Download Lecture Eleven (Powerpoint format)

Science 3210 001 : Introduction to Astronomy
Lecture 11 : Galaxies
Robert Fisher
Items
 Nathan Hearn guest lecture on dark matter on April 20th. Lunch in
the loop (on me) with Nathan following the lecture at Frontera
Fresco for anyone who wants to join us.
 Adler Planetarium field trip on May 4th - $16/person. Waiver
forms to be signed!!
 Final projects due May 11th, along with a short (5 minute)
presentation that day.
Final Project
 Your final project is to construct a creative interpretation a scientific
theme we encountered during the class. You will present your work in a
five minute presentation in front of the entire class on May 11.
 The project must have both a scientific component and a creative one.
 For instance, a Jackson Pollock-lookalike painting would fly, but ONLY if
you said that it was your interpretation of the big bang cosmological
model AND you could also demonstrate mastery of the basic
astrophysics of the big bang while presenting your work.
 Be prepared to be grilled!
 Ideas :
 Mount your camera on a tripod and shoot star trails.
 Create a “harmony of the worlds” soundtrack for the Upsilon Andromeda
system.
 Paint the night sky as viewed from an observer about to fall behind the
horizon of a black hole.
 Write a short science fiction story about the discovery of intelligent life in the
universe.
Review of Two Weeks Ago
 Stellar Structure
 Stellar Evolution
 Evolution of a low-mass star
 Evolution of a high-mass star
 Supernovae
Review of Last week
 Michelson - Morley
 Special Relativity
 General Relativity
Today
 Black Holes, White Holes, Wormholes
 Galaxies
 Distances in the universe
 Types of galaxies
 Ellipticals
 Spirals
 Irregulars
More Exotica From Relativity Theory
 Black holes are perhaps the most
exotic objects in the known universe.
 These solutions were originally
discovered by Karl Schwarzschild.
 Schwarzschild (1873 - 1916)
discovered the solutions while
serving in the German army on the
Russian front in WWI, within a year
after Einstein’s theory was published.
 Tragically, he died on the front
shortly afterward. He was, however,
survived by his son Martin
Schwarzschild, who made
fundamental contributions to stellar
structure.
Cygnus X-1
 The first strong case for the detection of a black hole was made in
the Cygnus X-1 x-ray emitting system in the 1970s.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Black Hole Physics
 In addition, as she nears the horizon, only those photons from
Alexis moving nearly vertically have a chance to escape; the ones
moving horizontally begin to fall into the black hole.
 This means that Bettie sees the signal from Alexis become more
and more highly-beamed as she moves further in.
 Alexis, on the other hand, sees the sky overhead begin to darken
to absolute black apart from a narrow cone above her.
Radio waves
Beyond the Horizon
 While Bettie will never see Alexis move behind the horizon, Alexis
actually falls behind the horizon in a finite time.
 What happens behind the horizon, and in particular what happens
as one approaches the center of the black hole is a matter of
intense speculation, but is not understood in the current
framework of physics.
 According to General Relativity, all of the mass of the black hole
is concentrated in a single point of infinite density -- the
singularity. This is in fact a breakdown of the theory itself, and so
General Relativity cannot be used to understand what goes on at
the location of the singularity.
White Holes
 The full weirdness of Schwarzschild’s
solution took many decades to sink
in.
 In particular, the most general
solution contains not only a black
hole, but also a mirror image on the
other side which ejects matter
instead of accreting it.
 The mirror image is known as a
white hole.
 The reality of white holes has been
debated over time -- no one has ever
seen anything in nature which
resembles a white hole.
Wormholes
 By joining a black hole to a white
hole, one can construct a “wormhole”
solution to the equations of General
Relativity.
 Such a solution was first discovered
by Einstein and Rosen in the 1930s.
 The neck of the Einstein-Rosen
solution, however, is unstable to
collapse.
 In 1988, Kip Thorne and his graduate
student Mike Morris showed that it is
possible to stabilize the EinsteinRosen wormhole solution using
“exotic energy” that exerts a negative
gravitational influence.
Kip Thorne (1940 - )
 Kip Thorne is perhaps the
leading figure in contemporary
General Relativity research in
the world today.
 He has contributed to virtually
every aspect of General
Relativity theory and has
supervised a whole generation of
students at the California
Institute of Technology.
 He also has an amazingly softspoken and kind manner and is
of the most genuinely nicest
people you could ever hope to
meet.
