So we're going to start with the disk and 
the disk contains as we said, 60 billion 
solar, masses of stars, orbiting, in a 
particular direction. 
They are of course, in Keppler orbits. 
Along with that is some gas and dust. We 
saw the, evidence of this, 
Within the disk there is a very exciting 
and interesting sub structure, 
which is difficult to tease out, and it 
turns out that the stars in the disc 
orbit in all poss, in many orbits, some 
of them more eccentric and some circular. 
But if you look for the concentrations of 
particular kinds of stars, blue stars, OB 
stars, the attendant ionized hydrogen, H2 
regions that come indicates star 
formation. and to some extent the 
molecular hydrogen clouds, those are not 
scattered uniformly over the disk, but in 
fact are concentrated in these spiral 
arms, these line, linear structures that 
wind through the disk. 
Now, how are you going to find that, if 
you're looking length wise through the 
disc, how are you going to recognize that 
there are discreet structures? Well, the, 
this is where 21 centimeter hydrogen 
line, gave us, great benefit. 
So, what happens is, that imagine if you 
will, that here we have our disc, and 
here is the center, and here about 1/3 of 
the way out is the sun, and imagine that 
the direction of the angular momentum is 
such that this is the direction of 
rotation, and what this, graph shows you 
is the observations that you would get by 
observing 21 centimeter emissions from, 
coming at the Earth from this direction. 
And what will happen is, I told you the 
21 centimeter line is a very sharp line 
and so, of course, because, relative to 
the sun's motion in it's Kepler orbit. 
objects, at different parts of the disc 
are moving. 
There will be a Doppler shift associated 
to the relative motion of, whatever is 
emitting the radiation, relative to the 
sun. 
We can, measure that Doppler shift, and 
what we find here is a spectrum of the 
observed 21 cm emission in a particular, 
from a particular direction. 
And, what we see is that this would be 
the correspond to the wave length, 21 cm. 
We see that there is somewhat red shifted 
light coming in, 
and then a lit, a light with a slight 
blue shift and then a whole lot of light 
received or radio waves received that 
have been blue shifted by various amounts 
and typically, the way we draw this, or 
we are shown this when we are students is 
a graph like this with 3 nice peaks in 
it, and we are given to understand, aha, 
the way this works is that there are 3 
clouds along this direction, each of them 
moving with the characteristic, Keplerian 
velocity, in it's orbit, and, these lead 
to three different Doppler shifts. 
And, by noticing the three discrete 
Doppler shifts, we can realize that, 
there were three clouds in the way And by 
reconstructing sort of doing a tumblr 
graphic analysis as we sweep the 
direction in which we are looking we can 
follow these clouds and reconstruct the 
structure of the disk. 
what's nice about this image is that it 
shows us more what real life is. 
These three peaks are in fact to be 
interpreted by, as a result of emissions 
by 20 discreet objects because we know 
something about the line shape. 
And you can sort of do a fitting. 
This gives you a more realistic image of 
what astronomers are up against. 
With all that they have come up with a 
pretty good picture of what the structure 
is in the milky way's disk and here is an 
artist's conception of what this looks 
like. 
this is an artist conception from circa 
2008 and in the past decade. 
Our understanding of this has changed the 
milky way was traditionally fought to 
have 4 large spiral arms, it turns out 
that now 2 of them, the norma and 
Sagittarius arms have been demoted to 
sort of secondary sub arms, and the Milky 
Way really, is thought to only have two 
large arms, the Scuttum Centauris arm and 
the Perseus arm that come, emerge from 
the edges of the, as I said, newly 
discovered, galactic bar. 
And You can ask where the sun is in this 
picture, that would be a good question. 
So the sun in this image sits about here, 
which interestingly is not on either the 
Scutum Centaurus arm or the Perseus arm. 
In fact it lives on some sort of branched 
structure over here that is called to 
Orion Spur. 
And, in addition, there's this recent 
discoveries, include these 2 3-kiloparsec 
arms, that are very closely wound near 
the galactic, bulge. 
So we've learned how to map the structure 
of the disk. It's important to remember, 
again, that what is concentrated along 
the disk are blue stars and, ionized 
hydrogen clouds. 
There are stars and certainly gas 
elsewhere in the galaxy. 
how do we know this? Well, it's easiest 
to look at a galaxy we can view this way. 
So let me cheat history for a while and 
ignore the fact that we're in the 20s. 
This is a beautiful spiral galaxy, M83. 
We can see the concentration of blue 
light indicating new hot stars, and red 
regions indicating ionized hydrogen 
emissions in the spiral arms of the 
galaxy, but when we look at the same 
galaxy on the right in the infared we see 
that the contrast between the arms and 
the rest of the disk is far reduced. 
There are stars, there is luminosity in 
the rest of the disk. It's just less of 
it is new blue stars and ionized 
hydrogen. 
Those are concentrated in the arms. 
It's very important to realize the space 
between the arms is far from being a 
vacuum. 
So we have the stars in the disc. Other 
than that, the disc of course is the 
location of most of the gas and dust in 
the inner galaxy. 
As we said, the gas is only 5 billion 
solar masses, so something like 7% or 8% 
of the mass of stars in the disk exists 
in the form of gas. 
This is mostly atomic hydrogren, some 
molecular hydrogen, some ionized 
hydrogen, and of course traces of 
everything else. 
the dust as I said is tracked well by 
the, the gases I said is tracked well by 
dust or the molecular hydrogen is tracked 
well by dust. 
in terms of mass, dust contributes very 
little to the mass of the galaxy. 
what is important about it, of course, is 
it's optical properties, it does 
contribute to extinction. 
the dust in the, gas, tend to be confined 
to the galactic plane, in general more 
tightly than the stars, 
though we saw that there are excursions, 
and much, of the dust and gas, is 
confined to within, what we call the 
solar circle. 
A circle of radius 8 kilo parsecs around 
the center of the galaxy, so about where 
the sun is. 
Past where the sun is, the dust and gas 
density drops off rather quickly. 
So that's the structure of the disk, what 
else is there?