So we have the disc in the center of the 
bulge, and around this we have a halo. 
The halo contains about 150 global 
clusters, typical masses of up to about a 
million solar masses and it turns out 
lots of field stars, so there are 
population 2 stars orbiting in a high 
inclination orbits that are not part of 
globular clusters, in fact we now know 
that 99% of the mass of the stars in the 
halo is outside the globular clusters. 
Globular clusters are very visible 
minority. 
furthermore about half of all these, 40% 
of orbit with inclinations larger than 90 
degrees, which makes them retrograde 
orbits, so if the stars in the disc are 
orbiting one way. 
These stars are orbiting in a, by and 
large in the other direction so the 
angular momentum of the halo in net is 
about zero. 
In terms of size remember Shapley had a 
large halo of 100 kiloparsecs taking into 
account absorption and correct distance 
measurements, all but about less than 10 
of the globular clusters and most of the 
field stars are within 50 kiloparsecs. 
So typically one says, that the halo's 
sphere of radius 50 kiloparsecs. 
You'll notice this is twice the size of 
the radius of the disc, so the halo is 
larger and envelops everything. 
Of course, some of these objects are in 
the disc, we detected them first as 
population 2 stars in the solar 
neighborhood. 
That just happened to be moving with 
wrong velocity and all of the stars in 
the halo, as we said, are old between 11 
and 13 billion years. 
What else is out there? Well, we knew 
that the halo was permeated by warm gas, 
we could obeserve that because warm gas, 
by warm I mean up to 100,000 or a few 
hundred thousand Kelvin absorbs visible 
light, and so we could see the visible 
light absorption from the halo. 
some of the hydrogen atoms are excited, 
some of the other atoms are inexcited, 
are, are there and have lower excitation 
energy than hydrogen so we could track 
the gas in the halo, and there's not that 
much. 
It's not a significant mass. 
a new discovery from September this year 
so I'm putting it in there with an 
asterisk because I don't know how 
verified it's going to be, is that there 
is another gas component in the halo. 
This is hot gas with temperature of a 
million Kelvin which makes it transparent 
to a visible light because there are no 
electrons with that excitation energy but 
it has been absorbed through x-ray 
absorption. 
By oxygen highly ionized oxygen atoms in 
this gas and if the Chandler result 
holds, then this halo might extend as far 
as 100 kiloparsecs so that's more than 
300,000 light years. 
And that's past to give you a sense, the 
Large Magellanic Cloud, and they contain 
as much mass as the entire stellar 
population of the thin disk. So, 60 
billion solar masses again, over again of 
the mass of the thin disk might be have 
been lost until recently in the form of 
this halo of hot gas. 
Note, that this is twice the radius of 
the globular cluster halo. 
So we're beginning to construct our 
model. 
I mentioned the large Magellanic Cloud. 
Indeed the Milky Way is not alone, there 
are satellite galaxies, a few dwarf 
satellites as well as the Magellanic 
Clouds orbit the Milky Way. 
I should point out that there are it's 
not quite clear whether the Magellanic 
Clouds have a positive, just positive or 
just negative Kinetic total energy 
relative to the Milky Way. 
In other words, are they inbound orbit or 
are they moving off and at some point 
will disappear, but not in my lifetime? 
in any event they are certainly, 
currently very near the Milky Way and we 
can see as we'll talk about the evidence 
of their tidal interactions with the 
Milky Way. 
Some other galaxies in the other some 
other of these dwarf galaxies are 
currently in the process of being ripped 
apart by the tidal forces of the Milky 
Way which are breaking them up into 
rings. 
The objects in the ring are being slowed 
down and eventually these dwarfs are 
merging to some extent with the Milky 
Way, the Milky Way gobbling up dwarfs and 
growing. 
this will see is part of the process of 
galactic dynamics so it pays to at least 
enjoy a look or two of our neighbors. 
To the left obviously the spectacularly 
beautiful Magellanic clouds. 
If you've never seen them and have a way 
to get to the southern hemisphere by all 
means do. 
It's a mind blowing experience, 
I've done it once and I can't forget it. 
to the right the Leo one dwarf. 
This is what a dwarf galaxy looks like. 
Of course, there's a long exposure 
picture, 
you probably would completely miss it and 
look right through it. 
The way that we identify these objects as 
being part of a separate galaxy is by 
tracking both their chemistry, their 
metallicity, and their proper motion, and 
radial motion we see a group of stars in 
a given direction. 
They all seem to be moving in concert, 
then we can study their random motion and 
see that they are in fact gravitationally 
bound to each other. 
And then this if the group is large 
enough dignifies them not as a star 
cluster, but as a small galaxy.