See class handout

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See class handout
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The Milky Way Galaxy

• In broad brush form, our galaxy is composed of
  three distinct structures (see handout)
• The central nucleus and bulge
• spherical distribution of stars about GC Dia 4
  Kpc
• The disk
• contains old and newly formed stars
• spiral arms newly formed, luminous, massive
  stars
• gas and dust of the interstellar medium
• about 30 Kpc in diameter and 1 Kpc in thickness
• The halo
• Composed of old, low mass stars and globular
  clusters

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Globular cluster
Circular orbits of stars in galactic disc
Inclined, eccentric orbits of Halo stars and
globular clusters
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The Suns location
Question How far away is the Sun from the
galactic center ? Answer Look at the
distribution of globular clusters

• Harlow Shapleys method (c. 1915)
• Determine distances to many globular clusters
• use 5 star formula and standard candle method
• Then calculate the distance to the center of the
  globular cluster distribution Suns distance
  from GC

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A globular cluster Composed of old low mass
stars N(stars) 105 - 106 Diameter 10 pc
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Globular cluster
Many globular clusters
Few globular clusters
Disk
Sun
Nucleus
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The mass of the galaxy

• 1917 Harlow Shapley
• Finds that the Sun is some 8000 pc away from the
  galactic center
• Also,
• Studies of star velocities indicate that the Sun
  has an orbital speed of 220 km/s about GC
• Hence, known radius and velocity ? orbital period

time to circle GC 2 p 8000 pc / 220 km/s 240
Myr
Sun has been 19 times around GC since it formed
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A real corker of a result

• From speed and position of Sun from GC, Keplers
  3rd law tells us
• Mgalaxy a3 / P2
• where a 8000 pc and P 240 million years
• Mgalaxy 1011 M?

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Galaxy classification

• Observations
• Three main types of galaxies are observed
• Spiral galaxies S and SB types
• a type large nucleus, tightly wound spiral arms
• c type small nucleus, loose, open spiral arms
• Elliptical galaxies E type
• Irregular galaxies Irr type

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Hubbles tuning fork diagram
Sa
Sc
Irr
E0
E7
SBa
SBc
Openness of arms
Gas and dust
Nucleus / disk ratio
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• Spiral galaxies
• bright nucleus
• spiral arms mapped out by bright stars
• many regions of active star formation
• Two types of spiral galaxy
• normal spiral
• arms radiate from a central spherical nucleus
• barred spiral
• arms radiate from a bar that extends either side
  of the nucleus

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• Irregular galaxies
• no obvious structure
• large quantities of gas and dust
• many regions of active star formation
• Elliptical galaxies
• football shaped
• no regions of active star formation
• very little gas and dust
• old, low mass stars only

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Galaxy collisions

• Separation of stars tens of millions of star
  diameters
• Separation of galaxies a few galaxy diameters
• Hence
• Stars virtually never collide, but galaxies do
  collide quite often
• During galaxy collision
• Large ISM clouds become compressed and star
  formation is triggered

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The Hubble flow
Edwin Hubble and Milton Humason (1920s) studied
distant galaxies

• Observations
• distances from standard candles
• velocities from Doppler shift

Milton Humason (1891 1972)
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Assume all elliptical galaxies are the same size
V from Doppler shift, d from standard ruler method
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Galaxy data V(km/s) vs d(Mpc)
Velocity (km/s)
Slope H 75 km/s/Mpc
Distance (Mpc)
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• Result - recall lab 5
• All distant galaxies have redshifts
• (i.e., are moving away from us)
• Hubbles law (1929) for galaxy recession
  velocities (another 5 star general!!)
• velocity H x distance ()
• H Hubbles constant

OOTETK
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km/s/Mpc
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An astounding result

• Hubbles law tells us that
• The Universe is expanding uniformly
• NB The galaxies are not moving into space but
  are moving with space - it is, in fact, space
  that is expanding carrying the galaxies along
  with it

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• Faintest
• galaxies in the
• image are some
• 1000 Mpc distant
• 10 day exposure
• over a 2 arc min. by 2 arc min.
• region of the sky
• Some 1500 galaxies
• have been
• identified in the
• image
• 55 billion
• galaxies in the observable universe

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The cosmological principal

• Hubble (again) 1936 in his book
• The Realm of the Nebulae
• Observations imply
• On the large scale ( 100 Mpc) the Universe is
  homogeneous (the same everywhere) and isotropic
  (the same in all directions)
• in other words there is nothing special about our
  specific location in the Universe

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• The Cosmological Principal in essence
• completes the Copernican Revolution
• 1543 Copernicus removes the Earth from the
  center of the Solar System
• 1917 Harlow Shapley removes the Sun from the
  center of the galaxy
• 1936 Edwin Hubble removes the Milky Way galaxy
  from center of the Universe