So how is it that we know so much more 
about the galaxy than [UNKNOWN] or 
Shapley? main reason that we have more 
information about the galaxy we live in 
is because they were using visible light 
to make optical observations, and we have 
access to the entire electro magnetic 
spectrum. 
And so in this clip we're going to look 
at the Milky Way through several 
different bands, and emphasize what it is 
that they teach us, each of them about 
the Milky Way, and then later we'll put 
together all that information and see 
what we know about the galaxy. 
So here's the multi-wavelength approach 
to the Milky Way, and each of these 
images is an image of the entire plane of 
the milky way, folded around, unfolded, 
so that the center is Sagittarius, the 
direction to the center of the Milky Way, 
is centered in the images. 
And we're seeing all of these except the 
bottom, are false color, 
and we're seeing images of the milky way 
as it appears in various 
frequency bands, and we'll start at the 
lowest frequency at the top. 
These are radio waves, 408Khz, the 
radiations that these wave lengths is 
produced, mostly, by relativistic 
electrons, propagating in the galactic 
magnetic field. 
So we see that there are a lot of 
emissions near the center of the galaxy, 
and we see some localized sources. The 
source on the left is Cassiopeia A, the 
super nova remnant, we talked about it. 
And the emission from there is in fact so 
brilliant that if you look carefully you 
can see a sort of triangle shaped around 
it, which are the diffraction images of 
the legs of the antenna, that is the 
radio antenna used to produce the image. 
And so this gives us a map of where high 
energy charged particles are found at a 
slightly higher frequency and a slightly 
shorter wave length. 
At 1.4GHz, we have an extremely important 
measurement the wave length corresponding 
to 1.4 GHz is about 21 centimeters, 
and this is an important frequency, 
because it allows us to see clouds of 
neutral atomic hydrogen. 
Now neutral atomic hydrogen is difficult 
to see because, if it is, unless it is 
very hot the atoms are mostly in the 
ground state, and, the frequency of the 
wavelength corresponding to excitation 
from the ground state to excited states 
is ultraviolet, so atomic hydro, cold 
atomic hydrogen can absord, has 
absorbtion spectrum in the ultraviolet, 
but there's not enough necessarily 
ultraviolet like to observe it. 
On the other hand, it can both absorb and 
emit, even at the relatively cool 
temperatures we're talking about, at this 
wavelength of 21 centimeters, this is not 
an atomic transition where the electron 
jumps from 1 energy state to the other. 
In fact, what this is, is the frequency 
with which the electron precesses. 
The electron carries angular momentum, 
intrinsic angular momentum in a magnetic 
moment, and the electron's angular 
momentum precesses around, the, direction 
perpendicular to its orbits. 
So, just like the Earth's axis wobbles as 
it orbits around the sun due to 
gravitational tidal effects, in this 
case, the electron's axis wobbles as it 
orbits the proton due to electromagnetic 
effects, and the frequency with which it 
wobbles or precesses is 1.4 GHz. 
And so this is a low frequency at, 
relatively low temperatures that are, 
these, atoms are excited. So they emit 
these radio waves. 
Two great properties of this, one is it's 
emitted by neutral hydrogen, and two, 
because it is a radio wave, dust and gas 
clouds do not obscure it. 
So, when we look at 21 centimeters at the 
galaxy, we can see through the clouds and 
the dust, and we can see the big gas 
clouds themselves. 
And you see that the 21 centimeter image 
of the galaxy shows us that hydrogen is 
scattered throughout with of course, 
increased density near the center. 
Being able to observe the neutral 
hydrogen clouds in, atomic hydrogen 
clouds in the galaxy is going to be 
extremely important. 
Moving to a higher frequency, a still 
shorter wavelength, the 2.7 GHz. 
notice that the, the wavelength of the 21 
centimeter spin transition is rather, 
sharply defined. 
You change wave of frequency, you don't 
see the atomic hydrogen anymore. 
2.7 GHz, we're looking at, emissions by 
ionized gas and also fast, albeit not 
relativistic. 
