1. Space Between the Stars: Properties of the Interstellar Medium Steven R. Spangler University of Iowa

2. An intuitive argument for the emptiness of space: the clarity of distant stars Yardstick for interstellar distances: the light year 1 light year = 9.46E+17 cm = 63,235 au Vega: 26 ly Altair: 17 ly Antares: 390 ly

3. How empty is interstellar space? A comparison with the Earth’s atmosphere

4. Point of talk: how we discovered the Interstellar Medium, and its properties Emphasis on results from radio astronomy

5. A physical argument for how interstellar space could be “invisible” If H is in the ground state, no Balmer transitions in absorption or emission For 1 % occupancy of n=2, need T=20,000K Demo

6. How to detect cold neutral hydrogen in interstellar space? A rare example of a theoretical search followed by observational confirmation Arose from “need” for a spectral line in radio astronomy The 21 cm line of hydrogen

7. Structure of the ground state of hydrogen Classical physics Quantum mechanical Angular momentum numbers Two states of H ground state N=1,S=0F=1 F=0 5.9E-06 eV

8. 21 cm HI spectra… University of Iowa SRT

9. 21 cm HI spectra

10. The galaxy has been mapped in the 21cm line

11. Properties of HI Gas in ISM n: 1-100 cc (also higher) T: 50-200 K (also higher) nT roughly constant at ~3000

12. Neutral hydrogen has also been mapped out in many other galaxies

13. visible HI

14. Topic 2: where do stars come from? Mass of star = 330,000 mass of Earth, mean density = 1.5 gm/cc

15. Hints from a century ago: star formation and Dark Clouds

16. Where is the gas from which the stars form?

17. Answer: gas is molecular and very cold Discovery was a contribution of radio astronomy Utilized observations of rotational transitions of molecules

18. Rotational transitions of (diatomic) molecules Classical rotational Kinetic energy Quantum mechanical Square of angular momentum Energy levels of rotator

19. Energy levels of a quantum rotator J=0 J=1 J=2 J=3

20. The sky in the glow of the carbon monoxide molecule

21. Physical properties of molecular cloud gas T=10 -70 K (and lower) N= 1.0E+05 - 1.0E+06 Numerous molecular species

22. The physics of star formation: the Jean’s Mass If a mass larger than the Jean’s mass is Compressed, gravity dominates gas pressure And it continues to contract

23. A Star is born….

24. A Star is born (Part 2) …

25. Molecular Clouds as Chemistry Sets in the Sky Number of molecules discovered in molecular clouds = 152 8 species with 10 or more atoms Deuterated species overrepresented

26. Interstellar chemistry as plasma chemistry Molecular clouds are weakly-ionized plasmas Ion-molecule reactions do not have formation barriers

27. The future of molecular cloud studies…ALMA 64 antenna interferometer 2010 August, “first science” 2012 December, “full science operations”

28. ALMA