The INTERSTELLAR MEDIUM

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 regionsO 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 NebulaeDust 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 nebulaeShells 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 RemnantsLots 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 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-3 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 8000 K < T < 12,000 K 100 < n < 1000 cm-3 very small fraction of ISM mass and volume detected via optical emission lines and radio lines

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 ofthe ISM and it is still the dominant/averageconstituent: partially ionized --1000 < T < 8000 K ; 0.01 < n < 0.1 cm-3 --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 --50 < T < 150 K; 1 < n < 100 cm-3 --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

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!