Electrons, and again, we see Cassiopeia A 
as a source. 
We see the center of the galaxy as a 
source. 
so these again are energetic, localized 
sources producing these. 
And, there is also a sort of galactic 
emission, but that's been subtracted to 
exhibit, these point sources better in 
this picture. 
The middle pane, is, 115 GHz, and this is 
again an important one. 
In fact, this corresponds to the energy 
of a transition in carbon monoxide. 
Carbon monoxide? Why do, what's, there's, 
where would we find carbon monoxide? 
Turns out that carbon monoxide, while 
rare, is a very useful tracer molecule. 
It's concentrations in various regions of 
the galaxy closely trace the 
concentrations of molecular hydrogen. 
Where you find molecular hydrogen clouds, 
there are small trace concentrations of 
carbon monoxide. 
Following the carbon monoxide, we get a 
mapping of molecular hydrogen, and we 
see, as I said. 
that molecular hydrogen is constrained to 
the plane of the galaxy, and we also see 
that in certain places there are a 
broadening of sort of places where the 
hydrogen clouds spill out of the plane, 
and we'll see those again in a In a 
second. 
At a slightly higher frequency between 3 
to 25 times 103 gigahertz is how it's 
represented. 
These are infrared emissions and these 
far infrared emissions in fact are given 
off. 
We see that there are not that many point 
sources, but sort of a. 
continuous emission from the entire 
galaxy, furthermore, we see that these 
emissions seem to track the H2 emissions 
and where there is added H2 there are 
molecular hydrogen clouds, there's this 
infrared emission indeed. 
At this wave length we are basically 
observing the dust. 
It is extremely important that we can see 
the dust in the Milky Way Because then we 
know how, what we need to subtract, 
because we can use this tracing of the 
dust and of the molecular hydrogen, to 
figure out what the extinction is and try 
to reconstruct the correct luminosity of 
objects as seen through the dust, and 
notice as I said that the dust tracks the 
molecular hydrogen rather well The image 
below that is a higher frequency, shorter 
wave length yet. This is the near 
infrared, and in the near infrared dust 
is not hot enough to glow. 
What we're mostly seeing is the emission 
of stars. 
Which stars emit a lot of infrared 
radiation? 
Red stars. 
Which stars emit most of the infrared 
radiation? Giant red stars, red giants. 
And so we're looking at cool giants here. 
We see in fact that we're seeing point 
sources, 
and only towards the center of the galaxy 
we're seeing sort of the far side of the 
galaxy in the center. 
In that directions we see distant stars 
melded together into this great big glow. 
But on the periphery, we're actually able 
to resolve individual sources. 
The important thing is that we can see 
these bright stars in the infrared, 
through the dust, because remember that 
scattering and absorption and dust is 
highly frequency dependent. The high 
frequencies scatter more, and it turns 
out are absorbed more, and so looking in 
the infrared, we can see through the dust 
and distinguish the stars that are behind 
it. 
Stars that produce lots of infrared will 
of course be more visible in this 
wavelength, but we can see through the 
dust. 
Dust. 
These are all the things that Kapteyn and 
Shapley did not have at their disposal. 
At the bottom we finally see a visible 
light image of the Milky Way, this is a 
familiar image. 
We see the dark dust lanes which traverse 
the Milky Way, where there are clouds of 
dust and, molecular hydrogen absorbing 
the light and we cannot see stars there. 
of course in the visible light it's the 
blue luminous stars that dominate what we 
see. 
But we cannot see them, and it's 
interesting to compare the lowest image 
with say the middle image, and see how 
closely the molecular hydrogen clouds 
track the gaps in visible light. 
the gaps in the Milky Way as we see it, 
are precisely at the positions where 
molecular hydrogen and the attendant, 
clouds of dust are obscuring the light. 
This is the, technology from space 
observatories and X-ray's and gamma rays 
that I haven't even included here, 2 
radio telescopes that were not available 
to [UNKNOWN], and it's thanks to that we 
have such a, so much better of picture of 
how the Milky Way is constructed. 
Let's see what it has all taught us.