Summary for Final
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Last week of classes 12/10 to 12/13
First hour (Wednesday):
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Second hour (Wednesday):
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Final, Part 3: SCANTRON Test (Form 882, #2 pencils),
70 questions, cumulative  60 from Review Questions
and 10 questions on SGA and Planispheres (70 pts)
During your Third Hour
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Planetarium Sky Quiz (30 pts)
Final, Part 1 (Deep Sky Object Quiz, 20 pts)
Final, Part 2: Group effort on questions relating to 3rd
hour (20 pts)
All extra credit due by 12/14 at NOON.
Also: HW due this Friday at 11:59 PM as usual
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Galaxy Classification
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Until the 1920s, we thought of our own
galaxy as the “Island Universe” and that
everything we saw lay in our galaxy
In 1924, Edwin Hubble found Cepheid
variables in three spiral nebulae, including
one in Andromeda, proving that they were
actually spiral galaxies.
The proof that galaxies existed outside the
Milky Way expanded the scope of the
universe.
The Hubble Classification
 Hubble divided galaxies into three basic
types: spiral, elliptical, irregular.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Galaxy Classification
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A spiral galaxy is one that is characterized by
bright spiral arms and a central bulge. They have
both a spheroidal and disk component
An elliptical galaxy is one of a class of galaxies
that have smooth spheroidal shapes. They have
only a spheroidal component
An irregular galaxy is a galaxy of irregular shape
that cannot be classified as spiral or elliptical.
Each major classification contains subdivisions
and come in variety of sizes
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Galaxy
Types
Dwarf galaxies: as few as 100 million stars
Giant galaxies: as great as 1 trillion stars
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M51 = Whirlpool Galaxy, face-on spiral galaxy +
Canes Venatici
M87 = Virgo A, giant elliptical galaxy @ 60 MLY
Virgo
The Large Magellanic Cloud = Irregular galaxy
Lecture 13: The Diverse Galaxies
Galaxy Classification
Spiral Galaxies
 Hubble divided spiral galaxies into two groups:
normal spirals and barred spirals.
 A barred spiral galaxy is a spiral galaxy in which
the spiral arms come from the ends of a bar
through the nucleus rather than from the nucleus
itself.
 Spirals are designated with an S; barred spirals
are designated with an SB.
 A few galaxies lack spiral arms but have a
spheroidal shape and dust and are designated S0
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Barred
Normal
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M 74, normal spiral
NGC 1365, a barred spiral
NGC 3992, a barred spiral
NGC 5866, a lenticular (S0) galaxy
Lecture 13: The Diverse Galaxies
Galaxy Classification
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Each type of spiral galaxy is then further
subdivided into categories a, b, c, and d
depending on how tightly the spiral arms are
wound around the nucleus.
Galaxies with the most tightly wound arms are
type a.
The size of a spiral galaxy’s bulge can also be
used to determine the subdivision.
Most spiral galaxies are from 50,000 to
2,000,000 light-years across and contain from
109 to 1012 stars.
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M104 = Sombrero Galaxy, edge on spiral galaxy
Type Sa
Virgo
M31 = Andromeda Galaxy, spiral galaxy @ 2.5 MLY
Type Sb
Andromeda
M51 = Whirlpool Galaxy, face-on spiral galaxy +
Type Sc
Canes Venatici
Lecture 13: The Diverse Galaxies
Galaxy Classification
Elliptical Galaxies
Elliptical
Irregular
 Elliptical galaxies are classified from round
(E0) to very elongated (E7).
 Most of the galaxies in existence are
ellipticals, but most of these are smaller than
spiral galaxies.
 A few giant elliptical galaxies have 1013 stars
and are thus larger than any spiral galaxy.
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Elliptical galaxies
E6
E4
M49
M110
E1
E0
M87
M89
Lecture 13: The Diverse Galaxies
Galaxy Classification
Irregular Galaxies
 Fewer than 20% of all galaxies fall in the
category of irregulars (designated as Irr)
 They are all small, normally having fewer than
25% of the number of stars in the Milky Way.
 Collisions between galaxies are not unusual
because on average galaxies are separated by
distances only about 20 times their diameter.
 What Collides?
© Sierra College Astronomy Department
Elliptical
Irregular
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Lecture 13: The Diverse Galaxies
Galaxy Classification
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Because of their great distances, galaxies
exhibit no proper motion. Evidence of past
collisions has to come from present
appearance.
When galaxies collide gas and dust clouds
interact, but there are few collisions between
individual stars.
Sometimes a larger galaxy may merge or
cannibalize (galactic cannibalism) a smaller
galaxy.
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The Antennae Galaxy – A galaxy merger
The Cartwheel Galaxy
Lecture 13: The Diverse Galaxies
Galaxy Classification
Hubble’s Tuning Fork Diagram
 Hubble’s tuning fork diagram relates the
various types of galaxies.
 Astronomers once also thought the
diagram represented an evolutionary
sequence, but this interpretation has been
discarded as old stars have been found in
all three types.
