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 < 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
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 gas
4. All of the above
5. 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 gas
4. All of the above
5. 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 rotationalvibrational 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!