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The Interstellar Medium (ISM)
Part II: Interstellar Gas
Gas Absorbing Light
In addition to dust grains, there is low-density gas between
the stars. This leads to the formation of additional
absorption lines in the spectrum of a star.
We want to determine the total amount and composition of
the gas between the stars.
But how do we distinguish the lines caused by interstellar
gas from those caused by the star’s own atmosphere?
Stellar Spectra: a Reminder
How Do We Detect I/S Gas?
One Diagnostic
Suppose you study a number of random stars in some
nearly common direction in space. Because they differ
in temperature (and possibly to a small extent in
composition as well), their spectra will generally not be
very much alike.
But suppose you may detect a common feature in all these
spectra. This suggests the presence of an interstellar
cloud of gas in the foreground, one that affects the
light from all those stars.
A Second Diagnostic
In the atmosphere of a star, individual atoms are rushing
randomly to and fro at various speeds – some towards
us, some away – because the gas is hot. Thanks to the
Doppler shift, each of them absorbs light coming out
of the star’s interior at a slightly different wavelength.
This means that the absorption lines in the spectrum of the
star itself are not sharply focussed at one very precise
wavelength, but instead cover a modest range of
wavelengths: they are said to be wide.
By contrast, an absorption line formed by interstellar gas
cloud will be very narrow because the gas between the
stars is so cold in the emptiness of space. The atoms
are scarcely moving.
Notice the Very Narrow I/S Lines
[Here, one star is observed through two clouds.]
Some Real Spectra
From these spectra, we know that there are
free-floating Calcium atoms in the ISM
Other common atomic species are found as well
Gas Emitting Light
Sometimes very conspicuous, like celestial neon lights!
“Fluorescence”
The gas absorbs energy in some form – here, from
an electric current through the tube – and re-emits
that energy as visible light.
What’s The Energy Source
in a Fluorescent Interstellar Gas Cloud?
There are two ways of inputting energy into a
cloud of gas. First, collisions:
1.
Material rushing out (perhaps from an
exploding star) collides with the gas. The
energy of those collisions can rip electrons right
away from the atoms. Shortly thereafter, an
atom may recapture an electron which ‘jumps
down’ from one orbital to another [see ASTR
101], releasing energy in the form of visible
light.
Very Familiar
This is what happens in
a fluorescent lamp or a
neon tube.
Electrons (the electric
current!) that are
rushing through the
lamp collide with the
low-density gas in it.
Alternatively: Ultraviolet Light
2.
A very hot star in the heart of the gas
cloud gives off lots of ultraviolet light.
These photons are so energetic that they
rips off electrons completely, ‘ionizing’
the gas.
When the electrons subsequently
recombine with the atoms, visible light is
given off.
Repackaging!
Here is the Orion Nebula, shining in visible light.
That glow is really just ultraviolet starlight that’s
been‘repackaged.’
The energy originates
in the extremely hot
young stars in the
heart of the nebula.
(Without those stars,
the gas wouldn’t glow.)
How About Cooler Gas
Between the Stars?
Interstellar gas that is far away from hot
stars is quite cool, does not fluoresce, and
emits no visible light.
If we want to study it, we need to do so at
longer wavelengths, using lower-energy
photons.
Radio Astronomy!
- occupied Holland, 1944
On theoretical grounds, van de Hulst predicts a yetundetected kind of emission from neutral hydrogen
atoms in interstellar space.
The Underlying Physics
a ‘spin-flip’ transition
The Unlikelihood
Such ‘spontaneous’ transitions are very unlikely.
If unperturbed, a randomly chosen hydrogen
atom would sit in interstellar space for 107 years
– yes, that’s 10 million years! -- before
undergoing this process.
Doesn’t this mean that there will be very few such
photons produced?
Saving Graces
1.
1.
The galaxy contains an enormous amount of
neutral hydrogen gas, multiplying the chances.
Moreover, ‘bumps’ (collisions) between atoms
can spark the transition.
