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.