The Milky Way

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The Milky Way

• Dr Bryce
• 2950

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Class notices

• Homework We are moving towards the end of
  semester, it is vital that you maximise your
  grade by completing all your homework
• CSP observing exercise
• Exam behaviour

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The Milky Way galaxy appears in our sky as a
faint band of light
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• All sky view
• The Milky Way in Visible light

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Dusty gas clouds obscure our view because they
absorb visible light This is the interstellar
medium that makes new star systems
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Interstellar Medium

• Can both absorb and emit light
• Most of the interstellar medium is gas and it is
  easiest to observe when it forms an emission
  cloud/nebula
• Good examples of this include the Orion Nebula
• Because the gas is predominantly hydrogen we see
  lines associated with atomic or ionized hydrogen

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We see our galaxy edge-on Primary features
disk, bulge, halo, globular clusters
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Globular clusters

• We know from our H-R diagrams that globular
  clusters are old
• One way to map the Milky Way is to consider the
  distribution of globular clusters

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Mapping Globular clusters
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If we could view the Milky Way from above the
disk, we would see its spiral arms
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• Our interpretation of the Milky Way
• Disk is thin and wide
• Note spiral arms and bar

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Stars in the disk all orbit in the same direction
with a little up-and-down motion
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Orbits of stars in the bulge and halo have random
orientations
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Suns orbital motion (radius and velocity) tells
us mass within Suns orbit 1.0 x 1011 MSun
Sun is about 8kpc from the galactic centre
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Orbital Velocity Law

• The orbital speed (v) and distance from the
  galactic centre (d) of an object on a circular
  orbit around the galaxy tells us the mass (M)
  within that orbit

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Star-gas-star cycle Recycles gas from old stars
into new star systems
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High-mass stars have strong stellar winds that
blow bubbles of hot gas
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HII regions

• H two
• Strong emission lines
• A central hot star emits UV photons which ionize
  the hydrogen
• When an electron is recaptured by a proton the
  HII line is emitted

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HII regions

• Require a hot star to have formed in a molecular
  cloud
• The hotter the star the larger the HII region can
  be
• HII regions tend to be red see the Rosette
  Nebula

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Lower mass stars return gas to interstellar space
through stellar winds and planetary nebulae
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X-rays from hot gas in supernova remnants reveal
newly-made heavy elements
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• The Milky Way at X-ray Wavelengths
• X-ray emission is produced by hot gas bubbles and
  X-ray binaries

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Supernova remnant cools and begins to emit
visible light as it expands New elements made
by supernova mix into interstellar medium
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Radio emission in supernova remnants is from
particles accelerated to near light
speed Cosmic rays probably come from supernovae
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Multiple supernovae create huge hot bubbles that
can blow out of disk Gas clouds cooling in the
halo can rain back down on disk
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Atomic hydrogen gas forms as hot gas cools,
allowing electrons to join with
protons Molecular clouds form next, after gas
cools enough to allow to atoms to combine into
molecules
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• Molecular clouds in Orion
• Composition
• Mostly H2
• About 28 He
• About 1 CO
• Many other
• molecules

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Gravity forms stars out of the gas in molecular
clouds, completing the star-gas-star cycle
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Radiation from newly formed stars is eroding
these star-forming clouds
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Gas recycling

• Stars make new elements by fusion
• Dying stars expel gas and new elements, producing
  hot bubbles (106 K)
• Hot gas cools, allowing atomic hydrogen clouds to
  form (100-10,000 K)
• Further cooling permits molecules to form, making
  molecular clouds (30 K)
• Gravity forms new stars (and planets) in
  molecular clouds

Gas Cools
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Interstellar gas temperature

• Molecular clouds are dense and at low
  temperatures (10K)
• Interstellar gas is much less dense and much
  warmer (10,000K)
• We also see very hot (1 million K) gas from
  Supernova shock waves, it is these regions that
  are responsible for the X-ray bubbles

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• The Milky Way at 21cm wavelength
• Neutral hydrogen in confined to the plane of the
  Milky Way

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21cm line

• Associated with the lowest energy level of
  Hydrogen
• Doesnt involve the hydrogen atom interacting
  with another photon so we can see this line
  anywhere in space

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Dark Nebula

• Associated with interstellar dust
• Dust particles block the photons from the stars
  behind them
• Dust will re-emit in the infra-red

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The development of our Model

• Galileo first observed that the Milky Way is made
  up of stars and many astronomers have tried to
  map it
• For example Herschel used star counts, see below

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Early models

• Were incorrect as they didnt include the effects
  of interstellar dust which will dim starlight
  (this effect is called extinction) and
  interstellar reddening
• It is for these reasons that we actually find it
  easier to study other galaxies rather than the
  galaxy in which we live

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We observe star-gas-star cycle operating in Milky
Ways disk using many different wavelengths of
light
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Halo No ionization nebulae, no blue stars

? no star formation
Disk Ionization nebulae, blue stars ? star
formation
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Halo Stars 0.02-0.2 heavy elements (O, Fe,
), only old stars
Halo stars formed first, then stopped
Disk Stars 2 heavy elements, stars of all
ages
Disk stars formed later, kept forming
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Much of star formation in disk happens in spiral
arms
Whirlpool Galaxy
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Spiral Structure

• We can easily observe spiral arms in other
  galaxies but within the Milky Way our view is
  hindered by the effects of interstellar gas and
  dust

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• Spiral arms are waves of star formation
• Gas clouds get squeezed as they move into spiral
  arms
• Squeezing of clouds triggers star formation
• Young stars flow out of spiral arms

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Density Waves

• Stars slow down in the spiral arms

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Our galaxy probably formed from a giant gas cloud
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Halo stars formed first as gravity caused cloud
to contract
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Remaining gas settled into spinning disk
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Stars continuously form in disk as galaxy grows
older
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Cloud collisions

• Collisions cause the flattening of the disk
• Upwards or downwards motions tend to be cancelled
  out

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Rotation

• Possible models for rotation
• Wheel or Merry-go-round
• Planetary or Keplerian
• Milky Way doesnt rotate like either of these
  models

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Milky Ways rotation Curve

• Is flat
• This means that the distribution of mass in the
  Milky Way continues outwards past the luminous
  material (stars)
• The dark matter could be brown dwarfs, white
  dwarfs, Jupiters, Black holes or elementary
  particles, they are not emitting light but they
  are exerting gravitational influence

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The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
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We can measure rotation curves of other spiral
galaxies using the Doppler shift of the 21-cm
line of atomic H
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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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Gravitational microlensing

• A dark object in the galactic halo (MACHO) could
  act as a lens because of the curvature of
  spacetime around it.
• Black holes would be the strongest type of
  microlens

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Infrared light from center
Radio emission from center
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Swirling gas near center
Orbiting star near center
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Stars appear to be orbiting something massive but
invisible a black hole? Orbits of stars
indicate a mass of about 4 million MSun
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X-ray flares from galactic center suggest that
tidal forces of suspected black hole occasionally
tear apart chunks of matter about to fall in