The Milky Way Galaxy

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The Milky Way Galaxy
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What does the Milky Way look like?
William Caroline Herschel
First to study the extent of the Milky Way Galaxy
Jacobus Kapteyn (1851-1922)
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Grindstone Model
Sun at center of irregularly shaped cloud of
stars.
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Harlow Shapely (1885-1972)
Studied globular clusters of stars and used
Henrietta Leavitts earlier work on Cepheid
Variable Stars to determine the true shape of the
Milky Way galaxy
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Cepheid Variable Stars
They are stars which change their luminosity
(reliably) over time.
The instability of stellar evolution produces a
reliable fluctuation of a stars absolute
brightness.
WHY?
Instability Strip
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Stars in this phase of their life are susceptible
to pulsations (size luminosity)
This behavior can help us determine distances!
If we know the intrinsic luminosity of a star we
can compare it to the apparent luminosity of a
star and determine distance.
Cepheids give us distance to objects and we can
then determine where things are.
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The Galactic Disk

• Most stars are here. Nearly all the interstellar
  gas.
• Old Stars (1010yrs) to Younger Stars (106yrs)
• Star Formation is occurring now.
• Composition Old Metal Poor to Young Metal Rich
  stars.
• Motions coplanar, direct, elliptical orbits.
• Spiral Arms (?)

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The Galactic Bulge Halo
The Galactic Halo

• Thin scattering of stars clusters
• Stars (OLD), globular clusters (OLD), no
  interstellar material.
• Metal Poor material (mostly H, He, very little
  else)
• Random eccentric orbits

The Galactic Bulge

• Like the halo, only more crowded

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