Galaxies

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Galaxies Island Universes

• A historic tour of the discovery of the dwindling
  significance of humans in the universe
• From the center of the universe towards the edge
  of an average galaxy amongst 100 billion others

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How do we know where we are?

• Obviously we are living on a flat Earth at the
  center of the universe, as a quick look tells
  us
• The stars, Sun, Moon and planets rotate us
• There is no apparent curvature of the ground
• The Milky Way is a band that surrounds us
• There are no signs for any movement of the Earth
  (like wind, or forces throwing us off)

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Logic to the Rescue

• How do we avoid these wrong conclusions?
• Sound data
• Flawed interpretation/reasoning
• ?Further observations are necessary to decide!
• Do we have to question everything?
• Yes, in principle.
• The signature of genius is to ask the right
  question, not necessarily to answer them.

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Exploring our own Island Universe The Milky Way

• A galaxy is a huge collection of stars, gas,
  dust, neutron stars, and black holes, isolated
  from others and held together by gravity

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Our view of the Milky Way

• Appears as a milky band of light across the sky
• A small telescope reveals that it is composed of
  many stars (Galileo again!)
• Our knowledge of the Milky Way comes from a
  combination of observation and comparison to
  other galaxies

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How do we know?
Obviously a bogus picture of our milky way!

• Question How can we say anything about our Milky
  Way, if we cannot see it from outside?

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Enter the Genius

• William Herschel (XVIII century)
• Simple model
• Assumed all stars have the same absolute
  brightness
• Counts stars as a function of apparent magnitude
• Brighter stars closer to us fainter stars
  further away
• Cut off in brightness corresponds to a cut off at
  a certain distance.
• Conclusion there are no stars beyond a certain
  distance

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Herschels Findings

• Stars thinned out very fast at right angles to
  Milky Way
• In the plane of the Milky Way the thinning was
  slower and depended upon the direction in which
  he looked
• Flaws
• Observations made only in visible spectrum
• Did not take into account absorption by
  interstellar gas and dust

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Discovering other Island Universes

• Data Lots of nebulous spots known in the
  nightsky
• Questions What are they? All the same? Different
  things?
• Need more observations!
• ? Build bigger telescopes

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The first nebula discovered to have spiral
structure M51
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M99 is a spiral, too!

• Q do we live in a spiral?
• Q Are we in the center of the spiral?
• Most probable answer No!

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Enter next genius

• Harlow Shapley used variable stars, e.g. RR Lyrae
  stars, to map the distribution of globular
  clusters in the galaxy
• Found a spherical distribution about 30 kpc
  (30,000 pc) across
• This is the true size of the galaxy
• Sun is (naturally!) not at the center its
  about 26,000 ly out

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Standing on the shoulders of Giants

• Shapley used methods developed by others to
  measure the distance to globulars
• Cepheid variables show luminosity-period
  correlations discovered by Henrietta Leavitt
• Shapley single-handedly increase the size of the
  universe tenfold!

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Structure of the Galaxy
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Intra-galactic Dynamics

• Three main parts of a galaxy
• Bulge (center of galaxy)
• Disk (rotating around center)
• Halo (orbiting around bulge with randomly
  inclined orbits)

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Properties of Bulge, Disk and Halo

• Disk Halo
  Bulge
• Highly flattened spherical
  football-shaped
• young and old stars only old stars
  young and old stars
• has Gas and dust none
  lots in center
• Star formation none since 10
  billion yrs in inner regions
• White colored, reddish
  yellow-white
• blue spiral arms

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An up-to-date Reconstruction
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Other Galaxies Hubble supersedes Shapley

• Edwin Hubble identified single stars in the
  Andromeda nebula (turning it into a galaxy)
• Measured the distance to Andromeda to be 1
  million Ly (modern value 2.2 mill. Ly)
• Conclusion it is 20 times more distant than the
  milky ways radius ? Extragalacticity!
• ? Shapleys theory falsified!

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Q How many galaxies are there?

• Hubble Deep Field Project
• 100 hour exposures over 10 days
• Covered an area of the sky about 1/100 the size
  of the full moon
• Probably about 100 billion galaxies visible to us!

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• About 1,500 galaxies in this patch alone
• Angular size 2 minutes of arc

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Other Galaxies

• there are 100 billion galaxies in the
  observable Universe
• measure distances to other galaxies using the
  period-luminosity relationship for Cepheid
  variables
• Type I supernovae also used to measure distances
• Predictable luminosity a standard candle
• Other galaxies are quite distant
• Andromeda (M31), a nearby (spiral) galaxy, is 2
  million light-years away and comparable in size
  to Milky Way
• Island universes in their own right

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Q How does our galaxy look like from the outside?

• Probably like others, so observe them!

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Hubble Classification Scheme

• Edwin Hubble (1924) grouped galaxies into four
  basic types
• Spiral
• Barred spiral
• Elliptical
• Irregular
• There are sub-categories as well

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Spirals (S)

• All have disks, bulges, and halos
• Type Sa large bulge, tightly wrapped, almost
  circular spiral arms
• Type Sb smaller bulge, more open spiral arms
• Type Sc smallest bulge, loose, poorly defined
  spiral arms

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Barred Spirals (SB)

• Possess an elongated bar of stars and
  interstellar mater passing through the center

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Elliptical (E)

• No spiral arms or clear internal structure
• Essentially all halo
• Vary in size from giant to dwarf
• Further classified according to how circular they
  are (E0E7)

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S0/SB0

• Intermediate between E7 and Sa
• Ellipticals with a bulge and thin disk, but no
  spiral arms