Consequently, at any given instant, there are huge
numbers of these photons being emitted. Our radio
telescopes pick this up very strongly. This is one of the
most important ways in which we study the universe,
because hydrogen is the dominant constituent!
An Important Benefit
These photons are produced at quite a long
wavelength (21 cm). This means that the
photons pass unimpeded through the gas and
dust in the Milky Way.
(To them, interstellar space is nearly transparent!)
Friendly Competition
Following the war, various
radio astronomers sought
the predicted radiation.
The Dutch were just beaten
to it by Ewen and Purcell,
working at Harvard, in 1951.
One Great Use:
Making Maps
And Exactly Where
is the Hydrogen Gas?
The bright features
indicate where the
photons come from, in
our radio-astronomical
studies of the Milky
Way .
We see conspicuous spiral structure, in a galaxy
that is about 100,000 light years across!
Yet More Gas:
Interstellar Molecules
An important distinction:


the dust grains, although very tiny in human
terms, each contain trillions of atoms. (We have
already considered those.)
now consider instead simple molecules,
containing at most a few hundred atoms!
How Might We Detect Them?
In molecules, atoms are held together by electrical
forces felt between electrons and nuclei. But a
molecule is not static: it ‘jiggles’ around (vibrates)
and ‘tumbles’ (rotates).
https://www.youtube.com/watch?v=HuSbLBDagdc
How Do They Emit Light?
A quickly-rotating molecule may change from a
state of rapid rotation to one of slower rotation,
losing energy. (Visualize a two-atom molecule,
like O2, spinning like a drum major’s baton
thrown into the air – but then suddenly slowing
down.) The lost energy shows up in the form of
a photon.
The same holds true for energy lost when a
molecule slows down in its vibration.
These are Quantized Transitions
In a molecule, many rotation rates are
possible, but not all. (Just as in a
food blender: only certain speeds
can be selected!)
In a molecule, slowing from one
allowed speed to another gives off
light of a fixed, determined energy
(wavelength). Detecting the set of
emitted photons at these specific
wavelengths tells us what
molecules are present.
An Example: Hydrogen Chloride
(gives off light of only the frequencies shown,
not any other!)
Interstellar Molecules
Were Not Expected!
In molecules, the bonds between atoms are quite
feeble. Any simple molecule sitting in empty
space will be quickly ripped apart by energetic
photons emitted by hot stars.
Consequently, sixty years ago, no astronomers
expected to find evidence of any complex
molecules ‘between the stars.’
Surprise!
Decades ago, the first very simple molecules
were found in the Interstellar Medium:
OH, NH3, H2O, CO, etc.
Subsequently, many very complex molecules
were discovered, including amino acids,
PAHs (polyaromatic hydrocarbons), etc.
Diagrams
for Some
of Them
…a Long and Growing List!
A Recent Exciting Discovery
There are ‘buckyballs’ (complex Carbon molecules known
as buckminsterfullerenes) in interstellar space.
Tiny ‘Footballs’
Built with carbon atoms
How Can Fragile, Many-Atom
Molecules Survive?
These complex molecules are not found in the
general emptiness of space, but rather deep
within GMCs (giant molecular clouds), which
may be up to 105 – 106 x the mass of the sun.
These clouds are quite cool in the deep interior, so
molecules can form and survive there, and are
shielded from potentially harmful collisions with
energetic photons streaming through space.
Importance for Life?
Molecules of amino acids are the building blocks of
proteins. Can their presence in interstellar space be
related to the later emergence of life on planets in the
universe?
Likely not in any direct way, since most of the gas later
condenses into stars (forming planets as this happens),
becoming hot enough to rip the molecules apart.
But we certainly learn that amino acids seem to be readily
and commonly formed in varied locations.
Final Conclusions
The abundance of the elements in interstellar
space reflects the distribution of material in the
cosmos as a whole.
As noted earlier, the heavier elements and the
grains are the products of a recurrent cycle of
star formation, life, and death, with extensive
recycling of material.