© Sierra College Astronomy Department
Tuning
Fork
Diagram
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Lecture 13: The Diverse Galaxies
Galaxy Classification
Classif.
Type
Designation
Elliptical
E0–E7
Spiral
Sa–Sc
Barred spiral SBa–SBc
S0
S0
Irregular
Irr
Description
Galaxies that appear
circular (E0) to very
elongated (E7).
Sa: large nuclei and tightly
wound arms. Sc: small
nuclei and open arms.
Spirals with elongated
nuclei.
Disklike; no spiral structure.
Do not fit into any other
category.
Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
Direct Distance Measures
 We have discussed two methods of
“direct” measurement
 Radar
ranging: bounce radio waves off of
an object and have it come back to the
Earth
 Parallax: The Earth’s motion around the
Sun cause nearby stars shift relative to far
away stars. The shift is inversely
proportional to distance
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
Indirect Distance Measures
 To get distances to objects further away we
must use indirect methods
 Astronomers often find standard candles or
objects of known luminosity to measure
distances
Recall: luminosity and brightness leads to
distance
 Example: A main sequence star with a spectral
class of G2 (like the Sun) should have a similar
luminosity as the Sun
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© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
Main
Sequence
fitting
Main sequence fitting
 Sun-like stars are relatively dim and hard to see
beyond 1000 light-years (ly)
 To measure distance to objects further away we use
the method of main-sequence fitting
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We find the distance to a star cluster using parallax
methods and plot its H-R diagram
To find the distance of a more distant cluster, plot the
more distant cluster’s H-R diagram and compare to the
closer star cluster.
The difference in relative brightness between the clusters
is related to the distance ratio between them via the
inverse square law
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
Cepheid Variables
 Some stars are intrinsically variable on a
Cepheids
regular or semi-regular basis
 Delta Cephei’s characteristic light curve rapid brightening followed by slow dimming.
 Doppler effect data showed that this star and
others are pulsating in rhythm with their
changes in luminosity.
 This fairly easy class of stars to identify were
named Cepheid variables or Cepheids.
© Sierra College Astronomy Department
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Cepheids2
Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
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Each Cepheid has a very constant period of
variation, ranging from about 1 day to about
3 months for different Cepheids.
In 1908, Henrietta Leavitt discovered
that for Cepheids the more luminous
variables have the longer periods.
Cepheids are important because the
period, which is easy to determine,
allows the absolute magnitude to be
determined.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
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Cepheids could be valuable distance indicators
if the distance to one could be determined.
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Can be see out to 100 million ly.
None are close enough to be measured
by parallax, but beginning in 1917,
Shapely worked out a complex statistical
method to determine distances to
Cepheids in our own galaxy.
Shapely’s work led to a periodluminosity diagram for Cepheid
variables.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
Distant Standard Candles
 Measuring distant objects more than 100
millions of light-years (100 Mly) requires an
even more luminous object
 Sometime large galaxies can be used as
standard candles
 The white dwarf supernovae is a very
consistent since 1.4 solar masses (the
Chandrasekar limit) is always exploding.
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However, these do not occur in a given galaxy
very frequently
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Measuring Distances to Galaxies
The Hubble Law
 In 1921, Slipher found that spiral
nebulae had redshifted spectra
indicating that they were moving away
from us at tremendous velocity.
 In 1929, Hubble and Humason
showed that there is a relationship
between the recessional velocities of
galaxies and their distances.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Hubble Law
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The Hubble law:
v = H 0d
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where v is radial velocity, d is distance, and H0 is
the Hubble constant (the 0-subscript refers to its
value today, and not the past).
The Hubble constant is the proportionality
constant in the Hubble law; the ratio of
recessional velocities of galaxies to their
distances.
The Hubble law is not ideal:
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Hubble
Law
Hubble
Spectra
Hubble
Spectra
It does not apply to nearby galaxies where gravity
dominates
It relies on a accurate measurement of Hubble’s constant
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Distance Chain
The distance chain summary
1. Radar Ranging
2. Parallax
3. Main-sequence fitting
4. Cepheid Variables
5. Distant Standards
6. Hubble’s Law
© Sierra College Astronomy Department
Distance
chain
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Lecture 13: The Diverse Galaxies
Hubble Law
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The Hubble law shows that the universe is expanding, and it is
the foundation for today’s theories of cosmology - the study of
the nature and evolution of the universe as a whole.
Modern day measurements of the Hubble constant place it about
22 km/s per million light-years or 73 km/s per megaparsec
For the most distant galaxies, the Hubble law must be used to
determine their distances.
Astronomy 25 provides an in-depth study of cosmology.
 Interested? Offered next Spring
 More advertising:
Astro 11, 14, and Astro 2, 5
Int 11 (or Astro 7) Astrobiology
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Hubble Expansion
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If the Universe is expanding uniformly
today, then galaxies must have been
closer in the past.
At sometime in the past everything must Expansion
have been contained at one point and Of universe
that everthing came into being at a
single moment – the Big Bang
The cosmological principle is the idea
that all matter in the universe is evenly
distributed without a center of edge.
