Lecture 13 - The Interstellar Medium (2/25/99)
Stellar Structure II --- | ---
Chapter 11-4, 15-1, 15-2 (ZG4)
pages 51 - 54
M42, the Great Nebula in Orion, a site of ongoing massive star
Bill Arnett/Jason Ware)
||What are the dark nebulae made of?
||The 21-cm HI line
- Interstellar Dust
- What does interstellar dust do to starlight?
- Why is reflected light blue, and transmitted light reddened?
- How is dust extinction related to optical depth?
- What is the typical size of dust grains?
- What does A_v = 1 mag of extinction imply for optical depth?
- What is the typical visual extinction in mag per kpc in the galactic
- What is the color excess in B-V corresponding to 1 mag A_v?
- What is the reddening vector in the H-R diagram?
- What effect does extinction have on Cepheid distances?
- What effect does extinction have on star counts?
- What sort of chemical reactions can take place on dust grains?
- What is the interstellar medium (ISM) and what is it
- H II Regions
- Why do hot O and B stars ionize the gas around them?
- What is an H II region?
- What wavelengths are relevant for ionizing photons to
make an H II region? (For O9 star, n_ion ~ 10^49/s)
- How does equilibrium between ionization and recombination
determine the size of an H II region?
- What is a Stromgren sphere?
- Why do we need only consider recombination to levels n > 1 above
the ground state? (a2 ~ 10^-19 m^3/s)
- What is the typical size for an H II region at T = 8000 K with
n_H ~ 10^3/m^3 and n_e ~ 10^9/m^3
- Atomic and Molecular Lines in the ISM
- Using the Boltzmann equation, what energy level do you expect the
atoms to be in for a cold cloud with T < 100 K? Do you expect strong
Balmer lines, for example?
- What is a hyperfine transition?
- Why are there two spin states for neutral hydrogen?
- What is the energy difference corresponding to the 21-cm line
at 1.420406 GHz?
- How does H I emission in the 21-cm line allow us to map the gas
in the galaxy?
- What are the vibrational and rotational transitions
- How do molecular lines allow us to probe the compositions of cold
- How many species of molecules have been discovered in the ISM?
- What is the most complex molecule seen?
- How do the rotational level populations for a transition at frequency
v (eg. CO 1-0 at v=118 GHz) follow the Boltzmann distribution?
- Why is CO an important tracer in the ISM?
The Interstellar Medium in Outline
- Gravitational Collapse
- Gravity is always attracting, there is no "anti-gravity" repulsion.
- What do you think happens if you start with a bunch of matter
(of any type) distributed far across space, and let gravity do
- Unless you make the matter distribution exactly uniform and infinite
then it will always collapse eventually, unless something
- What can stop gravitational collapse?
- The matter can hit something first, namely the rest of the matter
that is falling in also. These collisions cause heat and pressure
which resists the collapse. This is called gas pressure.
- If the collapsing cloud of matter becomes hot enough, and starts
emitting photons like stars do, then the light pressure can stop
the collapse. This is called radiation pressure.
- Some other sort of pressure, like that from magnetic fields, can
stop the collapse. This is called magnetic pressure.
- We know our solar system is stable and not collapsing, but the
planets are orbiting. They possess momentum, specifically,
angular momentum that keeps it spinning instead of
- It turns out that stars (and gas giant planets like Jupiter and
Saturn) use gas and radiation pressure to support themselves against
gravity. This is what defines the size of a star.
- It also turns out that in the early stages of star formation, when
large gas clouds have to collapse by many orders of magnitude in
size to form stars, that magnetic pressure (and to some extent
gas pressure) and angular momentum must be overcome.
- We will discuss the star formation process in more detail
in the next lecture.
- For now, where is the stuff from which stars are made?
- A View of the Interstellar Medium
- Early in the history of telescopic astronomy, it was noticed
that in addition to lots and lots of stars, there appeared to
be fuzzy patches, or clouds, in the celestial sky.
- These were called nebulae, or nebula in the singular,
which is Latin for "cloud".
- Catalogs for nebulae were made so that people would not confuse them
with fuzzy comets, which they were more interested in.
- What are the nebulae? What different kinds do we see? This is
the usual first step in observational science - classify the
objects that we find!
- Diffuse Nebulae - amorphous bright fuzzy blobs, sometimes
with one or more bright stars in the center. These turn out to
be gas clouds ionized and lit up by very bright young stars.
