What does our galaxy look like?

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What does our galaxy look like?
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The Milky Way galaxy appears in our sky as a
faint band of light
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We see our galaxy edge-on Primary features
disk, bulge, halo, globular clusters
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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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If we could view the Milky Way from above the
disk, we would see its spiral arms
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How do stars orbit in our galaxy?
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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

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Much of star formation in disk happens in spiral
arms
Ionization Nebulae Blue Stars Gas Clouds
Whirlpool Galaxy
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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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Saturns Rings Natural Color
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15km/s
lt 1cm/s
15km/s
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Unseen Influences

• Dark Matter An undetected form of mass that
  emits little or no light but whose existence we
  infer from its gravitational influence
• Dark Energy An unknown form of energy that
  seems to be the source of a repulsive force
  causing the expansion of the universe to
  accelerate

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Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
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What is the evidence for dark matter in galaxies?
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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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Contents of Universe

• Normal Matter 4.4
• Normal Matter inside stars 0.6
• Normal Matter outside stars 3.8
• Dark Matter 25
• Dark Energy 71

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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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Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
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All three methods of measuring cluster mass
indicate similar amounts of dark matter
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Clusters contain large amounts of X-ray emitting
hot gas Temperature of hot gas (particle
motions) tells us cluster mass 85 dark
matter 13 hot gas 2 stars
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Our Options

1. Dark matter really exists, and we are observing
   the effects of its gravitational attraction
2. Something is wrong with our understanding of
   gravity, causing us to mistakenly infer the
   existence of dark matter

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How dark is it?

• not as bright as a star.

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Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

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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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Detailed studies Halo stars formed in clumps
that later merged
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Our Galaxy
Earth 100 nm virus Sun
10 µm cell Earth orbit
¼ cm pin head Solar system 20
cm saucer Nearest star 250 m
lawn
Solar system
Galaxy Center
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NGC 3310 Spiral Galaxy
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NGC 5194 Spiral Galaxy
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NGC 1365 Barred Spiral Galaxy
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Interacting galaxies