The age of the universe can be estimated
by the inverse of the Hubble constant.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Hubble Expansion
Look-Back Time
 Look-back time is the time light from a distant
object has traveled to reach us.
 Objects have been detected that may be as far
away as 13 billion light-years.
 Look-back time complicates our interpretation of
galaxies because the farther out we look, the
earlier in time we are seeing them.
 However, if the speed of light was infinitely large,
we would not be able to gather data from the
past, data that gives us clues to the Universe’s
evolution.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Hubble Expansion
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The Look-back time is directly related to
the redshift of the galaxies
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This is an expansion of the geometry of
space and not the rushing of galaxies
through space
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expansion
No one galaxy - the Milky Way included - is in any
special, central position.
This is known as the cosmological redshift
The cosmological horizon marks a boundary
in time, not space and represent the edge of
the observable universe.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Galaxy Evolution
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Now turn how galaxies change over time
– galaxy evolution
Galaxies near us are 13 billion years old,
while galaxies seen with a lookback time
of 10 billion years, are 3 billions years
old
This allows to see what has changed
about galaxies over the many years,
though looking at distant galaxies is
more difficult because they are so far
away.
© Sierra College Astronomy Department
Family
album
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Lecture 13: The Diverse Galaxies
Galaxy Evolution
How did galaxies form?
 The most successful models assume the
following:
Hydrogen and helium gas filled all of space
nearly uniformly
 The distribution of matter was not perfectly
uniform, as certain areas of the universe
were slightly more dense than others.
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Starting with these assumptions, after
one billion years after the Big Bang,
protogalactic clouds started to form and
contract via gravity.
© Sierra College Astronomy Department
protogals
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Lecture 13: The Diverse Galaxies
Galaxy Evolution
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As the protogalatic clouds formed the coolest
and densest material formed into the first star
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These tended to massive stars which went
supernovae with 10 million years
This in turn lead to more star formation from the shock
waves of the supernovae
The disk population is a fairly flat plane with
uniform revolution
 The spheroidal population of stars (including
the globular clusters) was formed before the
galaxy collapsed into a disk and has more
randomly oriented orbits.
This explains the basic structure of spiral
galaxies
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© Sierra College Astronomy Department
protogals
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Lecture 13: The Diverse Galaxies
Galaxy Evolution
Why are galaxies different?
 If a galaxy did have a great deal of
angular momentum (which causes it to
spin) then it would not flatten out and
would form an elliptical galaxy
 An irregular galaxy is thought to be
caused by a collision of two or more
galaxies, though collisions may form an
elliptical galaxy as well.
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Some collisions can cause an incredible
increase of star formation. These galaxies
are called starburst galaxies.
© Sierra College Astronomy Department
protogals
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Lecture 13: The Diverse Galaxies
Active Galaxies
Active Galaxies
 An active galaxy is a galaxy with an
unusually luminous nucleus.
 Three main types of active galaxies:
 Radio
galaxies
 Have
greatest luminosity at radio wavelengths
with a double-lobed radio source.
 Radio galaxies often exhibit unusual jets in
visible light.
 Generally, they are elliptical galaxies.
© Sierra College Astronomy Department
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Lecture 13: The Diverse Galaxies
Active Galaxies - Quasars
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Seyfert galaxies
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A class of spiral galaxies having abnormally luminous nuclei.
The immense luminosity is spread over all wavelengths are
fluctuates rapidly.
Contain very fast moving gas clouds in some instances being
ejected in small jets.
BL Lacertae objects is another type of active galaxy
which have their jets point right at us
Quasars (Quasi-stellar objects or QSO)
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Blobs
Moving 
away
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Quasar
A small, intense celestial source of radiation with a very
large redshift (implying speeds close to c and at very large
cosmological distances).
Some are powerful radio sources and others eject hot gas
from their centers.
Often appear to lie within ordinary galaxies.
© Sierra College Astronomy Department
Jet
drawing
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Double Radio lobes of Cygnus A
Seyfert
Galaxy
NGC 7742
Intense core
Star formation
region
Lecture 13: The Diverse Galaxies
The Nature of Active Galaxies
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What Makes Some Galaxies Active?
 Current explanation: An accretion disk feeding
material into a supermassive black hole at
the galactic center.
 A supermassive
black hole is created in the early
years of a galaxy’s growth.
 As long as there is enough material in the disk to
feed to black hole, the galaxy remains active.
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All galaxies appear to have supermassive
black holes at their centers.
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The different types of active galaxies may be the
same basic object simply seen from different
vantage points.
Quasar model
Lecture 13: The Diverse Galaxies
Active Galaxies - Quasars
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From the Hubble Law, a large redshift
implies large distances and existence in the
past (era of the quasars).
Due to their large distances, quasars provide
an excellent testing ground for general
relativity through observations of
gravitational lensing and microlensing.
Era of
Quasars
© Sierra College Astronomy Department
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The End
© Sierra College Astronomy Department
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