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?