Galaxies

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Galaxies

• Dr Bryce
• 2950

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Class notices
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Edwin Hubble
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The Great Debate

• Before Hubble, some scientists argued that
  spiral nebulae were entire galaxies like our
  Milky Way, while others maintained they were
  smaller collections of stars within the Milky Way
• The debate remained unsettled until someone
  finally measured their distances

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Hubble settled the debate by measuring the
distance to the Andromeda Galaxy using Cepheid
variables as standard candles
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Distribution of Galaxies

• We find few galaxies in the plane of the Milky
  Way
• The Milky Way obscures our view

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Galaxies and Cosmology

• A galaxys age, its distance, and the age of the
  universe are all closely related
• The study of galaxies is thus intimately
  connected with cosmology the study of the
  structure and evolution of the universe

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Hubble Ultra Deep Field
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy
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halo
disk
bulge
Spiral Galaxy
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Disk Component stars of all ages, many gas clouds
Blue-white color indicates ongoing star formation
Spheroidal Component bulge halo, old stars, few
gas clouds
Red-yellow color indicates older star population
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Hubble Classification

• Tuning fork diagram
• Not an evolutionary flow (unlike HR diagrams)

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Hubbles galaxy classes
Spheroid Dominates
Disk Dominates
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Elliptical Galaxies

• Have a large range of sizes from 1 million solar
  masses to 1014 solar masses
• Stellar motions are randomly orientated, stars
  are orbiting the galactic centre but not in an
  organised fashion

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Line of Sight

• Ellipiticals are classified according to their
  appearance, but this actually depends on the
  angle through which they are viewed

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Elliptical Galaxy All spheroidal component,
virtually no disk component Few young stars Not
much interstellar medium
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Barred Spirals

• Called Sb
• Young stars in disk
• Significant amounts of interstellar gas and dust

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Barred Spiral Galaxy Has a bar of stars across
the bulge
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Spiral Galaxies

• Called S
• Young stars in disk
• Significant amounts of interstellar gas and dust

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Lenticular Galaxy Has a disk like a spiral
galaxy but much less dusty gas (intermediate
between spiral and elliptical)
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Irregular Galaxy
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Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
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Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
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Considering distance
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Step 1 Determine size of solar system using radar
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Step 2 Determine distances of stars out to a few
hundred light-years using parallax
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Luminosity passing through each sphere is the
same Area of sphere 4p
(radius)2 Divide luminosity by area to get
brightness
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The relationship between apparent brightness
and luminosity depends on distance We can
determine a stars distance if we know its
luminosity and can measure its apparent
brightness A
standard candle is an object whose luminosity we
can determine without measuring its distance
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Step 3 Apparent brightness of star clusters
main sequence tells us its distance
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Step 4 Because the period of a Cepheid variable
star tells us its luminosity, we can use these
stars as standard candles
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Cepheid variable stars are very luminous
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Step 5 Apparent brightness of white-dwarf
supernova tells us the distance to its
galaxy (up to 10 billion light-years)
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Step 6 Tully-Fisher Relation Entire galaxies
can also be used as standard candles because
galaxy luminosity is related to rotation speed
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Hubbles law
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The spectral features of virtually all galaxies
are redshifted ? Theyre all moving away from us
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Hubbles Law

• Hubbles Law velocity H0 x distance

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Redshift of a galaxy tells us its distance
through Hubbles Law distance
velocity H0
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Distances of farthest galaxies are measured from
redshifts
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Thought Question

• Your friend leaves your house. She later calls
  you on her cell phone, saying that shes been
  driving at 60 miles an hour directly away from
  you the whole time and is now 60 miles away. How
  long has she been gone?
• A. 1 minute
• B. 30 minutes
• C. 60 minutes
• D. 120 minutes

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Thought Question

• Your observe a galaxy moving away from you at 0.1
  light-years per year, and it is now 1.4 billion
  light-years away from you. How long has it taken
  to get there?
• A. 1 million years
• B. 14 million years
• C. 10 billion years
• D. 14 billion years

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The expansion of the Universe
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The expansion rate appears to be the same
everywhere in space The universe has no center
and no edge (as far as we can tell)
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One example of something that expands but has no
center or edge is the surface of a balloon
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The cosmic horizon

• The universe does not seem to have an edge
• But there is a horizon i.e. a place beyond which
  we cant see
• The cosmic horizon is a boundary in time NOT
  space
• We cannot see anything with a lookback time
  greater than 14 billion years

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Important points

• We observe the effects of expansion in virtually
  all galaxies
• Measurements of the rate of expansion tell us
  that it started 14 billion years ago
• Astronomers use many techniques to verify this
  rate, do not depend on just one set type of
  observation
• The Universe (Spacetime) is expanding and the
  galaxies are being carried along

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Cosmological Principle

• The universe looks about the same no matter
  where you are within it
• Matter is evenly distributed on very large scales
  in the universe
• No center no edges
• Not proved but consistent with all observations
  to date

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Hubbles constant tells us age of universe
because it relates velocities and distances of
all galaxies Age 1 / H0
Distance Velocity
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Deep observations show us very distant galaxies
as they were much earlier in time (Old light
from young galaxies)
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Distances between faraway galaxies change while
light travels Astronomers think in terms of
lookback time rather than distance
distance?
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Expansion stretches photon wavelengths causing a
cosmological redshift directly related to
lookback time