Chapter 15: The Milky Way cont'

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Chapter 15 The Milky Way cont.

• Dr. Michelle Arnold
• November 18, 2009

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The Milky Way Our Galaxy

• Hard for us to investigate since we are inside of
  it, and just see a streak across our sky
• Evidence that we are in a spiral galaxy
• We see stars in spiral arm type structures near
  by
• 21 cm hydrogen line
• travels far in the dust of our galaxy
• Doppler shifts can be used to determine the
  velocity of the hydrogen, and we can confirm that
  all the hydrogen in the disk is rotating in the
  same direction
• Logic
• We see the flat disk of our spiral galaxy!

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Structure of Our Milky Way

• Bulge
• No active star formation
• Only old stars remain (red, little heavy element
  content)
• Disk
• Most of the stars are here
• Active star formation occurring (gas and dust
  clouds)
• Old and young stars are here
• Halo
• No active star formation
• Only old stars remain
• Black hole in the center!
• Dark matter halo!

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Structure of Our Galaxy cont.
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BIG

• 2 x 1012 M
• 100 billion stars
• Determining mass by two methods
• Gravity observing how the galaxy rotates and
  how it effects other galaxies
• Stars knowing mass of different types of stars
  and counting how many of each type we have
• The mass of the stars is only about 10 of the
  mass that is present and exerting gravity
• This is consistent for other galaxies too
• Dark matter The name given to the 90 of the
  mass in the universe that is in an unknown form!

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Globule Clusters

• Stars in the halo have less heavy elements
  (metals) in them and more hydrogen than stars in
  the disk. Why?
• Globule Clusters
• Star clusters in the halo of the Milky Way, all
  stars in it made at about the same time
• Date it to be 13 billion years old. How?
• The universe MUST be at least 13 billion years
  old!

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Density-Wave Model
Explains why star formation occurs in spiral
arms AND Why the galactic disk is structured as
spiral arms.
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Center of our Galaxy

• Deep within the core lies an incredibly small (10
  AU diameter) radio source known as Sgr A
• A 106 M black hole may occupy the very center
  of the galaxy, although other explanations have
  been given

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Formation of the Milky Way

• We are still working this out ?

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Future of the Milky Way

• Eventually all gas finds its way into stars,
  which in turn will lock up material in stellar
  remnants
• Hundreds of billions of years from now the Milky
  Way will fade, slowly spinning in space, a dark
  disk of stellar cinders