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Welcome back everyone.
So, let's just talk a little bit about dark
matter again so we can understand its role in galaxies.
So recall that everything we know about space comes from light, what we can see.
And if we can't see it, we don't necessarily know that it's there, right?
The only way we can tell about perhaps dark material
is from the motion of the material we can actually see.
And that is really where the evidence
for dark matter comes from.
We talked about this before, but I want to go into it in a little bit more detail.
Basically, by tracking the orbits of material that we can observe, we can use
Newton's laws to back out from that the mass of the thing that is being orbited.
So, if I watch Jupiter's moons orbiting Jupiter, I can use the, how
long it takes for the orbit, the period of the orbit and the radius
of the orbit.
I can actually, from that, get the mass of Jupiter.
And that's actually the we know the mass of most astronomical objects.
Well, it turns out that we can use that to that information by looking at
the motions inside of galaxies to tell where,
how the mass in the galaxy is distributed.
And as we're going to talk about in a second, what we find from that
is that most of the mass of the
galaxy cannot be associated with the visible component,
there must be something else.
So now, let's talk about dark matter in the two different kinds of galaxies
that we know about, or that we've
talked about before, the spirals, and the ellipticals.
We won't really mention irregulars very much, but let's go on to the spirals.
So we've now talked about galaxies in a little bit of detail.
We've looked at our own Milky Way, and
now we've looked at other types of galaxies that
want the spirals that the Milky Way is
a member of that class, and elipticals and irregulars.
And the one really important thing to remember is, when you
look at all these beautiful pictures and
the reality is, that's not really a galaxy.
What we see in optical images, in, in visual images
is really sort of the after thoughts, so to speak.
It's the normal matter part of a galaxy.
But in fact, galaxies are really just a large collections of dark matter.
We spoke a little bit about dark matter beforehand.
It's this invisible component of mass that is tugging on, that is pulling on the the
visible component. That's how we know it exists.
We know it that it's there because of the motions of the luminous material.
Now for spiral galaxies, the way we can tell that there
are, that there's dark matter is when we look at the
rotation of stars around the center of the galaxy as we
move further and further out from the center of the galaxy.
What we find is, is that that material at
the outer parts of the of the spiral galaxy
is rotating far too fast to be that to be accounted for
by the mass interior, by the, the ga, the visible galaxy's own mass.
And so it must be that in fact that rotating galaxy
is in fact embedded in a much larger halo of invisible material.
And it turns out that for spirals, it's around 90% of the
mass of the spiral galaxy is in fact invisible, is dark matter.
And,
in ellipticals then, we don't necessarily see rotation, cause
there's not a whole lot of rotational motion ellipticals.
But just the motions of the of the the the average motions.
The, the plunging orbits back and forth because
in, elliptical galaxies, the stars are moving very
much like an orbits like globular clusters in
the the stellar or halo of our own galaxy.
So it's just from the average motions of the stars in, in elliptical galaxies.
We can also tell that there
is too much motion there to be accounted for just from the visible mass.
So again, ellipticals we see about 90% of the material must be dark matter.
So you have to remember that, when we look at this pictures, these gorgeous pictures.
You have to some how think to yourself, well that's not really a galaxy.
That's, that's really just some sort of pond scum so to
speak, floating on a much darker, much larger dark ocean of material.
Adam FrankProfessor
