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Quiz 3. Briefly explain how a low-mass star becomes hot enough to settle on the main-sequence. Describe what is solar weather and list two ways in which it can affect Earth. What are Cepheid variables and WD supernovae, and why they are important in astronomy?
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Quiz 3 • Briefly explain how a low-mass star becomes hot enough to settle on the main-sequence. • Describe what is solar weather and list two ways in which it can affect Earth. • What are Cepheid variables and WD supernovae, and why they are important in astronomy? • Briefly describe how nova and X-ray burst events occur.
Chapter 16: The Milky Way Galaxy Galactic Ecology Galactic Environments Galactic Motions The Galactic Center
The Milky Way Revealed • Medium-size spiral galaxy with over 1011 stars. • Flat disk of stars, gas and dust (ISM). D~30 kpc. • Bright central bulge. • Spiral arms of star formation. • A dim, roundish halo. • Few hundred globular clusters and three satellite galaxies (d~50 kpc).
Galactic Ecology • Chemical enrichment is the process of increasing the abundance of heavy elements in the ISM through the formation, evolution and death of generations of stars. • Material expelled by stars gets mixed with ISM and enters into subsequent events of star formation.
Evidence for ISM Stirring • High-speed winds from hot stars and SN excavate bubbles of hot, ionized gas that fill 20-50% of the ISM. • SN create shock waves, which heat the gas on the leading edge. • Superbubbles are created by multiple supernovae.
Atomic Hydrogen Gas • Atomic H emits at 21 cm because of hyperfine splitting of the ground state of the H atom. • Radio maps tell us that ~5x109 MSun of atomic H is distributed in the galactic disk. • The 21cm H-line is very narrow and provides excellent velocity maps of the ISM gas.
Molecular Clouds • Molecular clouds are the coldest, densest collections of gas and dust in the ISM. Total mass ~ similar to atomic H. • H2 is difficult to observe because it does not have strong lines. CO, H2O and NH3 are easily observed. • The structure of molecular clouds is usually filamentary and contains multiple dense cores.
Completing the Ecocycle • Newly formed stars erode the molecular clouds with UV radiation and fast winds. • Massive stars will explode in a few million years giving hot enriched gas to the ISM. • The ISM will again cool and coalesce into molecular clouds. • Part of the matter is locked in BDs and stellar remnants.
Multi- Snapshots of the MW • Atomic H is evenly distributed in the disk. • CO emission is concentrated in MCs, as well as mid-IR emission and gamma rays. • Near-IR picture is dominated by stars. • Visible light is obscured by interstellar dust. • ISM X ray emission comes from hot bubbles.