- Planetary Nebulae - shells or rings of bright nebulosity
surrounding a more or less hollow center. These turn out to be
shells of matter thrown off of a dying star.
- Supernova Remnant - veil like wisps, filamentary rings,
or a filled shell with lots of tendrils (often similar in appearance
to diffuse nebulae). These turn out to be the result of titanic
stellar explosions (supernovae) marking the death of a massive star.
- Dark Nebulae - dark clouds that block out the stars behind
them, and redden those few stars that manage to be seen through them.
Often look like "holes" or "gaps" in the Milky Way. These are
relatively dense clouds of gas containing dust grains that absorb
visible light. Some times dark nebulae are seen in conjunction with
bright diffuse nebulae, which seem to live on the edges of dark
- Diffuse nebulae can shine by their own light if they are hot,
then being called emission nebulae. Lines of hydrogen,
carbon, oxygen, calcium, and other atoms can be seen their
spectrum. Most bright diffuse nebulae are emission nebulae.
- Some diffuse nebulae shine by reflected starlight, and are thus
called reflection nebulae. Faint blue wisps around the
stars of the Pleiades, a star cluster in the constellation Taurus
visible to the naked eye are a refection nebulae caused by the
- The nebulae that we are interested in for star formation are the
dark nebulae, which contain large amounts of cold gas primed for
gravitational collapse and the birthing of stars!
- What is the Interstellar Medium Made Of?
- We call all this gas and nebulosity the interstellar medium,
the stuff "between the stars", like interstate means between states.
We abbreviate interstellar medium as ISM.
- From spectra of the nebulae, we know they are made of gas, just like
the photospheres of stars like our Sun.
- The gas in the emission nebulae often have compositions very similar
to the Sun's: mostly hydrogen and helium, with traces of other elements
like carbon, nitrogen, oxygen, and iron. In fact, it is as if they
came from stars (...hmmmmm...).
- There is also dust in the ISM, small grains about 1 micron
(10^-6 m) in size. About 1% of the ISM is in dust grains.
- Scattering of light off of dust grains is what reddens the light
of stars seen through dust clouds. The short wavelength blue photons
are more easily scattered than the longer wavelength red photons.
This same effect reddens the Sun at sunset - and after dust storms
or volcano eruptions or big fires sunsets are spectacularly red.
- The dust is made of solids, like ice, carbon compounds, silicates,
- Dust is formed in low temperature regions, below 100K or so,
since high temperatures will cause collisions and sputtering
of the grains thus destroying them.
- The surfaces of dust grains can act as a matrix to hold molecules
close together and allow chemistry to occur. Some very large
molecules have been detected in space, with more than ten atoms.
These tend to be chains of hydrocarbons and other organic molecules,
and there are claims of seeing some proteins, like those that make
up DNA, in some molecular clouds! These complex molecules are
believed to be made on the surfaces of grains. In the past decade,
the field of cosmochemistry has grown to study the formation
of these molecules.
- The densities in the molecular clouds are as high as 10^5 atoms/cm^3.
This is still very tenouous, these dark clouds are still better
vacuums than anything we can make here on Earth!
- Molecular clouds range in masses from 100 up to 10^8 solar masses!
You can make lots of stars out of that much stuff.
- Atomic Hydrogen Clouds
- Neutral hydrogen atoms (ionization state H I) make up about
22% of the interstellar medium.
- These clouds range in density and temperature from 3000K at
0.3 atoms/cm^3 for warm clouds to 100K at 50 atoms/cm^3 for
- The primary way of identifying clouds of H I is through the
21 cm line.
- The hydrogen line at a wavelength of 21cm in the radio spectrum
is caused by a slight energy difference between states of the
hydrogen atom where the proton and electron spins are aligned
- If you remember our discussion of the
hydrogen energy levels and quantum numbers, then you will remember
that particles like electrons and protons have a property called
spin (quantum number s = +/- 1/2) which is analogous
to picturing the particle as a little spinning top.
- Because the proton and electron are electrically charged, the spin
means that they act as little magnets. Because they are charged
oppositely, their magnetic poles are aligned oppositely with respect
to the spin in the electron versus the proton (if their spins are
aligned, then their magnetic poles are opposite).
- If you've played with magnets, then you know that they like to
be oriented oppositely - the opposite poles attract and the
like poles repel.
