
nature, origin, and evolution – is
called cosmology.
• The mathematical basis for
studying cosmology is general
relativity (Einstein’s theory of
general relativity)
Cosmology
• The fact that the universe is
expanding implies that it had a
beginning.
• The theory that the universe began
as a single point and has been
expanding since is called the
Big Bang theory.
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Cosmology
• Based on the best value for the
expansion rate (Hubble’s constant,
H), the age of the universe can be
pinpointed to 13.7 billion years.
• There are three possible outcomes
for the Universe based on the Big
Bang thoery.
Cosmology
• Open Universe: The expansion of
the universe will never stop
• Closed Universe: Expansion stops
and turns into contraction caused
by gravity.
• Flat Universe: Expansion slows
down to a point which seems to
stop.
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Cosmology
• In 1965, scientists discovered a
persistent background noise in their
radio antennae caused by weak
radiation.
• This noise is referred to as cosmic
background radiation
• The radiation is interpreted to be from
the beginning of the big bang.
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Cosmology
• According to every standard
model, the expansion of the
Universe should be slowing down
due to gravity
• Discoveries show that the universe
is actually accelerating.
• The acceleration is caused by
dark energy (an unknown force).
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