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.