Greetings and welcome to the
Introduction to Astronomy.
In this video,
we're going to talk
about the distribution
of galaxies
how were they
organized out in space
and how they tend
to group together.
So we can go ahead
and get started here
and what we find is that what
we want to look at essentially
is how the galaxies
are distributed.
Are they isolated or
are they in groups?
And if they're in
groups are they
in large groups or small groups?
So how are they
organized together?
And that leads us to what we
call the cosmological principle
the cosmological principle
states that the universe is
isotopic and homogeneous.
And what that means
isotopic means
that the universe looks
the same in all directions
homogeneous means that
every large chunk of space
looks essentially the same.
If you take a very
big area of space
it should look exactly the same
as another similarly sized area
of space elsewhere
in the universe.
Early observations
of this by Hubble
showed that approximately
the same number of galaxies
were found in any direction.
So as you looked
out into space it
didn't matter what
direction you looked
you found roughly the
same number of galaxies.
So kind of showing that
the universe is isotropic.
So let's take a look
at what this means
and how the galaxies
are distributed, then
and we can start off
close to us with what
we call our local group.
This is our local
cluster of galaxies.
It contains a little over
50 galaxies including
three large spiral galaxies
which are noted here,
the Milky Way the
Andromeda Galaxy
in the Triangulum galaxy.
Those are the three
large spiral galaxies
and there are many dwarf
elliptical galaxies
and irregular galaxies that are
also scattered around as well.
Now, even though we these are
the closest galaxies to us.
We are still discovering
new members of these.
And in fact, new
faint galaxies are
being found by
automated surveys that
look for concentrations
of stars and are
able to find some very
faint galaxies that
had been predicted but we're
only now being able to detect.
Now we don't this is a small
cluster with only 50 galaxies
we actually can have
much larger clusters.
And we have things
like the Virgo cluster
the Virgo cluster is the nearest
large galaxy cluster about 50
million light years away.
So this one unlike ours, which
contained about 50 galaxies
this has thousands of galaxies.
And when we look at images
like this almost every object
you're seeing here, including
some of the larger galaxies
but almost everything that
you're seeing in these images
is a galaxy.
So even some of the
small dots there
are actually galaxies
small galaxies
that are part of
this cluster M87
that the giant
elliptical galaxy is
the dominant galaxy towards
the center of this cluster.
There are also even
larger clusters
we have the coma cluster,
which is even larger.
And that, again,
is also dominated
by elliptical galaxies.
So it's an interesting
difference between our cluster
and things like coma
and Virgo clusters.
These are dominated by
elliptical or E type galaxies
whereas if you recall
our local group had
spiral galaxies as the
three most prominent members
the coma cluster contains
tens of thousands of galaxies
within it and is 250
million light years away.
And much like with the
Virgo cluster everything
you're seeing in this
image is pretty much
a galaxy that is part
of that Coma cluster.
Now what do we see when
we look in the clusters
how are the galaxies
distributed?
And that's something
that can be very
important for understanding
the evolution of galaxies
and what we see is that
within the galaxies
we have these social galaxies
and these shy galaxies.
So social galaxies are
the elliptical galaxies
these are found in the
crowded centers of clusters.
So when we look at the centre
of a big cluster of galaxies we
have a tendency to find lots
of elliptical galaxies there
spiral galaxies or the shy ones.
They are found isolated on
the outskirts of clusters
or in the very much, much
smaller clusters things
like our own local group.
So spiral galaxies do not like
to be in these big groups here
and we believe that
is because collisions
will destroy spiral galaxies
as galaxies cannibalize
each other.
They spiral galaxies
will be destroyed
and will be incorporated into
large elliptical galaxies which
is what is predicted to
happen with our Milky
Way and the Andromeda Galaxy
that they will eventually
merge into an elliptical galaxy
billions of years from now.
But in the outskirts
of these clusters
collisions are less likely.
So the spirals are able to
hang on for a longer time.
Now one of the ways we
can look at clusters
is through what we call
gravitational lensing.
Gravitational lensing
is a prediction
of general relativity.
What it essentially it says is
that mass bends space and time.
And that means that the light
paths that a star would follow
will deviate when they
pass near a massive object.
So we see here, this is
the foreground galaxy.
This is what is
doing the lensing.
We have a distant galaxy here.
So it's light heading
off in one direction
will then be bent by the
galaxy and come to earth
making us see an image
of in the other direction
and a second image.
