The Milky Way Galaxy

1
The Milky Way Galaxy

• Galaxy - definition
• Galactic distances
• RR Lyrae Cepheid Variables
• pulsating, variable stars
• characteristic light curves
• HR instability strip
• RR Lyrae - same luminosity
• much more common
• used to map MW halo
• Cepheids
• Period - Luminosity Relation
• distances out to 15 million pc

2

• Milky Way Galaxy - spiral
• disk mapped via 21 cm rad.
• Halo mapped via RR Lyrae
• bulge
• 30 kpc in diameter
• Sun 8 kpc from center
• Shape
• spherical halo r 30 kpc
• flat disk 300 pc thick
• bulge 4 kpc x 6 kpc
• football shaped
• Composition
• halo no gas or dust old stars red fewer
  metals
• disk gas dust young stars blue metal rich
• Population I Population II

3

• Orbital Motion
• disk differential rotation
• speed 220 km/s
• halo random orbits
• speeds comparable to disk's rotation at that
  radius, but in any direction
• Formation
• contracting cloud of gas
• merger of smaller systems?
• Large, irregular rotating object
• halo stars globular clusters
• no preferred direction for orbits
• disk forms from gas dust
• only stars remain in halo
• differential, ordered rotation

4

• Spiral Arms
• we reside in a spiral arm
• gas dust young stars, emission nebulae,
  prestellar objects
• site of star formation
• bright young stars gt arms are very visible
• "matter" arms would wind up quickly
• Spiral Density Waves!
• Triggers star formation
• pattern moves but material doesn't
• pattern rotates more slowly than material
• self-propagating star formation
• newly formed massive stars supernovae send
  shock waves through the gas
• new star formation is triggered
• only produces pieces of spirals

5

• Spiral Arms
• formation
• instabilities near bulge?
• Grav. Effects of nearby galaxies?
• Kicks from elongated shape of bulge?
• Mass of Galaxy
• galactic rotation
• Kepler's third law
• mass mass interior to orbit of Sun
• Galactic rotation curve
• 15 kpc radius where luminosity drops off
  sharply
• rotation curve does not!!
• at least twice as much mass lies outside 15 kpc
  as lies within it!!
• a large Dark Halo

6

• Dark Matter
• most of the matter in galaxy
• undetected in any wavelength of light
• only tracer gravitational pull
• not H, H2, ordinary stars
• Possibilities
• brown dwarfs white dwarfs very faint, very low
  mass red dwarfs
• exotic subatomic particles - almost impossible to
  detect created in Big Bang
• gravitational lensing
• an object is brightened briefly as another object
  passes in front of it
• 15 events observed in 10 years
• suggest low mass white dwarfs are very common -
  dark matter in inner galaxy??
• exotic particles in outer??

7

• Galactic Center
• galactic bulge center densely populated
• heavily obscured by dust, but IR radio obs are
  possible
• high stellar densities huge dust clouds
• ring of molecular gas surrounds center
• 400 pc across 30,000 solar masses 100 km/s
• Sag A (bright radio source) center
• strong magnetic fields
• rotating disk few pc
• Sgr A - exact center high energy, small size
  large mass
• supermassive black hole
• strong radiation matter in accretion disk
• strong fields generated in disk