- Thus, if the spins of the proton and electron in the hydrogen atom
are aligned (poles opposite), then there is a tiny extra attraction
which is like decreasing the energy of the ground
state level (making the electron slightly more bound).
- If the spins of the proton and elecron are opposing (poles aligned)
there is a tiny repulsion, which is like increasing the energy of
the ground state level.
- The energy difference between these two configurations of the ground
state is tiny: the photon wavelength to excite this transition is
21cm, compared to 121.6 nm for the transition from the ground state
to the next higher electron level. Q: How many times smaller
is this energy difference? What is this difference in electron volts
- If a photon of wavelength 21cm is absorbed or emitted, the spins
flip relative to each other.
- The emission (and absorption) of the 21 cm line by clouds of hydrogen
can be observed with large radio telescopes. The line was predicted
by the Dutch astrophysicist van der Hulst in 1945, and first observed
by Ewen and Purcell of Harvard in 1951.
- The 21 cm emission has been used to map out the H I clouds in our
galaxy, and to detect H I in other galaxies.
- Molecular Clouds
- If a cloud is cold enough, then the atoms have the chance to
come together and form molecules. This is able to happen in
clouds with temperatures of roughly 100K or less.
- These clouds tend to be buried deep within atomic H I clouds.
- These are the coldest (10K - 100K) and densest (10^4 - 10^5 atoms/cm^3)
- These clouds are made up mostly of molecular hydrogen (H2), which is
two atoms of hydrogen bound together. The dissociation energy
for H2, the energy needed to break apart the molecule, is 4.48 eV.
- Carbon monoxide CO is also seen in clouds. CO has a dissociation
energy of 9.6 eV, and it thus hardier than H2.
- There are many other diatomic (two-atom) molecules found in
molecular clouds, such as C2, CO, CN, CH, CS, SO, SiO, OH, etc.
- More complex molecules like H2O (water!), HCN (cyanide!), H2S,
as well as alcohols and molecules with more than 13 atoms have
also been seen in these clouds! As of the end of 1993, over
93 different molecules have been seen in the ISM. That number
has increased in the years since then.
- The presence of these molecules has been identified by observing
molecular lines, which like the atomic lines like the
Balmer series of hydrogen, correspond to energy levels in the
- The molecular energy levels correspond not to electron orbits like
in atoms, but to vibrations and rotations of the molecule.
- Vibrational lines are the most energetic, with typical
wavelengths of 0.1 mm or shorter. For H2, the series limit is
4.48 eV (277 nm), with the "alpha" transition at 2275 nm, or
2.275 microns, in the far-infrared. The CO "alpha" vibrational
transition is at 4.6 microns.
- Rotational Lines are lower in energy, and thus come out in
in the millimeter wave range. It turns out that to get a rotational
transition, you need two (or more) atoms of different mass in the
molecule - H2 has no rotational transitions, because it is symmetric!
The "alpha" rotational transition for CO is at 2.7 mm, this is one
of the primary lines used to map out molecular clouds.
- Because the molecular clouds are the coldest and densest of the
gas clouds, they are the places where stars are formed. They
are in many ways "stellar nurseries".
- The Orion Molecular Cloud
- One of the most spectacular and nearest large molecular clouds
is the Orion Molecular Cloud, which covers much of the
constellation of Orion, and contains the bright diffuse Orion
- The cloud is at a distance of 460 pc from us, and extends over
30pc in size. The total mass of molecular gas in the cloud
is around 2 x 10^5 times the mass of the Sun.
- The oldest stars formed out of the cloud are near the shoulder
of Orion, and are around 12 million years old. The youngest
stars that we see are in the center of the Orion Nebula itself,
and are about 2 million years old. The sun is about 4.5 billion
years old in comparison.
- Star formation in Orion uses less than 25% of the gas in any given
region, there rest is blown away.
- Because there seems to be a pattern to the ages, with the oldest
stars farthest away from the nebula, we believe that there is
progressive star formation where the formation of the first
stars triggers the next stars to form. Thus, star formation sweeps
through the cloud like a forest fire!
- Note that the Sun itself was not formed with the first stars, its
formation was triggered later on! In fact, basically all of the
elements heavier than helium were formed in stars, then blown out
in supernovae. Our bodies are made up of stuff that was cooked in
long dead stars!
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Steven T. Myers