1. The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 18

2. Exercise 17 – Dark Matter  Use Kepler’s Third Law to find the total mass of the galaxy   a = (20000 pc)(206000 AU/pc) = 4.12X10 9 AU  P = 460X10 6 years  M = (4.12X10 9 ) 3 /(460X10 6 ) 2 =  Mass-to-light ratio  Mass of stars = 1X10 10 solar mass  total mass/visible mass = 3.3X10 11 /1X10 10 =  The galaxy contains 33 times as much mass as we can see

3. Upcoming Events  Panel discussion: “The End of the Universe”  Tonight (April 25) Wallenberg Hall, 5pm  Planetarium Open House  Saturday, April 30, 8:30-10 pm  Roys Science Lecture: Dr. Ralph Harvey speaks on Martian Meteorites from Antarctica  Thursday May 12, 7:30pm, Olin

4. The Stuff Between the Stars   Called the interstellar medium  Concentrated in the disks of spiral galaxies   Exists with different temperatures, densities, size scales, and radiation environments

5. Composition  We can get an idea of the composition of the interstellar medium through spectroscopy  There are two components: gas and dust  Gas    Dust   Account for most of the opacity

6. Gas  Interstellar gas is abundant in hydrogen and helium    Heavier elements have condensed to form the dust  What form is the gas in?

7. Radiation Environment  Different parts of the ISM experience different external radiation fields   Location of bright stars   Density of cloud  The inner part of a dense cloud is shielded from most external radiation

8. Effects of Radiation  Clouds that experience little radiation are known as HI regions   Clouds in higher radiation environment are known as HII regions   Some clouds have so little radiation that they can form some molecules   Called giant molecular clouds

9. Dust  The clouds are much more opaque than you would expect just from the gas   Dust particles are small (~1 micron or 1 millionth of a meter)  Some different types of dust:     All these different types identified from complex absorption properties

10. Extinction Curve

11. What Does Dust Do?  Absorption   This causes interstellar extinction   Scattering   Dust scatters blue light better than red   Stars behind the cloud look redder

12. Reddening in the ISM

13. Reddening

14. Nebulae  What is a nebula ?    There are three basic types of nebula, each with a distinct appearance based on the way it interacts with light

15. Dark Nebulae  Dense clouds can completely block out the light of stars behind them   Can see stars in front of the cloud projected on it   Smallest are called Bok globules

16. Star Cluster and Dark Cloud

17. Emission Nebulae  If a cloud is near bright high mass stars it may shine as an emission nebula   The UV light ionizes the gas   Like a florescent light   Emission nebulae are HII regions

18. Reflection Nebulae  Dust preferentially scatters blue light   Same reason sky is blue   Need bright star fairly near-by to produce effect

19. Observing the ISM  With optical telescopes we see the effects of the ISM in different ways    We also need to observe at other wavelengths

20. Multiwavelength ISM  Radio   Penetrates dust and allows us to map much of the galaxy  Millimeter   Some are very complex and must be protected from UV radiation

21. More Multiwavelength ISM  Infrared   Dust is still very cold (<100K or -150 C)  X-ray   Formed from supernova

22. Types of Clouds  HI clouds   Molecular Clouds   Coronal gas   Intercloud medium  Hot, low density, about 50% of total mass  A lot of mass between the well-defined clouds

23. Structure of ISM  If we look in any direction we see many different parts of the ISM   The sun is actually in a large region of hot gas called the local bubble   The denser parts of the ISM are the sites of star formation 

24. Next Time  Read Chapter 24.1-24.4