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

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

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

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

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

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

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

21 cm HI spectra… University of Iowa SRT

21 cm HI spectra

The galaxy has been mapped in the 21cm line

Properties of HI Gas in ISM n: cc (also higher) T: K (also higher) nT roughly constant at ~3000

Neutral hydrogen has also been mapped out in many other galaxies

visible HI

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

Hints from a century ago: star formation and Dark Clouds

Where is the gas from which the stars form?

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

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

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

The sky in the glow of the carbon monoxide molecule

Physical properties of molecular cloud gas T= K (and lower) N= 1.0E E+06 Numerous molecular species

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

A Star is born….

A Star is born (Part 2) …

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

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

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

ALMA