1. The INTERSTELLAR MEDIUM
The ISM is all the stuff between stars; it’s about 10% of the mass in the galaxy.
It is also the stuff (gas and dust): from which stars are born and into which they throw off their outer parts when they die.

2. The ISM is Important NGC 604, w/ 200 young stars
The gas between stars is of VERY low density, with the densest ISM clouds far less dense the best lab vacuum
Nonetheless, it is from these gas clouds that new stars are born.
Old stars expel large portions of their envelopes into the ISM.
Heavier elements, which are cooked via nuclear fusion in stellar interiors, enrich (or pollute) the ISM.

3. Dimming and Reddening
Light traveling through these clouds will be absorbed and reddened (more blue light absorbed or scattered), so star light looks different than it did when emitted.
Black in center, redder at edges of dense clouds
Astronomers use spectral lines to “de-redden” a star’s light and can figure out its real brightness and distance.

4. Dust is Heated and Radiates
Young stars give off visible and UV light which heats dust in the surrounding dense cloud
The dust gives this heat back as IR light
The stars are often invisible because of absorption
Only the warm dust and the gas near it may be seen

5. Light Polarized by Dust Scattering
Dust particles < 1m in size, made of C, O, Si, Fe
If the molecules or dust grains are not spherical and are aligned, then light is polarized: E vectors (mostly) in one plane
Magnetic fields can align dust grains (which have some iron in them)
So mapping polarized light yields directions of magnetic fields in ISM

6. When starlight passes through interstellar dust
It gets fainter
The blue light tends to scatter sideways while the red continues to us
Wavelengths all get longer (redder)
All of the above
#1 and #2

7. When starlight passes through interstellar dust
It gets fainter
The blue light tends to scatter sideways while the red continues to us
Wavelengths all get longer (redder)
All of the above
#1 and #2

8. PHASES OF THE INTERSTELLAR MEDIUM
The cooler parts of the ISM are NEUTRAL. The hotter parts are IONIZED (some electrons ripped off atoms) The neutral phases include:
Molecular clouds, mainly made of H2 molecules. From these new stars form, but they make up a small portion of the mass of the ISM and an even smaller portion of the volume.
Atomic (H I) clouds and diffuse gas. These diffuse phases make up the bulk of both the mass and volume of the ISM.

9. IONIZED ISM PHASES
H II regions: parts of molecular clouds which are ionized by hot, young (O or B) stars, which pump out lots of powerful UV photons -- these are spectacular, but rare.
Shock heated ISM very low density makes up a big part of the volume, but only a small part of the mass of the ISM mainly heated by Supernovae sometimes called the galactic corona

10. CLOUDS or NEBULAE AS OBSERVED IN THE VISIBLE BAND
1. Dark Nebulae = Molecular Clouds: dust absorbs nearly all the visible light from stars behind the clouds so they look black on the sky.
Rho Ophiuchi (left), Antares (right) in V and IR

11. 2. Bright Nebulae
Emission Nebulae a.k.a. H II regions O or B stars (w/ lots of UV photons) ionize gas We see recombination emission lines, mainly from: O II, N II, and N III in red, pink and blue number density, n, at least 100 cm-3 (to 1000 cm-3) temperature, T~104 K (8,000 to 12,000 K)
Reflection Nebulae Dust scattering light near stars; Since dust preferentially scatters blue light, these nebulae look blue.

12. Milky Way with Dusty ISM
Emission nebulae;
Plane of MW is dashed line

13. Emission Nebulae: M8 & M20(Trifid)
Dust lanes trisect the emission nebula on the right
Blue regions are refection nebulae
Jet from protostar at lower left (about 0.5 pc long)

14. Nebula Structure & Spectrum

15. Bright Nebulae (continued)
C. Planetary nebulae Shells of gas ejected from old stars; Ionized by hot core of the star (will become a WD); Usually look ring-like because of greater column depth of emitting gas along edges of shell rather than through core (Chapter 20)
D. Supernova Remnants Lots of mass, blasted out at high velocities during the deaths of massive stars (Chapter 21). Seen in X-ray and radio bands as well as in visible. Fade after only about 100,000 years and supernovae are pretty rare events to begin with.

