Lecture 11 : Galaxies

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Lecture 11 Galaxies

• Robert Fisher

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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.

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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.

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Review of Two Weeks Ago

• Stellar Structure
• Stellar Evolution
• Evolution of a low-mass star
• Evolution of a high-mass star
• Supernovae

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Review of Last week

• Michelson - Morley
• Special Relativity
• General Relativity

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Today

• Black Holes, White Holes, Wormholes
• Galaxies
• Distances in the universe
• Types of galaxies
• Ellipticals
• Spirals
• Irregulars

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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
  Einsteins 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.

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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.

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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
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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.

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White Holes

• The full weirdness of Schwarzschilds 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.

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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 Einstein-Rosen wormhole solution using
  exotic energy that exerts a negative
  gravitational influence.

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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 soft-spoken and kind
  manner and is of the most genuinely nicest people
  you could ever hope to meet.

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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 novels 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.

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Wormholes as Time Machines
Accelerated Motion

• 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
  clearly-defined way in this context, for instance
  imagining billiard balls moving through the
  wormhole time machine.

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Surfing Spacetime -- Detecting Gravitational
Waves

• Einsteins 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.

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The Remarkable Binary Pulsar System PSR 191316

• 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 191316.
• 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.

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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.

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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.

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Galaxies
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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.

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M57 - The Ring Nebula
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M42 - The Orion Nebula
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Will the Real Nebulae Please Stand Up ??
Andromeda Galaxy

• 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.

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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.

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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
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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.

Herschels Universe (c. 1780)
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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.

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Shapleys Globular Cluster Distribution

• Shapleys results showed that the sun was far
  from the center of the galaxy.
• The modern accepted distance is about 8.5
  thousand parsecs (kpc) -- Shapleys 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
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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.

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Standard Candles
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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
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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.

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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 !!

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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.
• Shapleys 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.

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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 Leavitts 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)!!

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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

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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.

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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.

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M51 Spiral Galaxy
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Black-Eye or Sleeping Beauty Galaxy M64
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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.

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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

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NGC 1316
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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.

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Small Maganellic Cloud
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Large Maganellic Cloud
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Hubbles Tuning Fork Diagram
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Ellipticals into Spirals? Or Spirals into
Ellipticals?

• Hubbles 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.

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Galaxy Collision Movie
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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.

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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?