Part II Interstellar Gas Gas Absorbing Light In addition to dust grains there is lowdensity gas between the stars This leads to the formation of additional absorption lines in the spectrum of a star ID: 565272
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Slide1
The Interstellar Medium (ISM)Part II: Interstellar Gas
Slide2
Gas Absorbing LightIn 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?Slide3
Stellar Spectra: a Reminder Slide4
How Do We Detect I/S Gas?One DiagnosticSuppose 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.Slide5
A Second DiagnosticIn 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.Slide6
Notice the Very Narrow I/S Lines[Here, one star is observed through two clouds.]
Slide7
Some Real SpectraFrom these spectra, we know that there are free-floating Calcium
atoms in the ISM
Other common atomic species are found as wellSlide8
Gas Emitting Light
Sometimes very conspicuous, like celestial
neon lights!
Slide9
“Fluorescence”
The
gas absorbs energy
in some form – here, from
an electric current through the tube – and re-emits
that energy as
visible
light
.Slide10
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:
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. Slide11
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.Slide12
Alternatively: Ultraviolet LightA 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.Slide13
Repackaging!Here is the Orion Nebula, shining in visible light.That glow is really just ultraviolet starlight that’sbeen
‘
repackaged
.
’
The energy originates
in the
extremely
hot
y
oung stars
in the
h
eart of the nebula.
(Without those stars,
t
he gas
wouldn’t
glow.)Slide14
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.Slide15
Radio Astronomy!- occupied Holland, 1944
On theoretical grounds, van de
Hulst
predicts
a yet-undetected kind of emission from neutral hydrogen atoms in interstellar space.Slide16
The Underlying Physicsa ‘spin-flip’ transitionSlide17
The UnlikelihoodSuch ‘spontaneous’ transitions are very unlikely. If unperturbed, a
randomly chosen hydrogen
atom would
sit in interstellar space for 10
7
years – yes, that’s 10 million years! -- before
undergoing this process.
Doesn
’
t
this mean that
there
will be
very few
such photons
produced
? Slide18
Saving GracesThe 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!Slide19
An Important BenefitThese 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!)Slide20
Friendly CompetitionFollowing 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.Slide21
One Great Use: Making Maps Slide22
And Exactly Where is the Hydrogen Gas?The bright features indicate where the photons come
from, in
o
ur radio-astronomical
s
tudies of the Milky
Way .
We see conspicuous
spiral structure,
in a galaxy that is
about
100,000 light
years across!Slide23
Yet More Gas:Interstellar Molecules An important distinction:
t
he dust
grains, although
very tiny
in human terms, each contain
trillions
of
atoms. (We have already considered those.)
n
ow consider instead simple
molecules,
containing
at
most a few hundred
atoms! Slide24
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=HuSbLBDagdcSlide25
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 O
2
, 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.Slide26
These are Quantized TransitionsIn 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.Slide27
An Example: Hydrogen Chloride(gives off light of only the frequencies shown, not any other!)
Slide28
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.
’Slide29
Surprise!Decades ago, the first very simple molecules were found in the Interstellar Medium: OH, NH
3
, H
2
O, CO, etc.
Subsequently,
many
very complex molecules
were discovered, including
amino acids, PAHs
(
polyaromatic
hydrocarbons
),
etc.Slide30
Diagramsfor Some of ThemSlide31
…a Long and Growing List!Slide32
A Recent Exciting DiscoveryThere are ‘buckyballs’ (complex Carbon molecules known as buckminsterfullerenes) in interstellar space.Slide33
Tiny ‘Footballs’ Built with carbon atomsSlide34
How Can Fragile, Many-AtomMolecules 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 10
5
– 10
6
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.Slide35
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.Slide36
Final ConclusionsThe 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.Slide37