Chapter 14 Notes: Olivia Ward

Our Galaxy
14.1 The Milky Way Revealed
What does out galaxy look like?

• The Milky Way galaxy appears in our sky as a faint band of light.
• Dusty gas clouds obscure our view because they absorb visible light.
  
  • These clouds are interstellar mediums.
• We see our galaxy edge-on:
  
  • Primary features: disk, bulge, halo, globular clusters
    
    • If we could see the Milky Way from above the disk, we would see its spiral arms.

How do stars orbit in our galaxy?

• Stars in the disk all orbit in the same direction.
• Orbits of stars in the bulge and halo have random orientations.
• Sun's orbital motion (radius and velocity) tells us mass within Sun's orbit: 1.0 X 10^11 MSun
• Orbital Velocity Law
  
  • Mr = r X v^2 / G

14.2 Galactic Recycling
How is gas recycled in our galaxy?

• Star - Gas - Star Cycle:
  
  • Atomic hydrogen clouds → molecular clouds → star formation
    → nuclear fusion in stars → returning gas → hot bubbles → cycle repeats
  • Recycles gas from old stars into new star systems
• High-mass stars have strong stellar winds that blow bubbles of hot gas.
• Lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae.
• X-rays from hot gas in supernova remnants reveal newly made heavy elements.
  
  • A supernova remnant cools and begins to emit visible light as it expands.
  • New elements made by supernova mix into interstellar medium.
  • Multiple supernovae create huge hot bubbles that can blow out of the disk (convection).
    
    • Gas clouds cooling in the halo can rain back down on the disk.
• Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons.
• Molecular clouds form next, after gas cools.
• Molecular Clouds in Orion
  
  • Composition
    
    • Mostly H2
    • About 28% He
    • About 1% CO
    • Many other molecules
• Gravity forms stars out of the gas in molecular clouds, completing the Star - Gas - Star cycle.
• Summary of Galactic Recycling
  
  • Stars make new elements by fusion.
  • Dying stars expel gas and new elements, producing hot bubbles (~ 10^6 K)
  • Hot gas cools, allowing atomic hydrogen clouds to form (~ 100 - 10,000 K)
  • Further cooling permits molecules to form, making molecular clouds (~ 30 K)
  • Gravity forms new stars (and planets) in molecular clouds.
• We observe the Star - Gas - Star cycle using many different frequencies of light.

Where do stars tend to form in our galaxy?

• Ionization nebulae are found around short-lived high-mass stars, signifying active star formation.
• Reflection nebulae scatter the light from stars.
  
  • Why do reflection nebulae look bluer than nearby stars?
    
    • For the same reason that our sky is blue (blue scatters the most, compared to red).
• Halo: No ionization nebulae, no blue stars → No star formation
• Disk: Ionization nebulae, blue stars → Star formation
• Much of star formation in disk happens in spiral arms (whirlpool galaxy)
  • Spiral arms are waves of star formation:
    
    • Gas clouds get squeezed as they move into spiral arms.
    • The squeezing of clouds triggers star formation.
    • Young stars flow out of spiral arms. 

14.4 The Mysterious Galactic Center
What lies in the center of our galaxy?

• Infrared light from center: 200 light years
• Radio emission from center: 50 light years
• Swirling gas near center: 10 light years
• Orbiting stars near center: 1 light year
• Stars appear to be orbiting something massive but invisible: a black hole.
  
  • Orbits of stars indicate a mass of about 4 million MSun.
• X-ray flares from galactic center suggests that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in.