2. 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.

3. Chapter 16: The Milky Way Galaxy Galactic Ecology Galactic Environments Galactic Motions The Galactic Center

4. The Milky Way Revealed Medium-size spiral galaxy with over 10 11 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).

5. 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.

6. 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.

7. 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 ~5x10 9 M Sun 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.

8. Molecular Clouds Molecular clouds are the coldest, densest collections of gas and dust in the ISM. Total mass ~ similar to atomic H. H 2 is difficult to observe because it does not have strong lines. CO, H 2 O and NH 3 are easily observed. The structure of molecular clouds is usually filamentary and contains multiple dense cores.

9. 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.

10. Typical States of Gas in the ISM

11. 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.

12. The Big Picture