The Milky Way

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The Milky Way

• Our home galaxy, full of stars, gas and
  mysterious dark matter
• We decompose it into a disk and a halo and a few
  other parts

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Almost a View of our Milky Way
NGC 4526, a spiral galaxy like the MW but about
30 Mpc away it has a similar size, luminosity
and structure
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Edge-on View and View of MW
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Key Parts of the Milky Way

• HALO Contains most globular clusters, and most
  Pop II stars roughly 30 kpc (105 lt-yr) in
  diameter.
• THICK DISK roughly 5 kpc thick, and 30 kpc in
  diameter contains Pop I stars (but low density).
• THIN DISK 500 pc thick contains MOST stars
  includes spiral arms and great majority of
  luminosity.
• DUST DISK only 50 pc thick new stars are born in
  the molecular clouds found within this very thin
  disk.
• SPIRAL ARMS are wrapped within the dust/thin
  disk contain almost all hot, luminous (O and B)
  stars.

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Overall Structure of the Milky Way
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Inner Parts

• GALACTIC BULGE roughly 2 kpc in radius
  around center highest concentration of stars,
  including many globular clusters.
• GALACTIC CENTER in the direction of the
  constellation Sagittarius, some 8 kpc from the
  Solar System (SS).
• Multiwavelength Milky Way

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MAPPING the MILKY WAY

• Dust, mainly in molecular clouds, shrouds the
  Disk we see few stars beyond 2 kpc from SS in
  the thin disk, where the number of stars is much
  greater
• Originally astronomers thought the Milky Way WAS
  the Whole Universe SS central to it (because
  of visible light extinction by dust)
• Location of Globular Clusters in halo implied
  center towards Sagittarius and SS actually
  towards one side in early 20th century.
• Atomic Hydrogen gas sends 21 cm radio waves that
  allow us to map the far side of the galaxy, and
  the outer reaches where there are few stars

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Our Nearest Big Neighbor, M31, the Andromeda
Galaxy
Andromeda, about 30 kpc across down to nucleus
only 15pc
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A Limited Conception of the MW
Herschels map of the Galaxy from star
counts More in the MW plane, but thought the Sun
near the center and got the size too small
didnt understand dust
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Distribution of Globular Clusters
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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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Thought Question

• Why do orbits of bulge stars bob up and down?
• A. Theyre stuck to interstellar medium
• B. Gravity of disk stars pulls toward disk
• C. Halo stars knock them back into disk

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Thought Question

• Why do orbits of bulge stars bob up and down?
• A. Theyre stuck to interstellar medium
• B. Gravity of disk stars pulls toward disk
• C. Halo stars knock them back into disk