Science Fiction Begets Science
 When Carl Sagan was writing his
science fiction novel Contact, in the
early 1980s, he spoke with Kip to try
to come up with a plausible way to
rapidly transport the novel’s
characters over vast distances
without violating the laws of physics.
 Kip went to work on the problem and
actually worked out the details using
relativity theory. He suggested that
wormholes might work.
 Intringued, Thorne picked up the
wormhole problem over the next
several years and began pursuing it
as an active research project.
 Inspired by his bold lead on such a
far-out topic, other well-known
scientists like Stephen Hawking and
Igor Novikov also published work on
wormhole theory.
Wormholes as Time Machines
 Thorne suggested that it may be
possible to create a time
machine from a wormhole.
 The physics requires more
explanation than we have time
for, but as a result of
accelerating one end of the
wormhole, one has an effective
time machine.
 One can pose “grandparent”
paradoxes in a very clearlydefined way in this context, for
instance imagining billiard balls
moving through the wormhole
time machine.
Accelerated Motion
Surfing Spacetime -Detecting Gravitational Waves
 Einstein’s Theory of General Relativity predicts that spacetime
itself will form ripples which propagate at the speed of light.
 Where are these gravitational waves? Because gravity is a weak
force in comparison to electromagnetism, we have not yet directly
detected any gravitational waves.
 Physicists have searched for these gravitational waves both in
fantastically-difficult direct detection experiments on the Earth,
and in observations of the astrophysical objects.
The Remarkable Binary
Pulsar System PSR 1913+16
 Very strong indirect evidence for the existence of gravitational
waves was demonstrated by Taylor and Hulse, who were
measuring the properties of the binary pulsar system PSR
1913+16.
 Using the pulsed radio emission from the puslars themselves as
incredibly-accurate clocks, Taylor and Hulse were able to
demonstrate that the binary system is actually spinning
down, at precisely the rate predicted if the loss is due to
gravitational waves.
Direct Detection of Gravitational Wave -- The
Laser Interferometer Gravitational Observatory
(LIGO)
 Using an interferometer very similar to the one which Michelson
and Morley used in their classic experiment, scientists are
attempting at this very moment to measure the spacetime
distortion produced by gravitational radiation.
 The strongest conceivable sources of gravitational radiation are
coalescing binary black holes and neutron stars.
 Even with these incredibly intense and rare events, the expected
signal is minute -- about 1/100th of a proton diameter.
LIGO
 Two interferometers are place at two sites (one in Washington,
the other in Louisiana).
 If a signal is detected, its position on the sky will be triangalized.
Galaxies
The Question of the “Nebulae” -How Big is the Universe??
 For hundreds of years astronomers observed fuzzy “nebulae” (literally
“clouds” from Latin) in their telescopes.
 The precise nature of these nebulae was the subject of intense
speculation and debate.
 Since no one could see any individual stars in these using the smaller
telescopes and less sensitive photographic plates of the 19th century,
the consensus opinion was that all these nebulae were gas clouds in the
larger distribution of stars of our own Milky Way.
 Some of these nebulae are indeed known today to represent actual
gaseous regions nearby to us in our own galaxy.
M57 - The Ring Nebula
M42 - The Orion Nebula
Will the Real Nebulae Please Stand Up ??
 Other “spiral nebulae” turned out
to be entire galaxies like our own
Milky Way, like Andromeda.
 Viewed from a smaller
telescope, however, these
galaxies appear very blurred out
and nebulous just like the real
gaseous clouds in our own
galaxy.
 The issue reached a head in the
Great Debate of 1920.
Andromeda Galaxy
The Great Debate -A Universe of Galaxies, or a Galaxy Universe?
 The National Academy of Sciences
sponsored a debate in 1920 on the
scale of the universe, and invited
astronomers Harlow Shapley and
Heber Curtis.
 Shapley held that the Milky Way was
the entire Universe -- the “spiral
nebulae” were actually clouds of gas
within our own galaxy. He further
held that our sun was off-center
within that galaxy.
 Curtis held that the Milky Way was
only one of many galaxies in a vast
universe, and that the “spiral
nebulae” were enormously distant
from us. He held that our sun was
near the center of our own galaxy.
Not Seeing the Forest for the Trees -- The
Problem of Finding our Place in the Galaxy
 In understanding the problem of determining the shape of the
galaxy, consider an analogy.
