Okay. We have some understanding what 
goes on in the disk. Let's look towards 
the center. 
Of course, if you thought it was hard to 
figure out the structure of the disk, 
understanding the structure of the bulge 
is more difficult, yet, 
that is the direction where there is most 
obstruction by dust and so on. 
In fact as I said, we only realized that 
the Milky Way has a bar, this was only 
verified, there were suspicions earlier 
in the past decade, so we're still 
learning very much about the star 
structure of the Milky Way galaxy. 
In the bulge we find, as I said, stars of 
varying ages with varying populations an 
interesting reversal of what we're used 
to happens in the bulge. 
the oldest populations of stars in the 
galactic bulge end up having, turned out 
to have high metallicity, so they have 
high, relatively, concentrations of 
non-hydrogen and helium elements, whereas 
the newer stars are more metal-poor. the 
interpretation that we put on this is 
that presumably, early in the galaxy's 
history, there was this burst of star 
formation in the bulge. 
these, these formed many hot large stars 
which exploded as supernovae enriching 
the bulge, bulge region in metals, and 
therefore, subsequent stars that formed 
shortly thereafter were metal-rich. 
Subsequent to all of this, there must 
have been an infall of fresh unenriched 
matter from regions of a galaxy where 
star formation had not yet started. 
So, where there had not been supernova to 
enrich the environment, this brought in 
relatively low metal, metallicity 
hydrogen, and subsequent generations of 
stars formed in the bulge would have 
formed in a metal-poor environment, and 
the bulge is one of the regions in which 
ongoing star formation seems to be going 
on as we speak right now. 
I think it's hard to see the bulge, 
of course, the most interesting action in 
some sense is that the center, in the 
core seeing the core is harder still 
infrared observations allow us to peer 
through the dust to some extent and find 
that at the center of a galaxy is dense 
cluster of stars, 
including a bunch of stars with you'd 
call them type O and B. 
So, had blue stars, but with luminosities 
far exceeding what we expect for main 
sequence type O and B stars. 
luminosities of a million, solar 
luminosities per star. 
one interpretation of this is that these 
may be Wolf-Rayet stars, so older stars 
from a starburst episode about 10 million 
years ago, which are now very luminous. 
Remember, we're measuring their 
luminosity, the luminosity of an OB star 
in the infrared. 
These may be having extreme infrared 
emissions, because they are Wolf-Rayet 
stars. 
there are many puzzles about the on, 
goings on at the center of the galaxy. 
For example, there is evidence for a very 
dense mollecular cloud but no ongoing 
star formations, 
the conditions are right for star 
formation, but star formation does not 
seem to be going on. 
At the center of it all, of course, it's 
our 3 or 4 million solar mass black hole 
in the center of the galaxy. 
black holes in general, just a black hole 
in the middle of nowhere, is hard to 
detect. Indeed black holes are detectable 
when they create matter. 
the black hole at the center of the 
galaxy is creating matter at a modest 
rate. 
there is some x-ray activity there are 
occasional flares, but there is not large 
bursts of activity as we will see occur 
in some other galaxies. 
And to give you a sense for what infrared 
radiation does for us, 
we can watch this video courtesy of ESO, 
which zooms in on the center of a galaxy, 
invisible light. 
You see we're looking right into the 
heart of the constellation the, the tip 
of the teapot, of the spout of the 
teapot, that is Sagittarius. But more 
importantly, right into the heart of a 
dustling where we can see very little, 
and as we zoom in, the image goes pretty 
much greyed and dark. And the only way to 
see through this dust will be, this is 
the visible image, 
we are going to convert to infrared light 
and compare the visible image which is 
this. Not much. 
With, as the slider sweeps over, an 
infrared image, which allows us to peer 
through and realize that these few dim 
stars that we see in the Mac are in fact 
extraordinarily brilliant, of course, the 
colors to the right are all fake. 
This is an infrared image. 
But this is the way in which infrared 
light allows us to peer through the dust 
and distinguish the goings on at the 
center of a galaxy.