16. Planetary Nebulae
Cat’s Eye, Eskimo, Helix, & M2-9 PNs.

17. Supernova Remnants
Start on the next slide on 9/29
N132D & Crab SNRs

18. SUMMARY OF ISM CONDITIONS (in order of decreasing T)
SHOCK HEATED (or coronal gas): Highly ionized T between 105 and 106 K n between 10-4 and 10-3 cm About 1 percent of ISM mass About 1/2 of ISM volume first found only in 1970s via UV absorption lines and later by X-ray emission
H II REGIONS: Moderately ionized K < T < 12,000 K < n < 1000 cm very small fraction of ISM mass and volume detected via optical emission lines and radio lines
Start here on 9/29

19. HII Regions around Hot Stars
M16: (left) Eagle Nebula; (top) Pillars of cold gas in M16
M8: (right) Lagoon Nebula; (top) Core of M8: Hourglass

20. Cooler ISM Phases
WARM INTERCLOUD MEDIUM: In the old days this was considered to be the bulk of the ISM and it is still the dominant/average constituent: partially ionized < T < 8000 K ; < n < 0.1 cm Roughly half of both mass and volume of ISM --detected via 21 cm radio and UV absorption lines
ATOMIC CLOUDS: mostly neutral H, in atomic, or H I, form < T < 150 K; 1 < n < 100 cm roughly 1/2 of ISM mass but only about 1 percent of ISM volume Found first of all ISM constituents: via visble abs. lines; later, UV abs found and most easily mapped via 21-cm emission from H I
Start here on 10/4

21. Absorption Lines from ISM Clouds
Extra, narrow absorption lines are added to a star’s spectrum by intervening ISM clouds: at different redshifts

22. 21 cm Emission from H I
If protons and electrons have parallel spins the energy is slightly greater than when their spins are anti-parallel; the spontaneous spin-flip gives off photons w/ = 21 cm.

23. The COLDEST Phase
DARK (MOLECULAR) CLOUDS: mostly molecules of H2 , then He, CO, OH, CO2 , H2O, etc. some much heavier molecules, e.g., alcohol, formaldehyde, even amino acids are present, though much rarer
contains most of the dust grains that redden and absorb starlight
8 < T < 50 K
102 < n < 105 cm-3
less than 1 percent of ISM mass and volume

24. MOLECULES HAVE RICH SPECTRA
In addition to the electron energy levels that single atoms have, when combined in molecules:
there are quantized rotational energy levels;
there are quantized vibrational energy levels.
Both of these lead to lots of spectral lines, mostly in the IR, mm (or microwave) and radio bands.
Thus even rare molecules in space can be detected by tuning telescopes to particular frequencies corresponding to these rotational/vibrational transitions.

25. If gas and dust in space are dark, how do we know they are there?
We sometimes see absorption lines from interstellar gas
Infrared telescopes can see cool dust
Radio telescopes detect interstellar gas
All of the above
We can’t really be sure. Space may be empty.

26. If gas and dust in space are dark, how do we know they are there?
We sometimes see absorption lines from interstellar gas
Infrared telescopes can see cool dust
Radio telescopes detect interstellar gas
All of the above
We can’t really be sure. Space may be empty.

27. Molecular Rotational Transition in Formaldehyde (H2CO)

28. Molecular Lines Near M20
Most formaldehyde is found in the darkest, densest part of the molecular cloud

29. Molecular Lines Yield Cooling
Clouds can stay cool, even if collapsing, if all generated radiation can escape
Heat, or random collisions, excite rotational-vibrational levels
Photons emitted from them, esp. CO, keep clouds cool and cloud could keep contracting
Until: density gets so large that these mm and IR photons are absorbed many times before escaping.
Then the temperature will rise and gas pressure goes up rapidly.
This is a key process in star formation!