The%20Milky%20Way%20

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The Milky Way Our Galaxy
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Exam II

• A 25 or better
• B 20 or better
• C 17 or better
• D 15 or better
• F 13 or below
• Average 20.7

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Solutions to Exam III (updated)

• 1a,2b,3b,4d,5e,6a,7b,8c,9a,10c,11d,12a,
  13c,14c,15c,16c,17b,18d,19c,20a,21b,22c,
  23c,24e,25b,26d,27b,28c,29e,30a,31d,32b

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Toughest Questions

• Hydrostatic Equilibrium
• Temperature of Suns surface
• Star of absolute mag 4.6, apparent magnitude 3.2

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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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Famous Telescopes - Herschel

• Herschel detected Uranus (1781)
• (Uranus is visible with the unaided eye)

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Famous Telescopes Lord Ross

• 72 inch Reflector
• built during potato famine in Ireland
• Largest Telescope until Mt Wilson (1917)

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The first nebula discovered to have spiral
structure M51
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Lord Rosse (1845) M51
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Hubble Space Telescope (2007) 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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An observer far outside our galaxy would best
describe our galaxy and the Sun's position in it
as a

• a) disk of stars with our Solar System 2/3
  towards the edge.
• b) disk of stars with a bulge containing our
  Solar System.
• c) sphere of stars centered on our Solar System.
• d) sphere of stars with our Solar System near
  the edge.

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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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Activity Milky Way Scales

• Form groups of 3-5
• Work on the questions on the handout
• Hold on the the sheets until we talked about your
  findings
• Turn them in with your names on (one sheet per
  group)

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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

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Test What type is this galaxy?

• Spiral
• Barred Spiral
• Irregular
• Elliptical

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And this one?

• Spiral
• Barred Spiral
• Irregular
• Elliptical

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Type?

• Spiral
• Barred Spiral
• Irregular
• Elliptical

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Heres a weird one!

• Spiral Elliptical
• Barred Spiral Irregular

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Solutions

• Sb (Andromeda Galaxy M31)
• E2 (Elliptic Galaxy)
• SBb (Barred spiral galaxy)
• Ir II (Irregular galaxy M82)

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Q How do we know we live in a Spiral Galaxy?

• After correcting for absorption by dust, it is
  possible to plot location of O- and B- (hot young
  stars) which tend to be concentrated in the
  spiral arms
• Radio frequency observations reveal the
  distribution of hydrogen (atomic) and molecular
  clouds
• Evidence for
• galactic bulge
• spiral arms

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Rotation of the Galaxy

• Stars near the center rotate faster those near
  the edges rotate slower (Kepler)
• The Sun revolves at about 250 km/sec around the
  center
• Takes 200-250 million years to orbit the galaxy
  a galactic year

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How do spiral arms persist?

• ? Why dont the curl up?

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Spiral Density Waves

• A spiral compression wave (a shock wave) moves
  through the Galaxy
• Triggers star formation in the spiral arms
• Explains why we see many young hot stars in the
  spiral arms

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Density (Shock) Waves
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The Mass of the Galaxy

• Can be determined using Keplers 3rd Law
• Solar System the orbital velocities of planets
  determined by mass of Sun
• Galaxy orbital velocities of stars are
  determined by total mass of the galaxy contained
  within that stars orbit
• Two key results
• large mass contained in a very small volume at
  center of our Galaxy
• Much of the mass of the Galaxy is not observed
• consists neither of stars, nor of gas or dust
• extends far beyond visible part of our galaxy
  (dark halo)

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Galaxy Masses

• Rotation curves of spiral galaxies comparable to
  milky way
• Masses vary greatly

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The Missing Mass Problem

• Dark Matter is dark at all wavelengths, not just
  visible light
• The Universe as a whole consists of up to 25 of
  Dark Matter! ? Strange!
• What is it?
• Brown dwarfs?
• Black dwarfs?
• Black holes?
• Neutrinos?
• Other exotic subatomic particles?
• Actually Most of the universe (70) consists of
  Dark Energy ? Even stranger!

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Missing Mass Problem
Actual data
Hypothetical Keplerian motion

• Keplerian Motion more distance from center ?
  less gravitational pull ? slower rotational speed

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Galaxy Formation

• Not very well understood
• More complicated than stellar formation, and
  harder to observe
• Formation of galaxies begins after Big Bang
• Different than star formation because galaxies
  may collide and merge

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Galaxy Formation

• Galaxies are probably built up by mergers
• Contrast to break up of clouds in star formation
• Our own Milky Way is eating up the neighboring
  Sagittarius Dwarf Galaxy

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Galaxy Mergers

• Start with high density of small proto-galaxies
• Galaxies merge and turn into bigger galaxies

Actual photo (HST) lots of small galaxies
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Galaxy Interaction

• Galaxy Collision NGC2207 vs. IC2163

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Collision between NGC 4038 and NGC 4039
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The Tully-Fisher Relation

• A relation between the rotation speed of a spiral
  galaxy and its luminosity
• The more mass a galaxy has ?the brighter it is ?
  the faster it rotates ? the wider the spectral
  lines are
• Measuring rotation speed allows us to estimate
  luminosity comparing to observed (apparent)
  brightness then tells us the distance

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Active Galaxies
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Types of Active Galaxies

• Radio galaxies radiate a long radio frequencies
• Seyfert Galaxies between normal and active,
  compact core
• Quasars quasi stellar objects, very far away,
  maybe early stage of galaxy
• Note most active galaxies look normal in
  visible frequencies

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Seyfert Galaxies

• Look like normal spiral galaxies
• Energy output mostly in IR and radio frequencies
• Emitted from small region nucleus of galaxy
• Nucleus of Seyfert galaxy 10,000 times brighter
  than of normal galaxy

NGC 5728
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Energy Output

• Active galaxies emit most of their energy in
  radio frequencies

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Quasars

• Quasi-stellar objects
• Appear like stars on photographs
• Very distant objects
• Very high luminosity
• Essentially a quasar is a galaxy with
  exceptionally bright core
• Might be young galaxies

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A typical Quasar

• Very distant
• Very faint
• Appears star-like

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Beyond the Galactic Scale Clusters of Galaxies

• The Local Group The Virgo Cluster

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Superclusters
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Beyond Superclusters

• Strings, filaments, voids
• Reflect structure of the universe close to the
  Big Bang
• Largest known structure the Great Wall (70 Mpc ?
  200 Mpc!)