Download Galaxies and Cosmology

Galaxies and Cosmology
Attendance Quiz
Are you here today?
Here!
(a) yes
(b) no
(c) Cosmetology? Like hair and nails and makeup?
Next Tuesday, 5/30:
Dr. Jorge Moreno is unavailable,
so class will be cancelled
Final Exam
• The final exam is Thursday, 6/8, from 11:30am to
1:30pm (2 hours), in this room; please arrive early!
• The final exam will be comprehensive, i.e., it will cover
all the material you have studied this quarter
• It will be multiple choice, so make sure to bring a 100question (2-sided) scantron to class!
Today’s Topics
• Large Scale Structure in the Universe
• The Distance Ladder and Galactic Distances
• Hubble’s Law
• The Distances to the Galaxies
• Redshifts and Hubble’s Law
• Expansion of the Universe
• The Age of the Universe
Local Group
The Andromeda Galaxy and the Milky Way are
approaching each other at 119 km/s, and will
collide and merge into a giant elliptical
galaxy in approximately 5 billion years
2.5 Million ly
Local Supercluster
52 Million ly
Galaxy Clusters
100s-­1000s of galaxies
10s of Millions of ly
Abell 1060
Virgo cluster
Giant elliptical galaxies
Abell 1689
Large Scale Structure in the Universe
1 billion light years
The Distance Ladder
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Radar ranging (few AU)
Parallax (AU → few 1000 light years)
Main sequence fitting (100s → 10s of thousands of light years)
Cepheid variable stars (100s of thousands → 100s of millions of light years)
Distant standards (Tully-Fisher, White Dwarf supernovae; 10s of millions to 10+
billion light years)
Standard Candles
With the exception of radar and
parallax, all these methods rely on the
concept of a standard candle
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Recall, the apparent brightness of an
object is related to its intrinsic
brightness (luminosity) and distance
by
Luminosity
Apparent brightness =
4 π × (distance)2
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Thus, if we know an object’s
luminosity, and measure its apparent
brightness, we can find its distance
Interactive Figure
Standard Candles - Examples
• Main sequence stars (MS stars as SCs)
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Spectral type ⇒ Luminosity
Calibrated for stars near enough to
measure parallax
Many stars at same distance is better
(clusters) ⇒ Main Sequence Fitting
Calibrated by cluster near enough to
show parallax - Hyades
• Cepheid variable stars (Cepheids as SCs)
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Very bright stars which vary in
luminosity in a regular way
A relation exists between variation
period and the star’s luminosity
Calibrated using nearby Cepheids of
known distance
• White Dwarf (Type I) supernovae (SCs)
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All WD Sne cross the Chandrasekhar
limit and explode in the same way;
hence all have similar luminosities
Calibrated in nearby galaxies of known
distance
Galactic Distances Quiz
Cepheid variable stars are located in two different galaxies, A and B.
Both stars have the same average apparent brightness. The star in
galaxy A has a bright-dim-bright period of 10 days, while the one in
galaxy B has a bright-dim-bright period of 30 days. Which of the two
galaxies is at a greater distance from us?
Galaxy A
b) Galaxy B
c) They are located at the
same distance.
d) There is insufficient
information to tell.
a)
Edwin Hubble and Galactic Distances
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Before Hubble, the nature of so-called
“spiral nebulae” was unknown
• Other galaxies (“island universes”)
• Gas clouds in our galaxy
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The key missing piece of evidence was the
distances to these objects
Hubble, using the Mt. Wilson 100”
telescope in Pasadena (then the largest in
the world) observed Cepheid variable stars
in other galaxies
Using the period-luminosity relationship,
he found that these objects were much
further away than the size of the Milky
Way, meaning they are independent
galaxies
Redshifts and Hubble’s Law
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It was known since the 1910s that the spectra of most spiral galaxies were
redshifted, meaning they were moving away from us
Once Hubble found the distances to these galaxies he found something startling!
Rather than the motions of the galaxies being random, there is a relationship
between redshift and the distance to the galaxy
The more distant the galaxy, the faster it recedes (Hubble’s Law)
Expansion of the Universe
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If all the galaxies are moving apart, then
earlier, they must have been closer together,
i.e., the universe is expanding!
Since all the galaxies are moving away from
the Milky Way, does this mean that we are
at the center of the universe?
Our knowledge of the history of astronomy
(Copernicus, Shapley) should make us
skeptical of this idea
How else can we explain these facts?
If the entire universe is expanding then every
galaxy sees all others receding, and if that
expansion is uniform, then the rate of motion
apart is proportional to distance (Hubble’s
Law)
Note: because gravity holds them together,
individual galaxies do not expand as the
space they occupy expands
Space Expansion + Int. Fig. 20.22
Cosmology Quiz I
Consider three widely separated galaxies in an expanding universe.
Imagine that you are located in galaxy 1 and
observe that both galaxies 2 and 3 are moving
away from you with a speed proportional to
their distance from you. If you asked an observer in galaxy 3 to
describe how galaxy 2 appears to move, what would he or she say?
a)
b)
c)
“Galaxy 2 is not moving”
“Galaxy 2 is moving toward galaxy 3”
“Galaxy 2 is moving away from galaxy 3”
Lecture Tutorial:
Expansion of the Universe, pp. 161-162
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Work with one or more partners - not alone!
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Get right to work - you have 15 minutes
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Read the instructions and questions carefully.
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Discuss the concepts and your answers with one another.
Take time to understand it now!!!!
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Come to a consensus answer you all agree on.
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Write clear explanations for your answers.
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If you get stuck or are not sure of your answer, ask another
group.
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If you get really stuck or don’t understand what the Lecture
Tutorial is asking, ask me for help.
Cosmology Quiz I
Consider three widely separated galaxies in an expanding universe.
Imagine that you are located in galaxy 1 and
observe that both galaxies 2 and 3 are moving
away from you with a speed proportional to
their distance from you. If you asked an observer in galaxy 3 to
describe how galaxy 2 appears to move, what would he or she say?
a)
b)
c)
“Galaxy 2 is not moving”
“Galaxy 2 is moving toward galaxy 3”
“Galaxy 2 is moving away from galaxy 3”
Cosmology Quiz II
According to modern ideas and observations, what can be said about
the location of the center of our expanding universe?
The Earth is at the center
b) The Sun is at the center
c) The Milky Way Galaxy is at the center
d) The universe does not have a center
a)
Hubble’s Law and the Age of the Universe
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Hubble’s Law tells us that all galaxies
are moving apart at a rate proportional to
their separation distance
The slope of this relationship is called
Hubble’s Constant, or H0
If that rate stayed the same throughout
the history of the universe, we could run
a “movie” of the universe backwards to
find out when all the galaxies were on
top of each other
The faster the galaxies are receding now
(the larger H0), the shorter the time until
the “movie” reaches its beginning
Thus, the age of the universe is
proportional to 1/H0
This simple calculation gives an age of
the universe of 13.6 billion years
Interactive Figure 1: Estimating Age of the Universe
Interactive Figure 2: Dependence of Age on H0
Cosmology Quiz III
Astronomers currently believe that the Hubble constant has a value of
about 22 km/s/Mly. If some new measurement revealed that instead
the Hubble constant is closer to 60 km/s/Mly, what would this imply
about the age of the universe?
It is much younger than current estimates
b) It is much older than current estimates
c) This change would have no effect on estimates of the age of the
universe
a)