So, armed with all these observational 
tools, we have a pretty good idea of what 
the Milky Way is made of. 
In this first clip, let's sort of lay out 
the pieces, and later we'll study each of 
them in term and see what we can say 
about them. 
So, what is the Milky Way made of? Well, 
the Milky Way is mostly a disk of stars 
that are orbiting, and they have a sense 
in which they orbit much as in the solar 
system. 
You imagine some primordial angular 
momentum being conserved, so the Milky 
Way has a north pole and a south pole 
typically drawn to the top and the bottom 
of our images. 
Sorry, southern hemisphere dwellers. 
and in the disk, in fact, the disk is 
broken up into two different components, 
this is a relatively recent find. 
There is a thin disk only about a third 
of a kiloparsec, it's only about 300 
parsecs thick, a 1000 light years and, 
over the radius of about 25 kiloparsecs. 
And remember, the sun that way being 8 
kiloparsecs from the center is about a 
third of the way to the edge of the disk. 
the thin disk has the bulk of the mass 
the, the stars in the galaxy the stellar 
mass in a thin disk is about 60 billion 
solar masses, and the stars in the thin 
disk are, by and large, young. 
Their ages are less than 8 billion years, 
so the sun is certainly a young star. 
And the thin disk is surrounded by a 
thicker disk with about the same radius 
but 3 times the thickness. 
So, 3,000 light years, a kiloparsec in 
thickness. 
And, the total mass in the thick disk is 
neglible if you want, only 3 billion 
solar masses. But its charaterized this 
population of this thick disk and it's 
not that it's surrounded, of course. 
the thickness permeates the thin disk as 
well but the stars are scattered more 
diffusely and the thing that 
characterizes the thick disk population 
[LAUGH] is that the ages of the stars are 
older typically 8 to 11 billion years are 
the, or 10 to 11 depending on exactly 
where you draw the line, are the ages of 
the stars in the thick disk. So, two 
distinct stellar population but the bulk 
is in the thin disk. 
at the center is a bulge where the galaxy 
bulges to a greater thickness. 
Of course, the disk thickens out as it 
approaches the bulge, but there is a 
distinct structure called the bulge and 
as it has been discovered in the past 
decade, it's actually not a sphere as 
drawn in this image, but it has a bar 
shaped. 
and it's and therefore while its height 
is about 2 kilo parsecs, it's length 
along the long axis of the bulges 5 kilo 
parsecs, and this contains about 10 
billion solar masses. 
the stars in the bulge, have all kinds of 
ages, there are different populations, 
perhaps 3 different populations, 
including very young stars, 200 million 
years or younger, and some very old 
stars. 
And we'll talk about the bulge 
population. 
And, this is the disk with the bulge and 
then surrounding all of this is the halo 
in which, for example, one finds the 
globular clusters orbiting. 
These are stars, remember they don't sit 
there, they're orbiting. 
So, these are objects that are orbiting 
in, in orbits with high inclination to 
the galactic plane. And so some of them 
might at any given time be inside the 
disk, but they would be distinguished 
because their velocity would then be 
perpendicular roughly, to the direction 
of motion of the stars in the disk. 
And the math of the halo stars is again 
very small, only 3 billion solar masses. 
And the halo certainly contains the 
oldest stars in the galaxy. 
Their ages range between 11 and 13 
billion years, that's essentially the age 
of the galaxy. 
Remember, it's almost the age of the 
universe. 
The asterisk is because there's a 
component of the halo that I have not 
discussed here, it's been recently 
discovered. 
And we'll discuss it when the time comes.