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.

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.

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.

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

Light Polarized by Dust Scattering

• Dust particles < 1m 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

When starlight passes through interstellar dust

1. It gets fainter

2. The blue light tends to scatter sideways while the red continues to us

3. Wavelengths all get longer (redder)

4. All of the above

5. #1 and #2

When starlight passes through interstellar dust

1. It gets fainter

2. The blue light tends to scatter sideways while the red continues to us

3. Wavelengths all get longer (redder)

4. All of the above

5. #1 and #2

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.

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

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

2. Bright Nebulae

A. 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)

B. Reflection Nebulae Dust scattering light near stars; Since dust preferentially scatters blue light, these nebulae look blue.

Milky Way with Dusty ISM

Emission nebulae;Plane of MW is dashed line

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)

Nebula Structure & Spectrum

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.

Planetary Nebulae

• Cat’s Eye, Eskimo, Helix, & M2-9 PNs.

Supernova Remnants

N132D & Crab SNRs

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

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

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 --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

Absorption Lines from ISM Clouds

• Extra, narrow absorption lines are added to a star’s spectrum by intervening ISM clouds: at different redshifts

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.

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

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.

If gas and dust in space are dark, how do we know they

are there?

1. We sometimes see absorption lines from interstellar gas

2. Infrared telescopes can see cool dust 3. Radio telescopes detect interstellar gas4. All of the above5. We can’t really be sure. Space may be

empty.

If gas and dust in space are dark, how do we know they

are there?

1. We sometimes see absorption lines from interstellar gas

2. Infrared telescopes can see cool dust 3. Radio telescopes detect interstellar gas4. All of the above5. We can’t really be sure. Space may be

empty.

Molecular Rotational Transition in Formaldehyde (H2CO)

Molecular Lines Near M20

• Most formaldehyde is found in the darkest, densest part of the molecular cloud

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!