Perhaps out here.
So we can then see these
images of the galaxy
and get multiple images
of distant objects,
whether they be galaxies or
quasars the galaxy images will
be distorted as things
pass through the cluster.
And we will see as we
look at some of these.
If we take a look at what
these galaxies might look like.
Here's an example of
gravitational lensing.
And we can see that many of
the galaxies their images
have been distorted
that this does not
look like an ordinary galaxy.
This is a whole arc up here
circled and the galaxies have
been distorted in
this case, they're
being lensed not by
an individual galaxy
but by the combined gravity
of all of the galaxies
that we see here.
And the dark matter,
which cannot be seen.
This is another way to be
able to measure dark matter
and count how much
dark matter is
there is by looking at how the
lensing goes because we can use
then general relativity to
figure out how much mass must
be there in the cluster to get
what we see to get the bending
that we actually see.
But going back for
a second - looking
at how this works
again, what we see
is the light from
the distant galaxy
comes close to an foreground
galaxy and gets bent around.
That means that what
we see over here
is the image of this galaxy.
And this would be the foreground
galaxy here in the blue.
And then we see images
up on top of the galaxy
and down below the
number of images
will depend on the
exact alignment
the better the alignment the
better, you would better.
More images you will
get and in fact, you
can actually get a ring
of material around it.
If you had perfect alignment.
So what this shows us
then as we continue
on some of the
things we can look at
is looking at how these
are grouped together.
Is that will we be
what we begin to find
is that galaxies are grouped
into clusters of clusters
that we call super clusters.
So these clusters
of galaxies grouped
into even larger structures and
we call those super clusters.
So we see here a map
of the entire universe.
And it shows all
these various clusters
and through all these
various super clusters
that we look at used to see it.
So we saw things
like the coma cluster
while the coma cluster becomes
part of a super cluster
as well.
So they become even
larger groupings
and those cluster super
clusters are then just
clusters of clusters.
We also find that there are gaps
between these super clusters
that we call voids and there
are some large voids where
there do not seem to be
any galaxies or very, very
few galaxies.
So we see some of
them that we're
kind of used to seeing we
mentioned Coma and we mentioned
the Virgo cluster as well.
They also group
into super clusters.
But we also have these great
voids between the galaxies
and one of the things we
want to be able to understand
is how this type of things
these type of things
formed, how come we have all
these galaxies forming together
like this.
And all of these
big empty regions
with no galaxies in them.
Now we can use different
surveys to map the universe
and see what it looks like.
Mapping the distribution
of various galaxies
and what we find
is that the thing
is we need the distances
in order to get a good map.
We need to know the distances.
So we need to find
the red shifts.
Remember the red shifts.
We can then use Hubble's
law to be able to determine
the distances.
So Hubble's law
will then give us
the distances and what we find
as the galaxies are actually
concentrated into what
we call filaments.
So we get these
string like structures
that the galaxies
seem to form along.
We also find lots of these
empty voids big regions where
there are hardly any galaxies.
When we make this map of the
universe and what we're finding
is that 90% of the galaxies
only fill 10% of the universe.
So we talk about how
empty the solar system
is how empty the galaxy is
how empty the universe is.
The vast majority of
the galaxies 90% of them
are concentrated into just 10%
of the volume of the universe
making the universe even emptier
than we have considered before.
So let's go ahead and finish up
here as we do with our summary.
So what we find is that
galaxies grouped into clusters.
Those clusters group into super
clusters and the super clusters
will group into the filaments
between those filaments.
There are great empty
voids where there is hardly
any material very few galaxies.
Remember that those
voids only count
for 10% of the galaxies
in the universe.
And while there are a large
number of them compared
to what we see in the
other 10% of the universe
it is very, very empty.
And then we also looked at
the cosmological principle,
which said that the universe was
both homogeneous and isotopic
on the largest scales
not on the small scales.
And if you look at the
image before that we talked
about what we saw was
that the universe does not
look homogeneous or
isotopic because there
seem to be these great voids.
However, we are talking
about the very largest scales
when we do this.
We are not talking
about the small scales.
We are actually looking at
the very largest scales.
And if we take a whole big
chunk of the universe over here
and a big chunk of
the universe over here
we would find roughly
the same number
of galaxies in each of
these very large chunks.
So that completes our lecture
on the distribution of galaxies.
We'll be back again next time
for another topic in astronomy.
So until then, have a
great day, everyone.
And I will see you in class.
