chapter 20: galaxies and the foundation of modern cosmology · what have we learned? • how are...
TRANSCRIPT
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Chapter 20 Lecture
Chapter 20:
Galaxies and the
Foundation of
Modern
Cosmology
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Galaxies and the Foundation of Modern
Cosmology
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20.1 Islands of Stars
• Our goals for learning:
• How are the lives of galaxies connected
with the history of the universe?
• What patterns do we find among the
properties of the galaxies?
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How are the lives of galaxies connected with
the history of the universe?
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Hubble eXtreme Deep Field
• Our deepest images of the universe show a
great variety of galaxies, some of them billions
of light-years away.
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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.
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What patterns do we find among the
properties of galaxies?
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• Galaxies can be
divided into three
categories: spiral,
elliptical, and
irregular
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Spiral Galaxies
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Spiral Galaxies
• Here, the central bulge (part of the halo, or
spheroidal component of a spiral galaxy) can be
seen prominently.© 2017 Pearson Education, Inc.
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Spiral Galaxies
• In this barred spiral galaxy, seen face on, the
young, bluish-white stars associated with
ongoing star formation in the disk can be seen. © 2017 Pearson Education, Inc.
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Thought Question
Why does ongoing star formation lead to a blue-
white appearance?
A. There aren't any red or yellow stars.
B. Short-lived blue stars outshine the others.
C. Gas in the disk scatters blue light.
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Thought Question
Why does ongoing star formation lead to a blue-
white appearance?
A. There aren't any red or yellow stars.
B. Short-lived blue stars outshine the others.
C. Gas in the disk scatters blue light.
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• Lenticular galaxy:
has a disk like a
spiral galaxy but
much less dusty
gas (intermediate
between spiral
and elliptical)
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• Elliptical
galaxy: all
spheroidal
component,
virtually no disk
component
• Red-yellow color
indicates older
star population.
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• Irregular galaxy
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Blue-white color
indicates ongoing
star formation.
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Hubble's galaxy classesSpheroid
dominates
Disk
dominates
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• Spiral galaxies
are often found in
groups of
galaxies (up to a
few dozen
galaxies).
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• Elliptical galaxies
are much more
common in huge
clusters of
galaxies
(hundreds to
thousands of
galaxies).
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• Galaxies tend to
fall into two
groups, the blue
cloud and red
sequence.
• Can you guess
which types of
galaxies fall into
each group?
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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 patterns do we find among the
properties of galaxies?
• There are three types of galaxies: spiral,
elliptical, and irregular.
• Gas and dust are more abundant in spiral
and irregular galaxies.
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20.2 Measuring Galactic Distances
• Our goals for learning:
• How do we measure the distances to
galaxies?
• How did Hubble prove that galaxies lie far
beyond the Milky Way?
• What is Hubble's law?
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How do we measure the distances to
galaxies?
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• Brightness
alone does not
provide
enough
information to
measure the
distance to an
object.
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• Step 1
• Determine size of the solar system using radar.
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• Step 2
• Determine the
distances of
stars out to a
few hundred
light-years
using parallax.
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• Luminosity
passing through
each sphere is
the same.
• Area of sphere:
4π (radius)2
• Divide
luminosity by
area to get
brightness.
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• The relationship between apparent brightness and
luminosity depends on distance:
• We can determine a star's distance if we know its
luminosity and can measure its apparent brightness:
• A standard candle is an object whose luminosity we
can determine without measuring its distance.© 2017 Pearson Education, Inc.
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Thought Question
Which kind of stars are best for measuring large
distances?
A. high-luminosity stars
B. low-luminosity stars
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Thought Question
Which kind of stars are best for measuring large
distances?
A. high-luminosity stars
B. low-luminosity stars
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• Cepheid
variable stars
are very
luminous.
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• Step 3
• Because the period of Cepheid variable stars
tells us their luminosities, we can use them as
standard candles.
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• Cepheid variable stars with longer periods have
greater luminosities.© 2017 Pearson Education, Inc.
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• White-dwarf
supernovae can
also be used as
standard
candles.
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• Step 4
• The apparent
brightness of a
white dwarf
supernova tells us
the distance to its
galaxy (up to 10
billion light-years).
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How did Hubble prove that galaxies lie far
beyond the Milky Way?
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The Puzzle of "Spiral Nebulae"
• Before 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.
• The debate remained unsettled until Edwin
Hubble finally measured their distances.
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• Hubble settled the debate by measuring the
distance to the Andromeda Galaxy using
Cepheid variables as standard candles.© 2017 Pearson Education, Inc.
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What is Hubble's law?
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• The spectral features of virtually all galaxies are
redshifted, which means that they're all moving
away from us.
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• By measuring distances to galaxies, Hubble
found that redshift and distance are related in a
special way.
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• Hubble's law: Velocity = H0× distance
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• Redshift of a
galaxy tells us
its distance
through
Hubble's law:
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• Distances of the
farthest
galaxies are
measured from
their redshifts.
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• We measure galaxy distances using a chain of
interdependent techniques.
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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.
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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× distance
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20.3 The Age of the Universe
• Our goals for learning
• How does Hubble's Law tell us the age of
the universe?
• How does expansion affect distance
measurements?
• Why does the observable universe have a
horizon?
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How does Hubble's Law tell us the age of
the universe?
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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. 1 minute
B. 30 minutes
C. 60 minutes
D. 120 minutes
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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. 1 minute
B. 30 minutes
C. 60 minutes
D. 120 minutes
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• 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).
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• One example of something that expands but
has no center or edge is the surface of a
balloon.
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Cosmological Principle
• Matter is evenly distributed on very large scales
in the universe.
• It has no center or edges.
• The cosmological principle has not been proven
beyond a doubt, but it is consistent with all
observations to date.
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The universe looks about the same no
matter where you are within it.
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Thought Question
You 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. 1 million years
B. 14 million years
C. 10 billion years
D. 14 billion years
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Thought Question
You 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. 1 million years
B. 14 million years
C. 10 billion years
D. 14 billion years
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• Hubble's constant tells us the age of universe
because it relates the velocities and distances
of all galaxies.
~ 1/H0
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Distance
VelocityAge =
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How does expansion affect distance
measurements?
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• Distances
between
faraway galaxies
change while
light travels.
• Astronomers
think in terms of
lookback time
rather than
distance.
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• Expansion stretches photon wavelengths,
causing a cosmological redshift directly related
to lookback time.
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Why does the observable universe have a
horizon?
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• The Cosmological Horizon marks the limits of
the observable universe.
• It is a horizon in time rather than space. Since
looking far away means looking back in time,
there must be a limit – the beginning of the
universe!
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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.
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What have we learned?
• Why does the observable universe have a
horizon?
• We cannot see back to a time before the
beginning of the universe!
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