Jessie Horn: Chapter 14 Notes

Chapter 14: Our Galaxy
14.1 The Milky Way Revealed

• 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
• This is the interstellar medium that makes new star systems
• We see our galaxy edge-on
• Primary features: disk, bulge, halo, globular clusters
• If we could view the Milky Way from above the disk, we would see its spiral arms
• 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

Orbital Velocity Law

• The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass within that orbit

14.2 Galactic Recycling

Star-gas-star cycle

• 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 disk
• Gas clouds cooling in the halo can rain back down on disk
• Atomic Hydrogen gas forms as hot gas cools, allowing electrons to join with protons
• Molecular clouds from next, after gas cools enough to allow atoms to combine into molecules
• 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
• Radiation from newly formed stars is eroding these star-forming clouds
• We observe the star-gas-star cycle opening in Milky Way
• Ionization Nebulae are found around short-lived high-mass stars, signifying active star formation
• Reflection nebulae: scatter the light from stars
• 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
• Spiral arms are waves of star formation

Spiral arms of waves of star formation

1. Gas clouds get squeezed as they move into spiral arms
2. The squeezing of clouds triggers star formation
3. Young stars flow out of spiral arms

14.3 The History of the Milky Way

• Halo Stars 0.02-0.2% heavy elements only old stars
• Disk stars: 2% heavy elements, stars of all ages
• Halo stars formed first, then stopped
• Disk stars formed later, and kept forming
• Our galaxy probably formed from a giant gas cloud
• Halo stars formed first as gravity caused the cloud to contract
• The remaining gas settled into a spinning disk
• Stars continuously form in the disk as the galaxy grows older
• Detailed Studies: Halo stars formed in clumps that later merged

14.4 The Mysterious Galactic Center

• Stars appear to be orbiting something massive but invisible...a black hole?
• Orbits of stars indicate a mass of about 4 million M Sun
• x ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in