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=0 F=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: 1-100 cc (also higher)

• T: 50-200 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=10 -70 K (and lower)

• N= 1.0E+05 - 1.0E+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 pressureAnd 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 = 152

• 8 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