 Imagine that we find ourselves lost in a misty forest and we
attempted to find our location by mapping out the trees.
 Because of the mist, we only see those trees nearby us.
 Even if we were close to the edge of the forest, we would never
know so from this method.
Finding Ourselves in a Misty Forest of Trees
Not Seeing the Forest for the Trees -- The
Problem of Finding our Place in the Galaxy
 In determining the position of our sun within our galaxy,
astronomers were long confused by the fact that simply counting
stars, we appear to be at the center of the Milky Way.
 The problem with this method is that it does not take into account
the absorption and reddening of starlight by intervening
interstellar gas and dust, so the sun appears to be smack in the
center of the galaxy, regardless of its actual location.
Herschel’s Universe (c. 1780)
The Shapley Model of the Universe
 Shapley made a fundamental breakthrough in our understanding
of the structure of the Milky Way by using globular clusters
instead of individual stars.
 Shapley observed that globular clusters are evenly distributed
both above and below the plane of the Milky way, and therefore
they are associated with the Milky way itself.
 It follows that the globulars should be centered about the center
of the Milky way.
Shapley’s Globular Cluster Distribution
 Shapley’s results showed that
the sun was far from the center
of the galaxy.
 The modern accepted distance
is about 8.5 thousand parsecs
(kpc) -- Shapley’s value is off
because he did not properly
account for reddening, but the
basic conclusion is correct.
 How did Shapley measure
distances of thousands of light
years?? He used a method
which had been recently
discovered by Henrietta Leavitt.
Center of Milky Way
Henrietta Leavitt (1868 - 1921)
 Leavitt made fundamental
contributions to astronomy, and is
one of the unsung heroes of modern
science.
 Leavitt overcame enormous barriers.
Besides being a woman in an era
when science was almost exclusively
male, she was also deaf.
 After graduating from Radcliffe in
1892, she was hired as a “computer”
at the Harvard Observatory.
 Despite her initial position, she
persisted and made her own
discoveries. Shortly before the time
of her death she was the head of
photometry at the observatory.
Standard Candles
Variable Stars
 Leavitt most important discovery dealt with variable stars.
 While some stars have nearly constant luminosity (like our sun),
others vary their output brightness dramatically.
 In some cases (like explosive novae and supernovae) the
brightness is not systematic, but in others it is highly regular.
Brightness
Period
Time
Cepheid Variables
 Leavitt studied one type of variable star in particular -- a certain
kind of yellow giant called a Cepheid variable.
 When the star contracts, its atmosphere becomes more opaque,
absorbs more and transmits less light.
 When it expands, its atmosphere becomes more transparent,
absorbs less and transmits more light.
The Period-Luminosity Relationship
for Cepheid Variables
 Leavitt discovered that the intrinsic luminosity of Cepheid
variables was directly related to its period.
 One can easily measure the period of any visible Cepheid.
 Using the period, and knowledge of the relationship Leavitt
discovered, one can infer the intrinsic luminosity of the Cepheid.
 Knowing its intrinsic luminosity and its observed apparent
luminosity, one can determine the distance to the star !!
Where The Spiral Nebulae Are
 On the more fundamental issue of the “spiral nebulae,” however, it was
Curtis who was ultimately more correct.
 Curtis presented a number of lines of evidence in favor of his idea. In
particular, he
 counted the number of novae arising in the Andromeda “spiral nebula” and
found it to be larger than the rest of the Milky Way.
 measured the distribution of spiral nebulae on the sky and found it to be
concentrated away from the disk of the Milky Way.
 observed that the spectra of the “spiral nebulae” were indistinguishable from
other clusters of stars.
 Shapley’s argument was partially based on observations which would
later turn out to be incorrect (eg, that Andromeda was rotating rapidly
enough to be seen in a telescope) and partially on biases. In particular, it
was nearly impossible for astronomers of that time to accept that
galaxies were separated distances of hundreds of millions of light years,
even though this was precisely the case.
Hubble and the Conclusive Evidence

The conclusive evidence in favor of the
Universe of Galaxies came a few years
later when Hubble was able to resolve
individual Cepheid variables in the
Andromeda galaxy.

Using Leavitt’s period-luminosity
relationship, calibrated by Cepheid
variables in our own galaxy, he was able
to measure the distance to Andromeda
and conclusively demonstrate that it was
far outside our own galaxy.

Practically overnight on the scale of
history, our conception of the universe
shifted dramatically. Where space before
was just plain huge (tens of thousands
of light years across the Milky Way,
filled with a billion stars), now space
was now nearly unfathomably
enormous (billions of light years across
the observable universe, filled with the
light of millions of galaxies each with
billions of stars)!!
The Great Debate in Retrospect
 “The Shapley-Curtis debate makes interesting reading even today. It is
important, not only as a historical document, but also as a glimpse into
the reasoning processes of eminent scientists engaged in a great
controversy for which the evidence on both sides is fragmentary and
partly faulty. This debate illustrates forcefully how tricky it is to pick one's
way through the treacherous ground that characterizes research at the
frontiers of science." Frank Shu (contemporary astrophysicist)
 "As to relativity, I must confess that I would rather have a subject in
which there would be a half dozen members of the Academy competent
enough to understand at least a few words of what the speakers were
saying if we had a symposium upon it. I pray to God that the progress of
science will send relativity to some region of space beyond the fourth
dimension, from whence it may never return to plague us.” Abbot to Hale
Classification of Galaxies
 Like the O-B-A-F-G-K-M classification scheme of stars, it is useful
to classify galaxies.
 Classification is a bit like butterfly-collecting; it may at first glance
appear tedious, but in reality it is the first step towards
knowledge, by beginning to observe broad classes and trends.
 Once we have established classes and trends in galactic
systems, we can begin to ask meaningful questions about how
things got that way.
Spiral Galaxies
 Spiral galaxies are one of the two major types of galaxies.
 Spirals are distinguished by
 Bluish-light indicative of massive hot young stars.
 Current star formation.
 Complex spiral structure ranging from simpler two-armed spirals to richly-
complex “flocculent” spirals.
 Lanes of dark -- indicative of dust absorption -- mixed in with lanes of
starlight.
 Generally, broken into three components -- relatively thin disk of stars and
gas, a central “bulge” of stars, and a more weakly-defined spherical halo of
stars and globular clusters.
M51 Spiral Galaxy
Black-Eye or Sleeping Beauty Galaxy M64
Barred Spirals
 Many spiral galaxies have a central “bar,” varying from a very
weakly-defined bar to a very strongly-defined one.
 In some cases one can observe a nested bar structure, where
there is also an “inner bar”.
 The problem of determination of the Milky Way highlighted by the
Curtis-Shapley debate is complex enough that it took until the late
20th century before astronomers began to conclude that our own
Milky Way probably is a weakly-barred spiral itself.
Elliptical Galaxies
 Elliptical galaxies, along with spirals, are the second major class of
galaxies.
 Elliptical galaxies are distinguished by their
 Reddish light indicative of older stars
 Absence of current star formation
 Smooth centrally-condensed distribution of light, and absence of other
strongly-defined internal structure
 Generally few dust features and little interstellar gas content
 Frequently located in clusters of galaxies, particularly towards the cluster
center
NGC 1316
Irregular Galaxies
 Some galaxies do not fall into either major category. These are
the irregulars.
 Quite often they are smaller galaxies.
 In images of the distant (and therefore very young) universe,
these types of irregular galaxies also become more common.
Small Maganellic Cloud
Large Maganellic Cloud
Hubble’s Tuning Fork Diagram
Ellipticals into Spirals?
Or Spirals into Ellipticals?
 Hubble’s classification scheme is disfavored today as an
evolutionary scenario.
 The more likely evolutionary scenario is that elliptical galaxies are
the products of the collision of two (or sometimes more) spiral
galaxies.
 This scenario has been supported by computer simulations of
colliding galaxies.
Galaxy Collision Movie
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
But do Galaxies Actually Collide?
Arp 188 and Tidal Tails
 Halton Arp, a critic of the Big
Bang model, constructed a
catalog of “unusual” galaxies in
the 1960s.
 This catalog is now understood
to be an excellent source of
galaxies which have undergone
collisions in recent cosmic
history.
 The tidal tails seen in Arp 188
(located four hundred million
light years from the Earth) kin
this Hubble Space telescope
image are several hundred
thousand light years across.
Next Week : More Black Holes and Galaxies
 What would happen if two regions of spacetime were tied
together in a “wormhole”?
 What do we think happens at the very smallest scales in which
gravity and quantum effects both become important?
 And is there a black hole at the center of the Milky Way?